Retro Vintage - Gordon Rankin's Bugle - a Super 45 SET Amplifier

Gordon Rankin's Bugle - a Super 45 SET Amplifier

Introduction

Gordon Rankin (of Wavelength Audio) is reknowned in valve audio circles, no doubt heightened by a number of amplifier articles in Sound Practices magazine. One such article from 1997 concerns the famous 45 SET amp, named The Bugle. It's a simple amp: 5751 (or 6072) in SRPP as driver, capacitor coupled to a 45. PSU is dual choke input (a simple single stage LC) using a pair of small Hammond 157G 30H@40mA choke and a 100uF + 100uF Cerafine paralleled and bypassed with a 20uF SCR. But I should be fair to Gordon - I haven't built the circuit exactly as he specified it, so I should probably call mine "Bugle inspired". If you want Gordon's circuit it featured in issue 13 of Sound Practices - you can buy the complete back catalogue from various places, including direct from Joe Roberts through Ebay. Well worth the $30 for a CD of seminal articles (doesn't seem to be available in 2025). Or if you're feeling cheap and lazy download the PDF from DIYAudio...

So why did I want to build a Bugle? Well, I'd wanted to build a 45 SET for ages and reading around I came across the Bugle and liked the look of the circuit, and the positive reports of those that had built it. It's a simple circuit too, and I can't help feeling that simple is good; as long as it's good enough... Building the circuit is pretty straightforward, and was an opportunity to use my Experimenter's Chassis for the first time.

Building the Bugle

Having the sockets ready to go made construction very easy. The 45 stage was taken from my C3g-2A3/45 amp. Rankin specified a 22uF SCR cathode bypass cap, I just reused Blackgate 100uF NHs. For the coupling cap I used a Soviet 0.047uF teflon K72N from stock. The 5751 stage is exactly as per the Bugle, except I replaced the 100k grid leak resistor with a 100k volume pot. OPTs are AE 2.5k c cores.

I hadn't built the valve series regulated power supply by the time I built the Bugle so I went with Gordon's passive PSU. I didn't have Cerafines so I used a pair of 100uF + 100uF Blackgate WKZs instead. I didn't bother with the 22uF SCR bypasses, bypassing Blackgates is not generally recommended. I needed to drop the voltage with a pair of paralleled 390R resistors after the rectifier as my AE 400-0-400 mains transformer is a little too big. I added some 220nF snubbers to the chokes too for good measure.

And this is what it looks like wired up. Well, turns out I didn't have a photo of the insides, so here's one of the outside...

And that's it. Single ended triode simplicity. So how does it sound?

Performance

Now I've never built a power supply with such a large amount of henries and microfarads before. My own especially have been rather skinny in comparison following the principle of less turbulence in the stream being better. So I was very interested to see how it performed...

And after a few voltage checks it was time to connect the speakers and plug in to the Squeezebox. I'm not a huge fan of digital but I rather like my new shiny Squeezebox. Yes, it's convenient, but I reckon it sounds pretty good too.

So switching on, silence. No hum. That's either really good or really bad. Pressing the play button resulted in music, so it was a really good thing! I didn't get much chance to listen at first but later that night I settled down and listened to a few tracks.

Wow! Wow again! Big, bold, detailed, transparent and musical. I was blown away, particularly by the bass which was big, deep and tuneful. The amount of energy 2W of single ended 45 puts into the room is amazing. More extended listening is required but this is very promising. Maybe it's system synergy, or the WKZs, or all the Blackgates having a product greater than their parts? Dunno but it's good. I like this very much!

June 2008.

Amorphous Cores and 2A3s

I've lived with the Bugle for a little while now and I think it's a really nice amp; my first thoughts are confirmed and I like it very much.

My eagerly awaited 5k amorphous core OPTs finally arrived from AE. It was a long, long wait, but well worth it. They're well overspecced for the Bugle at 25W but they should be useful for many amps. They're still running in but I like them a lot - even from new there was an extra layer of detail and ambience that the excellent c cores don't have. The amp just got better.

Then one evening I was idly musing what the Bugle would sound like with 2A3s instead of 45s. So without any further thought I changed the cathode resistors to 750Ω and swapped over to a pair of RCA dualplate 2A3s. I was just a little surprised when I turned on again as the sound was remarkably similar, perhaps a little more detail (maybe?) but little else. Okay, I should remove the paralleled 390Ω resistors as the 2A3s draw more current than 45s and B+ is now down to 305V. It seems that the driver stage is quite dominant in forming the sound characteristic, or drives both valves well allowing them to perform similarly.

One day I'll get around to trying the series valve reg PSU in place of the passive PSU, and then I might just build a keeper, perhaps switchable between 45s and 2A3s...

September 2008.

December 2025 Update

I enjoyed the Bugle so much it was an amp I wanted to come back to some time, more so than others I'd built. From here I played with a 3-stage all DHT amp (26-10Y-300B) aka "Madness", cos playing with 3 stages of DHTs truly is madness. Then I had a dalliance with GM70, and rebuilt a 2A3 amp, which I still have as it's lovely.

But a couple of years ago I rebuilt the Bugle and it's the amp I still use. Playing with other valves allowed me to appreciate  the wonder of a 45. I thought the copper GM70 amp I built was the endgame for me, and it certainly had some stomp (as my friend Steve described it), but I was surprised by some additional detail when I went back to 45s. A caveat, I use it above 300Hz, directly coupled to the Fostex FE208EZ on a small open baffle (with T90A supertweeters hung off the 208EZs with a passive second order Butterworth high pass filter).

This time I used a pair of Onetics 4.5k impedance output transformers than Bud Purvine had wound for me to use with my KR 300BXLSs. The rest of the audio circuit used the same components as before if I remember correctly.

The power supply was similar - a 400V-0-400V AE mains transformer and the small Hammond 30H chokes, with 100uF+100uF WKZs and a GZ37 rectifier.

After I built the Bugle the first time around I emailed Gordon and he confirmed the point of the 20uF SCR wasn't to bypass the 100uF+100uF Cerafine but because he felt it needed more capacitance due to the amount of ripple.

