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 "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.

Design Of A Single Ended 2A3 Valve Amplifier - Part 2

In part 1 we looked at design of the 2A3 output stage of this amplifier, that was a very, very long time ago. Now we'll look at the D3a driver.

The Driver Stage

Output Stage Recap

Design of this driver stage is very similar to the output stage, but with slightly different goals. The purpose of the driver is to drive the output valve's grid, so let's have another look at the 2A3 anode characteristic curves to remind ourselves - here's the datasheet graph to which I added the safe operating zone.

If you remember, our operating point is 250V and 60mA, with a grid bias of -43.5V.

As this is a class A1 design the 2A3's grid should remain negative at all times (i.e. to the right of the red curve) so for full output the grid swings down from its bias point of -43.5V to 0V (and up to -87V). The driver is a common cathode stage, so the driver needs to swing + and - 43.5V on its anode, which is coupled to the 2A3 grid via a capacitor. Capacitors block DC voltage - we'll talk about this capacitor later.

Driver Stage Considerations

So how do we choose the driver? There are a number of considerations but if we want to use a single stage before the 2A3 then ideally we need to swing the + or - 43.5V from a standard 2V RMS input, such as a CD player. 2V RMS equates to 2 x square root of 2 x 2 = 5.66V peak to peak so + and - 2.83V. So in rough terms the driver needs a gain of 43.5 / 2.83 = 15.3. In reality a valve's datasheet amplification factor won't be fully achieved, maybe only in the region of 70%.

And we perhaps might prefer a little more sensitivity than + or - 2.83V, and + or - 1V gives us plenty of "welly" on the volume control. So let's aim for an overall gain in the region of 40.

It's not just a question of choosing a valve with the appropriate amplification factor, it has to be able to actually swing those volts too! And ideally we'd like it to have plenty of headroom i.e. some more swing in hand so it's not up against its limits and clip.

We also want to minimise distortion so might want to aim for areas of the anode curves with equally spaced grid voltage curves. (However, as the amplifier is the result of two stages, both stages have to be considered together as a combined system if we want to fine tune minimum distortion. But that would need some test equipment and is beyond the scope of this blog.)

And of course we need to obey the safe operating characteristics of the valve.

The D3a

So back to the D3a. It's a "special quality" pentode that was used for telecommunications. Here's the datasheet. Pentodes are generally newer than triodes (which is somewhat relative these days!) but can make good drivers as they typically have higher gain than triodes. Sometimes too high. But they can be wired as triodes with lower gain, and some think they can sound better this way.

Not all pentode datasheets contain triode connected info but fortunately the D3a does. Looking at page 3 the triode amplification factor is given as 77, so ~70% is a real world gain of about 50, which is nice. Now let's look at the second graph on page 7 for the average anode characteristics.

The curves look rather like the 2A3, but the values of voltage and current are very different. That's because this is a signal valve rather than an output valve.

The Operating Point

So let's look at the operating point and start with the safe operating requirements, see page 5 of the datasheet. Helpfully the maximum anode power dissipation of 4.5W is already shown on the datasheet graph. In addition we want to stay to the right of the 0V grid line, and not exceed the maximum anode voltage of 220V and maximum anode current of 30mA. So here's the graph again, with the safe operating zone shown.

Now we can choose our operating point. We want to swing 2 x 43.5 = 87V peak to peak on the anode, and we want a nice place on the grid voltage curves with equal spacing to minimise harmonic distortion. But also, unless we're going to have a separate power supply we need to bear in mind the B+ supply voltage that we determined in our output stage design.

Now if we go back to our 2V RMS input voltage from a CD player, which is 5.66V peak to peak, if we look at the curves above we can't adequately bias our valve for this swing - the grid voltage curves only go as far as -3V. In reality we would need a meatier driver.

But it's not necessary to swing the full 5.66V - music rarely reaches the full 2V RMS. If we go for 2V peak to peak then the amp will have decent sensitivity. So let's bias the valve to the right of the 1V curve. This does mean of course that there is the potential danger that at full bore the amp could overdrive, but in reality the volume control would never be turned up that high to permit the full 2V RMS swing in a well balanced setup system.

