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.