After rebuilding the Bugle more recently I dropped Gordon another email as I was interested in what he thought the best 45 amp is he's heard/built. As part of that discussion Gordon strongly recommended going from dual LC to dual LCLC supplies. I looked at what chokes I had at my disposal and figured I could actually try dual LC-split-LC to the 45 and LC to the 5751. So reconfiguring the two WKZs so that each was common to both channels allowing me to make the star earth at the second C (using the first would not be good) I ended up with 14H-100uF-split to 30H-100uF to the 45 and 150H-10uF to the 5751. I don't know if I could hear much of a difference. Maybe tuning of the two resonant circuits (LC and choke resistance) might help, but perhaps because I use the amp above 300Hz the difference isn't so obvious?

This really is a cracking amp. If you can manage with just a couple of watts you should build one.

Retro Vintage - Experimenter's Chassis

Experimenter's Chassis

Introduction

Each time I think about building an amp I'm faced with the whole breadboard/valve socket/chassis conundrum. Cutting, drilling, filing aluminium plate, mountings etc. etc. So I thought building an Experimenter's Chassis might be a good idea - short term pain for long term gain. So I procured a sheet of 2mm ally and carefully marked it out. Then I started to drill holes. And more holes. And yet more holes. Then I had a break to let the drill cool down. And then I drilled some more holes. Enough for two each per channel of UX4, UX5, B4/5, octal and noval. Mounting holes for a choice of AE OPTs, speaker terminals and a couple of umbilicals completed the drilling.

With the protective film...

...and without

The plate was screwed to sides made from 120mm x 18mm softwood from a DIY shed.

Inside I mounted miles of tag strip fixed to valve socket fixings. A small plate on the front for phonos and volume pot finished the job. For the PSU I've been a little more pragmatic choosing to go open breadboard to facilitate playing with chokes and big caps. But eventually I think I might just build a Steve Bench-style valve series reg (Steve's (now Stephie) website went down years ago, but his/her contribution is so highly valued there are a few places where the pages have been hosted since) along the lines of the one I played with when breadboarding the C3g/Aikido phono stage (dead link) to give me easily variable B+.

Back to the breadboard for the PSU, to link it to the audio stage I made a pair of umbilical cords. I wasn't convinced it was a great idea to go with umbilicals but without trying I wouldn't know. I wanted plenty of flexibility so each cord has six lengths of wire, which I first soldered to the male part of the connector. This is a reasonably delicate job and one I would have made a real mess of if I'd tackled it when I first started DIYing...

A little worried about the possibility of shorts considering flexing of the cord I added some shrink wrap which finished the job of nicely.

Finally I wrapped both umbilicals in some expandible black nylon braid. It's not perhaps as good a solution as I would hope for as there is little protection for the cables when plugging in and removing the socket. But it's a start and hopefully I'll find a sturdy metal socket that has a high voltage rating as well as current rating for HT and heater duties.

Here's a photo of the PSU breadboard, from when I was playing around with PSUs for the Bugle amp.

So now when I want to try something I should be able to knock an amp up quite quickly. Or that's my hope at least.

September 2008.

December 2025 Update

I never really used this. I should have used it a lot more. But I didn't. It's still in the loft somewhere, no doubt I'll probably rob sockets and tag strip from it eventually.

Retro Vintage - C3g Driven 2A3 Single Ended Valve Amplifier

C3g Driven 2A3 Single Ended Valve Amplifier

Introduction

The WE91s are sounding really lovely and I'm thinking that I have the makings of a pretty good system now. Of course, that doesn't mean I'm happy to live with what I've got for the rest of my days when there's so many more valves to try...

But before trying new valves I felt I had a little bit of unfinished business with the 2A3. The Loftin White had some real strengths but the one I built at least was limited at the frequency extremes. Whilst I had been building the WE91s James D had posted on the WAD forum about his 2A3 amp, which had originally started out in life as the WAD 2A3 PSE. Richard Higgins had also built a variant of this amp (but using a C3m pentode) and was very impressed by the sound.

James had altered the amp somewhat by removing one of the output valves and changing the driver valve to a C3g, a German telecom pentode noted for its linearity. A voltage regulator tube on G2 of the pentode was another notable change.

I emailed James to discuss the possibility of building his amp using the bits I had left over in the Loftin White amp. James was very enthusiastic and helpful, drawing up a slightly revised circuit to suit my components, i.e. a Hammond 374BX mains transformer, Hammond 1627SE output transformers and GZ34 rectifier.

The amp requires four chokes, two in the power supply and one in each channel of the audio circuit. I needed at least three more chokes and weighing everything up I decided this was a good opportunity to get some more iron, and AE Europe in the Netherlands had a good reputation for quality transformers at a reasonable price. The only downside was the delivery time, which turned out to be five months. This was largely due to their popularity in selling excellent iron for a good price. Fortunately things are a little better now, taking eight to ten weeks apparently.

So in went an order for four 6 chamber EI chokes and a pair of 2.5k SE c core output transformers, as well as a 100uF+100uF Blackgate WKZ for the power supply and a couple of 1uF Mundorf silver PIOs for coupling. And I waited. And waited. And waited. And after about six months I got a rather heavy carboard box full of beautifully made chokes and handsomely potted output transformers. Fortunately the wait had given me plenty of time to procure the other bits I needed. Blackgate NH caps for the cathode bypass caps, cheap enamelled wirewounds for cathode resistors, and an Alps Blue 250k pot. Alps Blue are really good, aren't they? Commercial amps costing thousands have them in, so they must good, right? Well I got mine from ebay for 8 quid so they're not exactly dear... But it was a good way to get started.

Construction

I wasn't building a previously-prototyped amp this time so it made sense to breadboard. I don't have the space to have one large breadboard, nor separate pieces for the PSU and audio circuits, so I cut a single piece just smaller that the top of the rack, and then made a platform for the output transformers and mains transformer to sit on at the back. A pair of aluminium plates were cut, filed and punched for each channel's C3g, 0A2 and 2A3, plus another plate for the GZ34 rectifier. The plates were elevated above the breadboard such that I could place a "cover" over the guts of the amp and allowed the filament transformers to be mounted on the breadboard beneath the 2A3 sockets.