Now it's an iterative process of looking at operating point and anode loadline. An operating point of 150V and 20mA, which is a grid voltage of approximately -1.38V, looks a decent compromise. You can see I've sketched in the approximate -1.38V line at the operating point.

Loadline and Anode Load

When we looked at the output stage we went with the datasheet loadline of 2500 ohms, which is approximately 3x the anode resistance. Our choices for a driver are slightly different as the higher the load resistance generally the better as it will tend to reduce distortion. But the higher the load resistance (i.e. the larger value of the anode resistor) the more voltage will be dropped across it, which in turn means the higher the required B+ voltage.

The datasheet for a triode connected D3a gives the internal resistance as 1900 ohm. If we apply the 3 x rule: 3 x 1900 = 5700 ohms. So we're looking for a resistor of at least 5700 ohms, and ideally we want to use a standard value with suitable rating. So 6800 ohms looks pretty good, and the power dissipated in it will be the current squared x the resistance, so 0.02^2 x 6800 = 2.7W. We derate by 3 x to 5 x, so we actually want a 6800 ohms resistor rated between 8W and 13W, maybe a nice 12W Mills again.

The loadline is shown in magenta, and the thicker section shows the swing about the operating point - up to a maximum of -2.56V where the maximum allowable anode voltage of 220V occurs. As the grid voltage can swing up from -1.38V to -2.56V (1.18V) it therefore swings down to -1.38 - -1.18 = -0.2V.

Now if we look at the anode voltages at these points on the grid voltage we can see the anode voltage will swing up to 220V and down to 75V i.e. -75V and +70V from the operating point. Ideally those values would be the same as the difference reflects a little distortion - that's the consequence of unequally spaced grid lines. But a difference of 5V is pretty good in reality.

If we now think back to our 2A3 we need + and - 43.5V swing on its grid, and the D3a as we have it configured here can swing +70V and -75V, more than actually required. But as most music is recorded with less than 2V RMS, in reality we'll be able to use more of the volume control, without needing a pre stage. And if the music is too loud so the driver stage starts to clip it will sound a little unpleasant and the volume will inevitably be turned down.

Cathode Resistor

Nice and easy, we want to raise the cathode to 1.38V to achieve the correct bias and there will be 20mA through it, so using our friend, ohm's law, R = V / I = 1.38 / 0.02 = 69 ohms. Let's say 68 ohms as a standard value. Power dissipated through the cathode resistor = 0.02^2 x 68 = 0.03W, so we can happily use a half watt rated resistor.


Cathode Resistor Bypass Capacitor

Right, time for some more maths again. First let's work out the cathode resistance (check back to the 2A3 page for more explanation):

rk = (ra + Rl) / (mu +1) = (1900 + 6800) / (77 + 1) = 112 ohms

So R = 1 / ((1 / rk) + (1 / Rk)) = 1 / (1/112 + 1/68) = 1 / (0.0089 + 0.0147) = 42.4 ohms

As this is a driver stage and earlier in the reproduction chain we should aim for a much lower f-3 frequency than the output stage as once the bass frequencies have been lost they can't be magiced back. 1Hz isn't unusual.

And therefore C = 1 / (2 x pi x f-3 x R) = 1 / (2 x 3.142 x 1 x 42.4) = 3754uF.

Which is a big capacitor. But I had some Panasonic 2200uF electrolytics which gives an f-3 of less than 2Hz. Still lots of uF, but not unreasonable.

As the voltage at the cathode is approximately 1.38V a capacitor rating of 5V is fine.


Grid Leak Resistor

The datasheet suggests 500k ohms as a maximum, I went with 100k ohms, 1W is more than adequate.


Grid Stopper and Screen Tie Resistors

There are two more resistors that are very important. The first is called a grid stopper and is soldered as close to the valve socket pin for the control grid (G1) as possible. This stops valves oscillating. Higher gain valves, like the D3a have a habit of oscillating which is a bad thing. You might not audibly recognise oscillation but it will affect the sound to some degree. Sometimes it can be present but at very high frequencies, much higher than we can hear, but it can still have an affect on the music. A value somewhere between 300R and 2200R usually does the job. I used 1000R, and half a watt will be adequate.

The other resistor ties the screen grid (the middle of the three in a pentode) to the anode. This is what make the valve work like a triode rather than a pentode. 1000R half watt again.


