I have a humble opinion too and my experience has been that it is easier to build a simple PP amplifier than a SE one. It takes some effort to defeat hum-induced power supply noise in a SE amp, vs. a Push-Pull amp, unless you faithfully reproduce a fool-proof kit/design (Like the Tubelab designs, which I have not done).
Yes, the 6Π3C-E (6p3s-e) tubes are still affordable. And they are perhaps (in practice) more of an equivalent of the 6L6GC/7581 than 5881. But they are rapidly demanding higher prices (after being discovered). I have personally used and sometimes preferred those to EL34s in my EICO HF-87.
The current bargains (if you need 6L6 type tubes) are the unqualified 6Π3C (6p3s) tubes. While they can not tolerate the higher plate voltages and dissipation of the -E tubes can, they still equal 6L6GBs in most respects.
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Assuming you mean a power transformer with “a single transformer 250 W” I suggest that you decide what amplifier design you would like to build first, based on what your needs and preferences are (budget, power requirements, PP, SE, Pentode, Triode, DHT, music and listening preferences, speaker sensitivity and drive requirements, etc.) Then ask specific questions about what power supply requirements might be, and explain why 4-channel.
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If building for a four channel end result, I would probably build a pair of stereo amplifiers, more versatile later on and not as cumbersome to move around and work on.
I built a six channel tube amp a few years back, and really wished that I had split it into several stereo amplifiers.
Well I took your advice and ordered a couple of PP EL84's (8k) for US$133 shipped to Michigan. These seem to be the size of the PP EL34 (115mm x 65mm) I think I will use them for a PP 6L6 amp. I think 8k would be fine, as an 8 ohm speaker could spend some time around 6.5 ohms. I would have bought the PP KT88 but they had only 1 in stock. I will keep checking.
The original poster had written p-p for ‘point to point’, so I used the same notation. I agree push pull is more straightforward.
My apologies! Shame on me that I did not realize that at the time of my post.
The more I look at it, the more I feel that this is the way to go. Currently I feel that it makes sense to go with the easier more thoroughly documented route even if it's not necessarily the highest performance. (simply due to my inexperience with valves and debugging them) I feel that the most sensible choice then is to pump greater funds into buying really high quality OPTs, so the Sowter UA21 PP might be my best bet as a very solid base to build upon. I can then "hot rod" the amp further down the road once I have something that hopefully requires minimal fettling to get working.
Whilst I've been looking through the components list, how critical are some of the values as some capacitors are awkward to find in non preferred values. (250nF vs 270nF and 50nF vs 51nF) Obviously modern tolerances are much tighter than they were in the 50's so I'm assuming that if the value is close enough with tight tolerance, it should be fine.
Thanks for that Alan, very handy. A few more questions on this:
How critical is the choke inductance for L1? I've found an off the shelf triad CX-14 that has the appropriate DC current and DC resistance but it's 6H. The schematics I've seen vary from 8-10H and the Claus Byrith psu shows 5-10H 200mA on the optional choke.
For the output transformer, is it best to go for 16/8/4 in separate windings or 16 ohms tapped at 8 and 4?
Hopefully that's all the info I need. I'm going to dig out my film cap stash later and see if I already own some suitable parts which save a few bob. Otherwise, I've chosen mostly metal film resistors with a normal operating voltage of 500V or higher. I may already have a few items here that would also save buying new.
Just my opinion, you will get others I hope, that the Triad choke would be OK.
The original Mullard had small value (8uF) paper in oil caps for the smoothers (C12 and C15 in the Mullard book), so a larger inductance was useful. If you are going to use electrolytic caps you can bump up their values to 32uF or 47uF, but no larger for the first cap if you are using a GZ34. Just make sure the choke can handle 180mA at least, which ever you choose.
The output transformer, is difficult to advise.
Rule of thumb is the heavier and the more winding sections the better and more expensive.
If you can afford the best, buy them. You can re-purpose them and try other configurations with them.
As this is a toe in the water, perhaps buy the cheapest and not try to exceed their specification?
If you know you will want to try other circuits, e.g. fixed bias like the Claus Byrith, then maybe go middle ground and say 30 Watts minimum. That will be fine for the 'standard' cathode bias 5-20 to start with and give you a bit more headroom for fixed bias if you go that way.
It is a tricky choice... Alan
The original Mullard had small value (8uF) paper in oil caps for the smoothers (C12 and C15 in the Mullard book), so a larger inductance was useful. If you are going to use electrolytic caps you can bump up their values to 32uF or 47uF, but no larger for the first cap if you are using a GZ34. Just make sure the choke can handle 180mA at least, which ever you choose.
The output transformer, is difficult to advise.
Rule of thumb is the heavier and the more winding sections the better and more expensive.
If you can afford the best, buy them. You can re-purpose them and try other configurations with them.
As this is a toe in the water, perhaps buy the cheapest and not try to exceed their specification?
If you know you will want to try other circuits, e.g. fixed bias like the Claus Byrith, then maybe go middle ground and say 30 Watts minimum. That will be fine for the 'standard' cathode bias 5-20 to start with and give you a bit more headroom for fixed bias if you go that way.
It is a tricky choice... Alan
I was looking at using 10uF 630V epcos MKP film caps in place of the paper in oil/electrolytic, primary reason being they were actually cheaper than the 8uF version and being plastic film they ought to outlast the amp. Good to hear the triad choke should be ok as that saves me around £60 vs a custom wind or VVT. The hammond 159R is virtually identical to the triad item albeit £3.50ish more each.
The plan, even though it's a very expensive, is to go with the sowter UA21 output transformer. I know it's a quality item that will not hold back anything I build or upgrade to. I'll treat it as an investment. It's rated to 40W @ 20Hz, the frequency response -3dB points are 5Hz-50KHz and THD is low at the rated power output. It's also pretty much twice the weight of the others that I was looking at. The hammond 1650PAP was looking quite good until shipping was added. Doesn't look like it's available in the uk either. I'm prepared to pay the sowter price as it looks to be worth the extra outlay.
In other news, the hunt for components that I already own has been a bit poor. I've found some 47pF ceramic caps (orange coded), some 330pF ceramic caps (violet coded), some 470nF 600V MKP caps, some 1K 1/2W oxide resistors, 3 octal sockets, 2x fuse holders with fuses and some standoffs/spacers.
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I've committed myself to the project and bought the sowter UA21 outputs as 16 ohms with 8 and 4 ohm taps. They take 6-8 weeks to build and ship so I'll save buying the other components until next month. I'll be sorting the mains transformers out today, as I expect they take a similar amount of time to manufacture.
The 6197 tube is a good sub for the 12BY7. Plentiful and cheap, with excellent curves, pentode or triode mode ( Mu 22 like the 6CG7 or ECC99).
Or there is the 6HZ8 which gives you a 6197 pentode and a nice linear Mu 70 triode in one 9 pin bottle. Have been on the $1 list at ESRC.
Or there is the 6HZ8 which gives you a 6197 pentode and a nice linear Mu 70 triode in one 9 pin bottle. Have been on the $1 list at ESRC.
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