The plates for input phonos and speaker conectors were attached to the back and then the significant components, chokes, caps and filament transformers were laid out.

Then it was just a question of working through and wiring everything up. I used a number of star earth washers to star earth the amp. One for the power supply, one for each channel, and one for each output stage, all starred to a "master" to which the earth lift resistor is soldered.

Switching On

Always a nervous time the first switch-on. Although nothing went bang my nerves were well founded. The 0A2s which should light a nice purple glow when the voltage reaches its striking voltage instead flashed and clicked, what transpired to be oscillation of the C3g. Connected to the speakers with a little volume set the oscillation caused a rather unpleasant thumping type sound through the speakers. Interestingly, when I tried to check some voltages with my multimeter as the probe approached the valves the multimeter clicked in time with the oscillation of the 0A2s.

This was a little disconcerting as I'd not come across oscillation before. The more I thought about it the more I figured it was oscillation, but I wasn't sure what to do about it. What was needed was a trip to the doctors for a good coat of looking at. Fortunately the good "Dr." Nick Gorham was taking on new patients and an appointment was made to visit his "surgery" in Halifax.

Arriving at Nick's I was greeted by what seemed like a dozen rather friendly lurchers; Nick's wife runs a lurcher sanctuary. Then there were the chickens, rabbits and ferrets - a real conservation area.

Arriving at the "surgery" was another experience - Nick's listening/hi fi room is a haven for all of us who have to use the dining room table to do any soldering and then share any of our creations with the family in a communal living space. Nick had two large-ish James D-style open baffle speakers about a metre from the rear and side walls with a pair of "Russian trawler" 6C33C monoblock amps on the floor next to the speakers. Between these amps was his breadboarded 300B amp with regulated variable power supply, and then against the back wall was his monster 211 breadboaded amp. Nick likes valves and breadboards, and his amps are invariably some of the very best I've ever heard.

Then to the side there is his Technics SP10 with Hadcock arm and Denon DL103R cartridge, his own LCR phono stage (which is still the best phono I've heard) and a stash of vinyl. Then there's a narrow strip of carpet to tiptoe through to the listening seat, or beyond to the workbench. Yes, this is a functional room.

So, the patient was placed on the workbench and the scope attached. Nick diagnosed the oscillation pretty quickly and the addition of 300R? carbon resistors to G1 of the C3g pentode sorted the really high frequency oscillation.

But there was still some low frequency oscillation which was manifesting itself as oscillation of the speaker cones. I'd guess it had a frequency of about 50Hz and although the oscillation wasn't causing any sound to come from the speakers it shouldn't be there.

Nick had a pretty good idea what was causing the oscillation, suspecting that the 22uF cap in the feedback loop next to the C3g needed increasing. Clipleading a pair of 120uF caps, one in parallel with each channel to give 142uF stopped the remaining oscillation in its tracks. Playing further at home I found that 54uF on each channel was enough.

Listening

I like this amp. I like it a lot. And after the C3g heaters were referenced to ground it's completely quiet through my 97dB Fostex. The sound is detailed and well balanced, and the improvement from switching from the monoplate Sovteks to the dualplate RCAs was even more discernable than with the Loftin White.

Comparing to my other amps I would say that this amp is a little better than the Loftin White overall, but not quite as good as the WE91s with the KRs.

Improvements

Well, the RCAs are very nice. A margin better than the Sovteks.

Nick did suggest an alternative. When he had been playing around with this front end Nick found that he preferred taking the 0A2 to ground rather than the top of the cathose resistor. I tried it but couldn't hear any difference.

Nick also suggested trying one choke instead of two in the power supply. Playing around with different values of capacitance I found that 10H and a large 30uF motor run cap was enough to give a ripple small enough that I couldn't hear any hum.

Sticking with the WKZ I could hear a small difference between a single choke and two. Whereas the two chokes had a firmer, more beefy, bass the single choke seemed to remove a veil: it was more detailed with a lighter touch. So I wired 10H 100uF up and lived with it for a while. After about a month I went back to 10H 100uF 10H 100uF and thought I preferred the extra beefyness of the second choke. I think it's probably a suck-it-and-see thing from amp to amp. More valve welly might be better suited to a single choke perhaps?

I did add some choke snubbers to the first choke, a pair of 220nF 1250V Wima PPs from Maplins. These had a subtle effect on the sound, removing a veil and allowing more detail through. I got a similar improvement by removing the C3g gridleak resistor and just using the volume pot.

And that Alps Blue pot that's used in all those high end amps? I replaced it with an altogether better one. I was looking for a Panasonic pot as these are supposed to sound rather nice. Benny at Aquablue (dead link - used to be diyparadiso.com) in Belgium used to supply them but he only had 50k pots and I didn't want to go as low as this. He did have a 100k pot which he reckoned was on a par with the Panasonic, though he didn't remember who made it. So I gave it a whirl, and sure enough it's a lot better than the Alps.

A Diversion Into 45s

I have a few old 45s I bought on ebay for the day I get round to building Gordon Rankine's Bugle amp; a well regarded amp which uses a 6072 or 5751 in SRPP to drive a 45. But thinking about the C3g driver stage I thought it would probably drive a 45. Checking with James he said give it a go, so I did, changing the cathode resistor to 1k5R to suit the 45. Whereas 2A3s like a 2k5 impedance in the output transformers 45s prefer 5k. Unfortunately I didn't have any 5k opts so I used the 4ohm tap instead of the 8ohm. Then all I needed was a little tweak to the first power supply cap to bring the HT voltage down a touch, and that was it.

This was the first time I'd heard a 45 amp at home so I was really interested to hear how they sounded. The first thing that struck me was that the 45 sounded rather warm and a little coloured in the bass after the 2A3. Initially I found the 45 a little uncomfortable to listen to such was its apparent colour. But I decided to live with it for a while to give me chance to get accustomed.

