Gordon Rankin's Bugle - a Super 45 SET Amplifier
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 parallelled 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. Or if you're feeling cheap and lazy download the PDF from Gordon's website...
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?
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 turbulance 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!
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 switcheable between 45s and 2A3s...
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