Capacitor Coupling the Two Stages

Now we have the basic design of the driver stage we need to consider how we couple the driver stage to the output stage. The signal is taken from the anode of the D3a so we know the voltage there is 150V (our D3a operating point) plus the voltage we have raised the cathode by (1.38V) so ~151V. If we were to attach the anode of the D3a directly to the grid of the 2A3 there would be 151V on the 2A3 grid, which would be a very bad thing. Remember, from our 2A3 design there's 0V on the grid, -45V relative to the cathode, so applying 151V wouldn't do the 2A3 any good at all.

So we need a way to ensure this doesn't happen. The three usual methods are capacitor coupling, interstage transformer coupling, and direct coupling. Capacitor coupling is the most common, and for good reason; it's the easiest to implement. And whilst some say other methods sound better others are inclined not to agree; you need to build for yourself and see what you think. Capacitor coupling works in this application because capacitors block constant voltage but allow changes in voltage to pass, so using a capacitor blocks the 151V but allows the signal to pass to the 2A3 grid. We just need to work out how big a capacitor we need.

Whilst almost any capacitor would block the 151V on the D3a anode the coupling capacitor forms a high-pass filter with the 2A3 grid resistor, so if the value is too low the f-3 corner frequency could be too high and we would lose our bass again.

The high pass filter capacitor value is given by:

capacitance (farads) = 1 / (2 x pi x corner frequency f-3 (hertz) x resistance (ohms))

C = 1 / (2 x pi x f-3 x R)

Using the 40Hz f-3 we designed the 2A3 stage for gives:
C = 1 / (2 x 3.142 x 40 x 100000) = 0.4uF

0.4uF isn't a common value, but 0.47uF is. And it has the advantage that as it is slightly larger it drops the f-3 frequency to around 34Hz. Whilst beneficial as it drops the high pass filter a few Hz it also doesn't compound high pass filters at the same frequency, which wouldn't be ideal in some respects.

But you could also choose to use a 1uF coupling capacitor which would drop f-3 to 17Hz. This is perhaps a nicer place to be, but it requires a bigger value capacitor, and some audibly prefer smaller. More capacitance generally costs more too.

There is roughly 151V across this capacitor so the capacitor needs a higher voltage rating. Generally, you want to be using a nice film capacitor here. Some swear by paper in oil, others prefer PP. But most film capacitors used in valve amplifiers are rated for 630V, some 250V, so more than adequate.

Volume Control

This just leaves the volume pot. There is a balance to be struck here, 50k is pretty good but others can be used.

And now it's confession time. It's so long since I first started preparing these two blogs I've lost the drawing file somewhere on an old computer which had the circuits and values :-(. But if you've made it this far then you have enough persistence to fill the values in for yourself :-). So no final glory shot of the full amp circuit, but who knows, maybe in another seven years I might do a blog about the power supply...

Design Of A Single Ended 2A3 Valve Amplifier - Part 1

This is a simple single ended valve amp I "threw together" a couple of years ago. It was meant for a colleague to try valves at home, but never found it's way out of the house. It's been in the project room ever since (as attested by the obligatory dust to be found in all the best male domains) and makes a particularly good match for the Quasars.

There's nothing fancy about the design - a triode strapped D3a pentode capacitor coupled to a 2A3, both in self bias with AC heating. The HT power supply is a simple LCLC passive filter with a GZ37 rectifier. Here's the audio schematic without part values, all straight forward stuff.

But it has decent parts and sounds very good. Surprisingly good really. Not the best amp I've built (that's a copper GM70 SE amp, but that's another story), but it fits in a single chassis which makes a change from most of my more recent builds... And I haven't felt the need to replace it which speaks volumes. And in the spirit of openness here's a photo of the inside just to prove how thrown together it was. I certainly wouldn't encourage anyone to study my wiring!

The chassis was actually recycled from my very first amp build - a Bluebell Audio 2A3 Loftin White built 10 years ago. Parts were supplied by Philip Ramsey using Shishido san's circuit. I just needed to drill a few more holes to accommodate different output transformers, chokes and a volume pot.