Over the space of a month or so I grew to appreciate what a 45 can do. It has a magical top that a 2A3 can't match, nor many others for that matter. But the bass is just a little woolly, in this amp at least. Maybe the PSU needs some tweaking to get the best out of the 45? But somehow I can't help feeling that the 45 sounds rather like a tabletop radio, the very application it was intended for. Anyway, I definitely need to build a Bugle to compare.

Interestingly, when I wired up for 2A3s again I had problems with the low frequency instability again, even though I had only reconnected what I had used without problems earlier. Anyway, adding 200uF to the 54uF cap of one channel solved the problem.

Conclusion

Overall, this has been a very interesting amp to build, and I've learned a great deal. The high transconductance C3g pentode has not been without problems, and the low frequency instability has been frustrating at times too. But I played with a few new things and have a great sounding amp to listen to my 2A3s. I think the next thing to try is a nice triode or two in place of the C3g.

February 2008.

December 2025 Update

Gosh, there's so much to think about with this one. Ultimately it didn't survive the cut but it was an important amp in my journey as there were a number of new construction issues to understand, particularly the C3g pentode oscillation. I love the look of glow tubes and went on to play with octal versions. I haven't used them for years for some reason, probably because they're just another valve to accommodate, but I really do love the glow.

I'm pretty sure that Alps pot from ebay was a fake. But Alps aren't that good anyway.

I never posted James's circuit, here it is

Nor did I ever post any photos, but I found some buried on a hard drive.

This shows the AE potted c core OPTs

This one shows Hammonds, must have been a clipleaded experiment

 

Retro Vintage - Bluebell Audio "WE91" Single Ended Valve Amp Monoblocks

Bluebell Audio "WE91" Single Ended Valve Amp Monoblocks

The Loftin White 2A3 has pride of place in my system. After a couple of decades of solid state grain I was enjoying the wonder of direct coupled RCA 2A3s (dualplates admittedly). I'm having to use my old Exposure pre amp to control it unfortunately but it sound's great and I'm really happy with it. Why do I need anything more?

Well, because I'd become hooked... without realising it. That I was happy with the Loftin White was irrelevant, I needed to smell solder fumes. Nobody warned me how addictive this hobby could become. And I was now aware of how many valves I could try in so many configurations; single ended of course!

During the course of one of my conversations with Philip at Bluebell Audio, Philip had raved about a pair of 300B monoblocks based on the old WE91 circuit which he had just finished after sitting incomplete on his shelves for a few years. Over the following few phone calls Philip's enthusiasm for the WE91s didn't wane at all. Here, it seemed, was a 300B SE amp that could do bass and treble as well as the renowned midrange... Here was my next project.

Circuit Description

The actual incarnation of the WE91 is Philip's so it's not appropriate to post it here, but it's very similar to the Angela circuit at www.Angela.com. If you speak to Philip he'll no doubt be very pleased to discuss it with you!

If you have the first issue of Sound Practices the circuit is in there too. You don't have the first issue of Sound Practices? Have a look on ebay and US$30 buys a CD of the entire Sound Practices output from Joe Roberts himself.

Basically, the monoblocks have a 6SJ7 pentode driver stage coupled with a 0.1uF Audio Note copper capacitor to the 300B. Blackgates for both cathode bypass caps, and Cerafines in the PSU. Mains transformers and chokes are Hammond, and I was able to save a few bob by reusing the Hammond 1627SE output transformers from the Loftin White.

As the WE91s are monoblocks some method of control is needed. The beauty of 97dB loudspeakers in a typically sized British living room is that they don't need a lot of power, so I built a simple passive preamp to control them. This was the easiest way (rather than an active preamp) and allowed me to control the headphone amp from the same box too.

Construction

Starting with a Hammond 12" x 10" x 2" black powder coated chassis the first job was to cover the outside surface with 2" wide masking tape. Experience has taught me that with the best will in the world the metal swarf from all the drilling is likely to mark the chassis unless protected.

Then the layout of the iron and valve sockets can be attached. I drew the layout in AutoCAD first , then printed it out and attached to the chassis along two adjacent sides so I could clear the swarf out periodically. Guess I find it easier this way than marking out on the masking tape.

Then it was a matter of labouriously drilling each hole, starting with a 2.5mm drill bit and opening up with increasing diameter bits 0.5mm at a time. Hole punches were used for the larger diameter holes, such as valve sockets etc. I used a captive power lead rather than an IEC socket. It's much easier to use a circular hole punch for a cable gland than mess about fashioning an odd shaped hole for an IEC socket. Here's the chassis drilled and punched...

And with the masking tape removed...

On to the more interesting bit, building the amplifier. First job, install the iron, valve sockets, tag strips and rest of the hardware. The 1627SE output transformer isn't attached to the chassis yet as I was still using them with the Loftin White...

And inside showing the 300B filament transformer and tag strip...

Then start to install the components and wire up. I sketched out the layout of all the components before I even thought about marking out the chassis layout. This allows me to refine the layout to optimise wiring etc. First lay the components out to see how they actually fit...

Then it's a relatively simple job to build the amp and provided I've got the sketched layout correct I have a master plan to compare my build to. Double and triple checking of the sketch helps!

Finally the amp is complete. Now repeat for the second monoblock...

Listening Impressions

After the amps were installed into the system controlled by the passive control unit it was time to plug in the valves. The 300Bs were Chelmer badged - they look a little like Valve Art, but who knows what their heritage actually is? Philip supplied some nice NOS Sylvania 6SJ7s, and a pair of new production JJ GZ34 rectifiers to complete the compliment.

Power up, and fortunately no exploding electrolytic caps! Unfortunately one of the GZ34s was arcing. There have been a number of reports on the web about dodgy new production JJ rectifiers and I can now add my own. So I switched the dodgy JJ for the Mullard GZ33 from the Loftin White and we were away.

Straight away I could hear there was more detail and control than the Loftin White, but it wasn't until the amps had been running for a few days that the bass opened up. Now, in addition to the nice midrange and treble there was real weight and extension to the bass. If the WE91 is the equivalent of FM radio the Loftin White seemed like AM radio. I was quite surprised. Not just that the WE91s sounded so good, but that in comparison the Loftin White sounded rather limited in the frequency extremes. So much so that I'd like to rebuild the Loftin White sometime to see just how good I can make it.

The WE91s had taken me a little further in the journey towards musical reproduction realism. I was very pleased.

Improvements

The easiest place to start is with some valve rolling. Reading on the net suggested that some nice RCA red can 5693 valves would be a decent replacement for the 6SJ7s and some were duly procured through a famous online auction site. Sure enough these were a small but worthwhile improvement. And they look kinda funky in red too.

Weighing up my options for the dodgy GZ34 rectifier I considered a pair of new production Sovteks, but in the end got another Mullard GZ33 from Philip to match the existing one.

Then for the 300Bs. I tried some Electro Harmonix (not the Gold Grid version) first they were a little different but not really any better. Weighing up options, two likely candidates were EH's Gold Grid version (which can be had for under 70 quid a pair) and TJ Meshplates costing a little more. I ummed and ahhed for a while, and after concluding that I could end up spending quite a few quid trying different 300Bs, I took a deep breath and bought some KR 300B XLS from Rod Burman. These are the turbo charged versions and are not cheap, but I really, really like them. These valves have transformed the amps. There is more detail, more control, more tone, more everything.

Lovely KRs

Monoblock with revised valve contingent - KR 300B XLS, RCA 5693 and Mullard GZ33

The only other change I made was different output transformers. I had ordered a pair of 2.5k c cores from AE Europe in the Netherlands as part of a larger order for another 2A3 amp, and whilst I was building the new amp I broke the c cores in with the WE91s. AE have a good reputation amongst DIYers, both for sound quality and value for money. I could tell as soon as thy arrived they were beautifully made, but how did they sound?

When I first clipleaded the c cores in to the WE91s they sounded very similar to the Hammond 1627SEs they replaced. About a week later I had put an album on and disappeared through to the kitchen when I stopped in my tracks, turned around and went back in to the living room. Where had the bass come from? Deeper and more extended, very nice. Treble is also improved. These are very nice output transformers which also happen to be very good value for money. I had to wait 5 months for them as AE was so busy, but I know that one day I'm going to order some amorphous core output transformers from AE. The only question will be silver or copper wired...

Conclusion

I've been listening to these amps for about eighteen months and I really like them. The KRs and AE c cores transform very nice amps to really excellent amps. They aren't particularly cheap to build, probably in the order of 1100 quid, but for those of us not able to design the amps we'd like this is a fine way of achieving really good sound. Just don't try to use them with inefficient speakers.

Future Developments

300Bs seem to need rather a lot of current to drive them properly and I rather suspect that the passive pre I use is not letting the amps achieve their full performance. An active pre amp or another stage may be a worthwhile improvement perhaps. I wonder about an Aikido-style driver between the 6SJ7/5693 and 300B...

And I must try the really nice looking mod that Paul Barker magiced up, adding a voltage regulator tube to the 5693 and doing away with the cathode bypass cap and associated gubbins around the 5693. I haven't plucked up the courage to tweak the amps yet for fear of breaking them. I need another amp so I can have a play without being musicless.

August 2007.

Heaters Update

Over the last few months I've been musing why the WE91s didn't sound as good as when I'd first put the KRs in the amps. I reasoned that it could be that my ears had caught up with the performance and I could now hear the deficiencies in the sound. But I felt that the sound had actually deteriorated quite considerably, to a much greater extent than simply me becoming very familiar with the amps. Specifically, the sound had become laid back to the point of me wanting to push the music along. Not slow exactly, but not pacey as I knew the KRs are.

Thinking about things though, it occurred to me that maybe there was potentially a problem with the heater voltages. Philip had specified a simple CRC DC rectification for the 300B heaters as he had some hum when prototyping. The R of 0.1 ohms was set to give the required 5V. So if the KRs draw more current that cooking 300Bs then the heaters may be under voltage. It would have made more sense to have checked the voltage when I first put the KRs, still, better late than never!

4.4V DC. Not good. It seemed very believable that the under voltage has slowly caused the sound to deteriorate. The question is, would upping the voltage reverse this?

Removing the bridge and CRC filter and AC heating the KRs direct from the 5V Hammond filament transformers gave 5.5V AC. I was a bit surprised that the filament transformers should be so much over the required 5V, but regardless of this I plugged the amps into the system and turned on.

After listening to the amps with completely silent DC heating the tiny amount of hum was something of a surprise. Initially the amps sounded quite thin, plenty of "brightly lit midrange", but the bass was very light. But with time the sound evened up and returned to the wonderful sound I knew. Bodging together the 0.1R resistors and a couple of 0.47R resistors I had in stock I was able to get the voltage down to 5.07V. I think it sounded better at 5.5V but I'm trying not to think about that as burning up the KRs prematurely would make them very expensive valves... Still, it's very pleasing to get the sound back to where it was, fortunately valves seem rather tough and tolerant of misuse.

What has been really interesting about playing with the WE91s again is that I have spent quite a bit of time of late playing with 45s, and whilst 45s have a magical top they seem to be a bit vague in the bass. Maybe it's my amp, and I need to play further, but rediscovering the KRs has been really good and spurred me on to tweak further with the VR tube mod and the driver stage. I'd like to try choke input in the power supply too, but that's going to need a couple of rather large mains transformers.

December 2007

December 2025 Update

These were nice amps, really nice. Yes I worried about it not being a choke input power supply, and a little about the AC heating. But tackling these would have made them very different amps, much bigger, and I would have needed at least new power transformers, and probably another choke.