Preamble over, let's talk about the design. Bear in mind this is intended to be a simplified explanation of simple valve amp design - if you want the in-depth theory there are many good sources, try Valve Wizard's website or Morgan Jones Valve Amplifiers. I would also highly recommend reading Gordon Rankin's write up of his Bugle 45 amp - a great amp and really good design primer. First though, recognition must go to Nick Gorham as it was a circuit he posted that inspired me to build this.

The Output Stage

The usual place to start when designing a valve amplifier is the output valve, in this case a 2A3. By looking at the datasheet we can see whether it's likely to be suitable for our system, or what we might need in our system to make it "work". Assuming we have some reasonably efficient speakers a 2A3 makes a lot of sense.

Anode (Plate) Characteristics

Let's look at the anode characteristics graph on the datasheet

This graph shows a series of curves representing the relationship between voltage across the valve and the current through the valve for different grid voltages. Let's consider the classic 2A3 operating point, i.e. how the valve is "set up": 250V, 60mA, -45V on the grid. If we look back at the first page of the datasheet we're even told what might be a good operating point ;-). So looking at the graph above, 250 plate volts and 60 plate milliamps just happens to intersect with where a line representing -43.5 grid volts would be. (Okay, that's not quite -45V but it represents the lower limit of the AC heating, 2.5V.)

If for instance you maintained 250V across the valve but went with -35 grid volts you'd draw around 115mA. And burn out your 2A3 pretty quickly. So we need to check that our proposed operating point is within the valve's rating, and for a class A1 amplifier the datasheet tells us that we shouldn't go above 300V across the valve, and we can plot this line on the graph.

The datasheet also tells us we shouldn't let the anode (plate) dissipate more than 15W. So we can add this to the graph too as a curve with a series of points where voltage x current = 15W.

To draw the 15W curve we could plot a point at 100V and 150mA (which is 15W) say, and another at 200V and 75mA (= 15W), and another at 300V and 50mA (also = 15W), and so on for as many points as we feel necessary. For safe operation of the valve we need to operate in the area of the graph below the curve.

As we're designing a class A1 amplifier the grid should always remain negative relative to the cathode too, so we also need to be to the right of the 0V grid line too.

And if we plot all three on the same graph we find the acceptable area of the curves that we want to work within, the green shaded area below.

 And 250V anode voltage and 60mA is right at the limit of this region, thus maximising the power obtained from the valve. In general it's usual to maximise the power obtained from a valve, if you're lucky enough to have rare old monoplates you might choose to be a little kinder to them and maybe run them at 50mA instead...

Loadline

The next thing to consider is the loadline, an example of which is also shown on the datasheet, rather conveniently. It's the sloping straight line labelled "LOAD RESISTANCE = 2500 OHMS", centered about the -43.5V grid line, ranging between 0 grid volts and -87 grid volts. This loadline represents the load resistance of the output transformer, which in the case of the datasheet is 2500 ohms. But why 2500 ohms? You could just leave it to the RCA engineers and accept they knew what they were doing, which they most certainly did, but the longer answer is it's a compromise.

What do we want from our amplification stage? Usually we want to maximise power and we want to minimise distortion. The relative importance of the two depends on what we're trying to achieve, but as this is an output stage we want a good balance of the two.

If we consider distortion we can see how linear the valve is (or isn't) at our operating point by looking at how evenly spaced the grid lines are along the load line. Between 0V and -60V the spacings look fairly even, but beyond -60V the grid lines tighten a touch. So when the music signal is larger than + or - 15V (so lower than -30V and higher than -60V) the distortion will increase.

In general, if we wanted to reduce distortion we could increase the load resistance which would flatten the loadline i.e. make it closer to horizontal. But if we do this we will lose some power. And that's the compromise. We're considering the classic 2A3 operating point, and there's good reason for doing so as it's a good balance.

The rule of thumb for an output valve's load resistance is 3 x the anode (plate) resistance. So if we look at the datasheet again, the plate resistance is given as 800 ohms, which multiplied by 3 gives us 2400 ohms, or our 2k5 output transformers. 3k5 output transformers are often seen used in 2A3 schematics and trade a little output power for slightly lower distortion.

If we look back at our graph of anode characteristics with our limiting conditions marked on we can see the loadline is right at the very top of the 15W line. In fact part of the loadline actually crosses the 15W line but it's transient and overall the valve dissipates 15W.