I might have experimented further but sadly one of the monoblocks met with an accident one Christmas Eve afternoon, in 2016 I think. One of the Alpair 12P MLTLs I built (and are featured in this blog somewhere) thought it would be amusing to fall over, smashing a KR300BXLS, a GZ33, and probably a 5693. I weighed up buying another pair of 300BXLS, but they were already considerably more expensive than the £300 I'd paid for them. I wondered about "lesser" i.e. cheaper 300Bs, but even cheap ones weren't cheap. In the end I did nothing and pursued other valves. 

That's not quite true. The monoblocks are good candidates for using 6CB5s instead of 300Bs, which are dirt cheap, but do have a top cap. I bodged one up to try but was underwhelmed by the sound and put them on the shelf for another day. Maybe. (Thomas Meyer has written about a number versions of 6CB5 amps in his Vinylsavor blog.)

Retro Vintage - TubeCAD two-tube OTL SE headphone amplifier

TubeCAD two-tube OTL SE headphone amplifier.

So I have a lovely direct coupled Loftin White-esque 2A3 SET amplifier which sounds great through my Fostex FE208EΣ rear loaded horns. But I also have neighbours, and, if I'm completely honest, my better half really doesn't enjoy the footy commentaries on my local radio station in the same way that I do... So the next project was to be a headphone amplifier. After discussions with Philip at Bluebell Audio (dead link)he suggested John Broskie's output transformerless single ended headphone amplifier, which appeared on John's website (dead link - try the WayBack Machine) in December 1998.

My implementation of John Broskie's OTL SE headphone amplifier

According to the website, it's an amplifier capable of driving 300Ω Sennheisers as well as 32Ω Grados, and uses 2 number 6922 dual triodes per channel. Philip knocked up a power supply using a small Hammond 369AX 250V centre tapped mains transformer with two IN4007 diodes for rectification and a CRCRC smoothing arrangement after, the final RC being separate to each channel.

Construction

The plan was to use a black powder coated 13.5" x 5" x 2" Hammond chassis longways. Holes were drilled on the back for the input phono sockets, fuse holder, and cable gland. No more cutting IEC connectors for me!

Rear view of headphone amp. Note captive mains lead!

The mains transformer was mounted at the back, with an on/off toggle switch on the right side and a green led on the left. Then some holes to attach the power supply capacitors, and then the four miniature 9 pin Noval sockets. Philip had some concerns that the 6922s may be a little microphonic so supplied the sockets with skirts and spring-loaded cans. Finally, a Switchcraft 1/4" headphone jack was mounted on the front.

Internally, the tag strip was mounted on the fixings for the valve sockets or beneath the transformer to minimise visible fixings on the top surface of the chassis. This needs to be planned otherwise it's pot luck whether the tag strip fixings will mate up.

Internal view

Close-up of audio circuit

Close up of power supply

The compact nature of the chassis helps to make the amplifier look really quite neat and smart, but I had a nightmare building the audio circuit as space and tag strip turned out to be in very short supply. If I was building it again I would try to get some more space for the audio circuit, though I'm very taken with the external look of the amp and wouldn't want to change its appearance greatly.

Switching On

Well, this was the third valve amp I had constructed, but I was still nervous of things going bang. Fortunately nothing did, but, not thinking, I turned it on without headphones plugged in. Inserting the headphone jack with the amp switched on didn't seem to do it any good; I was greeted with loud rustling and then silence. It's probably testimony to the ruggedness of valve amps that after about five minutes of panic I switched on again and had sound! Phew, what a relief. I hadn't trashed the thing before I had started.

Listening Impressions

So the amp's working and my heart rate has started to settle down. The first thing I noticed was the dreaded hum. I checked the voltages which seemed reasonable, although the cathode voltages weren't particularly balanced between channels. I tried moving the four number 6922s around but this only had a small effect on voltage, and none on the hum.

Tackling the hum, I found judicious broddling of the cables inside sorted the worst of it out. Interestingly, it was the shielded cable running from the phono inputs to the input of the audio circuit at the front of the amp that was picking up the hum. Mental note, shielded cable doesn't provide complete immunity from hum. Much better to choose a cable routing regime that avoids the interference altogether. The remaining hum I could perhaps equate to AC heaters; I can hear it if there is no music playing, but as soon as there is (even footy commentary) the hum is no longer noticeable.

I only have a pair of 20 year old Sennheiser HD414s, not a pair of the 650s I fancy, but the sound is very nice. There's that wonderfully fluid grainless sound that valves produce, and that makes it very relaxing to listen to. There's no shortage of detail though, and there's plenty of texture too. I think the 414s are the weak link at the moment, but the combination is a very good alternative to my Loftin White and Fostex horns.

Improvements

Thanks to Steve Shiels I replaced the original Chelmer 6922s with some nice NOS Mullard E88CCs. Very nice, a notable improvement over the Chelmer 6922s. I suppose I really should rebuild the audio circuit too and see if I can balance the cathode voltages, but the amp sounds pretty good as it is and I haven't felt the need to do this yet.

Chelmer 6922 and NOS Mullard E88CC flanked by spring-loaded screening cans

Reading John Broskie's TubeCAD blogs, JRB has published a few Aikido circuits including headphone amps, so I want to try one of these. It's a good excuse to start playing around with the Aikido topology. Then I'll treat myself to a pair of 650s too.

December 2025 Update

Another amp that didn't stand the test of time. I couldn't get rid of the hum sufficiently, I suspect because of my lack of experience of amp building at the time and because it was a more demanding application and layout. It was a useful step on my amp building path though.

The nearly bare chassis sits unloved in a cupboard somewhere.

As the page is no longer available here's JB's circuit.

Retro Vintage - Bluebell Audio "Loftin White" Style 2A3 Single Ended Triode Valve Amplifier

Bluebell Audio "Loftin White" Style 2A3 Single Ended Triode Valve Amplifier

With the WAD PHONO II and PSU II safely under my belt I could get to building what I really wanted - a low powered single ended triode valve amp. The beauty of 97db efficient loudspeakers is that they don't require oodles of power to get a nice sound from them. Still being rather unadventurous and a novice in the amp building game I had intended to build the WAD 2A3PSE kit. Unfortunately as soon as I was in a position to construct one WAD went through an "internal restructure" and the 2A3PSE was dropped.

After a bit of head scratching and a few prompts on the old WAD forum I considered a Loftin White style direct coupled amp, inspired by Keith Garrett's website (dead link). The originator of this take on the original Loftin White amp is Philip Ramsey of Bluebell Audio (dead link - Bluebell Audio is no more). After an initial phonecall in which Philip encouraged and reassured me that I could build an amp from scratch I placed an order, and soon after two big boxes full of strange looking bits arrived. No turning back now. This was the start of a telephone friendship with a man incredibly passionate about valves and music, without whose patience I probably wouldn't be listening to valve amps now. I suppose I should add that I have no connection with Philip or Bluebell Audio other than I provide the cash and they provide the components. Just a happy customer.

Construction

So, I had two big boxes full of bits. One was very heavy and contained all the transformers, the other was smaller and was full of components, valves, fixings etc. Now I had to work out what to do with them.

I spent a long time studying some photographs Philip had sent me of one of his Loftin White builds and sketching out what I thought was going on. It's probably my nature but I wanted to think it all through so I knew what the end would be, not just the beginning. Once I'd got things sorted out, I started to draw the transformers etc. up in AutoCAD and planned how I would lay the amp out. Once happy I printed out the layout and attached it to the 17" x 14" x 4" black powder coated steel Hammond chassis.

Then it was a combination of drilling, filing and hole punches to prepare the chassis. The secret is to be methodical, take your time, and increment the drill size by 0.5mm. The temptation is to jump by a couple of millimeters but a better finish can be achieved by the smaller increment. Generally this creates less swarf so less deburring is needed with a countersink bit. Perhaps more significantly, at bigger diameters an increase of, say, 2 millimeters is quite a lot of material to remove and can result in some work hardening of the steel chassis. Not to be recommended. The worst bit is forming the rectangle-ish shape for the IEC socket. I try to avoid these now and use captive leads that require a simple circle. Much, much easier.

The finished chassis. Philip recommended that I should cover the chassis in masking tape before I started drilling to protect the finish. I rather foolishly thought that by covering it with the printed layout I was doing the same, but all that did was trap swarf between the layout and the surface and inevitably lead to some scuffing. I still use AutoCAD to draft out the layout but I cover the chassis entirely with masking tape and I frequently lift two sides of the layout from the chassis to clean the junk out.

Next job was mounting the transformers, choke and valve sockets. The amp is starting to take shape now. The small 9 pin sockets were probably the most fiddly bit of the whole amp. The holes for the bolts are very close the to hole for the socket and in one case it actually "broke through" when fettling the bolt hole to fit the bolt.

On the rear, the on/off switch, IEC socket (boo!), slow blow fuse holder, phono sockets and speaker terminals.

Internally, the tag strip, Cerafine cap in the PSU, the filament transformers for the 2A3s (AC heated), and the twisted earth bar. I tried to use as many existing valve socket fixings as possible to mount the tag strip, mounting them beneath transformers where there were none.

The first few components connected up - the PSU and output stage.

Now finished, including some mahogany side pieces.

Switching on

Gulp! It's an understatement to say that I was a little nervous the first time I switched the amp on. Thoughts of 400V rippling through incorrectly wired electrolytic capacitors filled me with a little terror, even though I'd checked my wiring, but I thought it important to look confident in front of my better half!

So, standing at arm's length, I flicked the toggle switch and waited. The GZ34 rectifier started to warm up, then there was blue arcing inside and everything went dead. The slow blow fuse had gone. Hmmm, that wasn't supposed to happen.

I tried again, exactly the same thing happened and the fuse blew. Rather frustrating. Over the next few days I had a few conversations, checked all the wiring, scratched my head and waited for Philip to send down another GZ34 and some more fuses.

The following Friday the bits arrived. I got home from work, plugged the new bits in, held my breath and switched on. Success! A smidge of hum from the AC filaments through the Fostex FE208EΣ was very reassuring. I never had another fuse blow after this.

Initial Listening Impressions

It's Friday night, I've been abandoned at home for the weekend, and the neighbours have also gone away. I'm Home Alone with my new amplifier... great! A trawl through all my favourites proved that this amplifier really was as musical as I had been promised. Philip assured me of an "awesome" sound thanks to the direct coupling of the driver stage to the output stage, and that is exactly how it sounded after all those years of solid state amplifiers. The sound seemed so solid, all the grain I heard with solid state gear had been lifted. And even with a seemingly meager 3.5W on tap there was more volume than I could handle at two o'clock in the morning with no-one to wake up. Hell, even Oasis sounded good that night.

Improvements

I had a trip over to see Steve Shiels with the Loftin White. This was the first time I'd heard his Lowthers and rather more esoteric valve amps. Steve has a very nice collection of valves (I'm not doing it justice here) and he was generous enough to try a few different valves. First up was a Mullard GZ32 rectifier which glowed like a light bulb which concerned me a little as I didn't want one of Steve's nice Mullards to expire! It sounded very nice though, quite an obvious difference. I was surprised.

Next up were a few replacements for the ECC83s. Most didn't make much difference to my untrained ear, except for a nice pair of Tungsrams Steve produced. Turns out these were actually made by Mullard. Very nice. Once I got home and heard them in the context of my normal system I could tell they were much nicer than the new production Svetlana 12AX7s they replaced. Result!

Talking to Philip, he said that the components in the amp were already a pretty good spec and without spending significantly there was little to change. He did suggest that I could try some NOS RCA 2A3s though. Which I did...

Dualplates as you can see, don't think I've even seen a photograph of a monoplate. Were they an improvement over the cheap Sovtek monoplate 2A3s? Well, yes I think so, but there's not a lot in it, at least in this amp. The Sovteks are a real bargain though.