Cathode Resistor

Okay, that was a bit heavy, sizing the cathode resistor is much easier now we've decided how we want to operate our valve. As we're designing a simple valve amplifier we're going to use self bias. We know that there will be 250V across the valve and we need to bias the grid at -45V relative to the cathode. Ohm's law is our friend

voltage (volts) = current (amps) x resistance (ohms)

or

V = I x R

Rearranging Ohm's law gives us R = 45 / 0.060 = 750 ohms. The datasheet suggests 750 ohms, there's a surprise!

We also need to calculate the power rating of the resistor.

power (watts) = current (amps) ^2 x resistance (ohms)

or

P = I^2 x R

(Blogger doesn't seem to allow superscript for the squared term)

Which gives us P = 0.060^2 x 750 = 2.7W

But we must derate this as a 2.7W resistor (even if we could find such a thing) would burn up very quickly. Typically we derate by 3 to 5 times, so our 2.7W becomes 8.1W to 13.5W. Even at 3 x derating the resistor will get very hot and I prefer to go to 5 x if I can find something suitable. In this case I would go for at least 12W, maybe a nice Mills if you're feeling flush.

Our output stage is starting to take shape now. Under the cathode we have a 750 ohms 12W resistor which raises the potential at the cathode to 45V. As there's 250V across the valve (i.e. between the cathode and anode) there will be 250 + 45 = 295V at the anode.

Cathode Resistor Bypass Capacitor

If we don't use a capacitor to bypass the cathode resistor the stage will have lower distortion and higher headroom, but more importantly for an output stage it will have a small fraction of the gain. This is why common cathode stages usually have a cathode resistor bypass capacitor. Calculating the size is a little involved.

capacitance (farads) = 1 / (2 x pi x f-3 (hertz) x R (ohms))

or

C = 1 / (2 x pi x f-3 x R)

f-3 is the frequency at which bass will have rolled off by 3dB. 3dB represents a halving of the bass output in this case so we need to set f-3 somewhere below the frequency at which we want full bass output. If we want full bass output down to 40Hz, not unreasonable for an output stage, let's set f-3 at 20Hz.

R, unfortunately, isn't simply the value of the cathode resistor. It's actually the cathode resistor in parallel with the cathode resistance, rk.

cathode resistance = (anode resistance + load resistance) / (amplification factor +1)

or

rk = (ra + Rl) / (mu + 1)

So rk = (800 + 2500) / (4.2 +1) = 3300 / 5.2 = 634.6 ohms

And therefore R = 1 / (1 / 634.6 + 1 / 750) = 344 ohms

Finally we can calculate the value of the cathode resistor's bypass capacitor

C = 1 / (2 x pi x f-3 x R) = 1 / (2 x 3.142 x 20 x 344) = 23.1uF. So we'd choose 22uF as it's a commonly available size. If you had a 47uf capacitor to hand then f-3 would be 10Hz. And 10uF would give a f-3 of 46Hz. I had a couple of 100uF capacitors handy so used those.

As you can see, if we had simply used the value of the cathode resistor, Rk, instead of R then we would have made the cathode bypass capacitor more than twice the size it actually needs to be.

As there's 45V at the cathode the capacitor needs to be rated higher than this. 50V is perhaps a little too close for my liking and I would choose at least a 63V rated capacitor and possibly 100V.

Grid Leak Resistor

We're nearly there now, just the grid leak resistor to specify. It's purpose is to tie the grid to ground and provide the cathode bias via the cathode resistor. The datasheet usually specifies a maximum value, in our case 500k ohms. Higher isn't necessarily better, but it can't be too low otherwise it would draw significant current and we want to keep the current draw under a milliamp. 100k is a reasonable value for the 2A3, and 1W should be more than enough as it should see very little current.

Final Output Stage

And here it is, our finished output stage. If you're wondering why there are two resistors between the cathode resistor and the cathode, this is in lieu of a humpot. A humpot allows any hum caused by AC heating of the valve to be minimised. But instead of a hum pot two resistors will put the cathode resistor at the centre of the potential difference between the two ends of the filament. In practice 2A3s are virtually hum free with decent construction of the amp and I've never felt the need for a hum pot.

In a later blog I'll look at design of the driver stage.