Living with the Loftin White

For a year I was really happy with the amp. If it hadn't been for Philip raving about his recent incarnations of the Western Electric WE91 pentode driven 300b monoblocks I might never have listened to another amp. But Philip was so effusive of his new amps I had to try them, and in comparison the Loftin White is limited, particularly in the bass. I had intended to replace the output transformers which I had robbed to build the WE91s and get the Loftin White up and running again. But as time goes by that's not going to happen, slowly I'm taking more bits and it's dying an ignominious death. A little sad really for my "first amp". But it really is a grand little amp, and is so much better than so many commercially available amps it's laughable. Maybe one day in the future I'll rebuild it out of sentimentality. Now, taking inspiration from the Japanese a fully tricked-up Loftin White might be something else entirely...

December 2025 Update

Well I don't think I'll ever rebuild a Loftin White. It's a nice amp for sure, and one that has a special place as the first power amp I built, but it was too limited for my taste compared to other amps.

The chassis lives on though, in my D3A-2A3 amp, which I think is a much better amp. And the output transformers were reused in the WE91 clone 300B amps.

Retro Vintage - World Audio Designs PHONO II and PSU II

World Audio Designs PHONO II and PSU II

After the success of the Fostex horns I worked up the courage to have a go at the now defunct World Audio Design (WAD) PHONO II and PSU II kits. I hadn’t really used a soldering iron before but was encouraged by the supposedly easily construction thanks to the pcbs and reassured by the thriving community on the old WAD BB.

The kits consisted of an RIAA stage using 3no. ECC83 double triodes remotely powered from a solid state power supply. A matching PRE II preamp was also available using a 6922 double triode.

Everything required was included in the kits, from chassis, brushed aluminium front plates and cables to pcbs, components and valves. Being somewhat cautious with my first build I followed the instructions closely and took my time. Construction was pretty straightforward: populate the pcbs and solder the components on both sides of the pcbs. Even a novice like me managed. No problems.

Having read the warning in the instructions that electrolytic caps wired up with incorrect polarity can explode I was rather nervous the first time I switched on. Fortunately my trepidation was unfounded and it worked first time! This was a bit of a surprise, though, in reality, following the instructions and taking plenty of time is all that was needed to complete a successful build.

Listening Impressions

These kits make a very nice sound, no doubt about it, and are an improvement over the inbuilt stage in my Exposure IIV pre amp. Having heard other valve phono stages since, to make a significant improvement requires significant expenditure.

Improvements

There are a number of improvements that could be made, from valve rolling to a complete rebuild with fancy bits. The only changes I have made so far (apart from adding Lundahl LL9206 MC step-ups) are to the valves, which I guess is a measure of how good the phono stage is in its basic form.

Perhaps fortuitously, one of the originally supplied Ei ECC83s died after about a year, so I replaced it and another of the remaining good Eis with Svetlana 12AX7s. This improved the sound – more detailed and perhaps a little tauter and refined. The improvement left me thinking the Eis were rather uninspiring.

A much bigger improvement was changing all three valves for some old 5751s. These are similar to ECC83s but with less gain. They are also reputed to have a nicer sound and they certainly improved my PHONO II. I have some Mullard ECC83s, I should compare them to the 5751s sometime...

Future Improvements

In its standard form the PHONO II does not correct for the 3.18us RIAA equalisation time constant. This is sometimes used to protect the cutting head during recording, and there is an FAQ on the World Designs forum for correcting this. Reported results are mixed; some hear a small improvement others hear none. I have the resistors, just waiting for a period of stability in my system (ha!) so I can see if it makes any improvement.

The caps in the PHONO II are prime candidates for upgrade and I really must get around to trying some fancy ones.

Lundahl LL9206 MC Step-ups

As I experimented with my Garrard 401 the time came where I had to try the infamous Denon 103. As my existing cartridge, a Dynavector 10X4, is a high output MC I didn't have any step-ups. Initial experimentation with the 103 without step-ups was promising: a good sound but the system ran out of gain on quieter pressings. Looking on the WD forum the LL9206 MC step-ups had some good feedback and didn't cost the earth. There is no Lundahl supplier in the UK so I got mine direct from Sweden. There are suppliers in Europe, but it seemed easier to go direct to Lundahl.

The LL9206 have an amorphous core and mu metal can. The datasheet says that the primary side must have a ground reference. From experience I can confirm this, unless you like listening to lots and lots of hum! They can be configured 1:5, 1:10 and 1:20 for gain. 1:10 seemed to fit my system best, so I used schematic C (no center tap).

The small pins protruding from the cans are frustratingly fiddly to solder to, a right royal pain in the posterior. I had seen on a couple of websites that small pcbs are available to facilitate installation. Not appreciating how difficult it would be to solder them I didn't bother, but if I was soldering them for the first time again I would be sorely tempted to get a couple of the pcbs. Once soldered I attached each one to the chassis with a zip tie.

For cartridge loading, the 103 has an output impedance of 40R but wants to see, say, 6 to 10 times this, or 240R to 400R. Some 103 users report good result up to 1000R. The 1:10 ratio transforms the impedance by 100 (the gain squared i.e. 10 x 10), so the stock 47k resistor at R13/R14 should be replaced by one between 24k and 40k. As this is pretty close I have left the 47k resistors alone, but will eventually try 20k and 100k, and tune from there if there is an improvement.

December 2025 Update

I still use the PHONO II and PSU II. That can't be a bad sign. I have been working on a C3g-E88CC Aikido on and mostly off for 20 years, but that's another story. I don't remember ever implementing the 3.18us RIAA equalisation time constant, but I did change the two coupling caps. Not to anything fancy, just cheap PP. I should probably investigate some "better" caps, but life's perhaps too short.

I must have written this before 2008 as I bought a Denon 103pro in Japan that year and it hasn't been supplanted since.

I used the Lundahl MC step ups until earlier this year. I started playing with JFETs a year or so ago and after a couple of iterations I drew up a pcb for the front end of Wayne's Pearl 3 phono stage, using 4no. paralleled LSK170s rather than 2SK170s. I was really quite surprised how much better this was than the Lundahls, at least with the PHONO II. It will be interesting to see how it matches with the C3g-E88CC Aikido, if I ever get it finished......