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Help deciding on a low cost 15-20 watt triode push-pull amplifier design.

I have single ended triode and pentode amps and push pull pentodes, so got to thinking about building a push pull triode amplifier.
Target output: 15 - 20 watts no feedback.
Ideally something just a little bit more exotic than trioded el34, 84 6L6 etc.... and got plans for PL36s EL500/4/9 already so not them. UK based which seems to limit the (cheap) availability of a lot of American triode sweep tubes that would be suitable. :(
Don't want to spend much money if possible.
Seems the days of super cheap tubes are gone, initial thoughts, in an ideal world, would be something like a 2A3 amp, but even 6B4G quite expensive nowadays.
Initially the 6P21s seems like it could provide some DHT goodness, but I hear reservations about the longevity of the filament. It is a "relatively normal" tube though? so if that was a problem it would not be a total loss to redesign the amp with say a 6L6 type of valve. I could use a fairly normal 6k ish A-A output transformer.
I also have some 12E1 tubes. I get the impression they are not the happiest in triode mode though, but they are more powerful than I need so some room for flexibility, but seems moving towards lower voltage lower Z transformer.
And then there are the (Russian) regulator triodes, and 6080s. On reflection I could imagine paralleled 6080s (2) into a carefully chosen toroidal mains transformer, although not aesthetically what I am after, could have the potential to deliver the kind of sound I think I would like, clean and tight...
any thoughts.
 
I'm in the UK too so I know the cheap tube choices. First on the list would be 6C4C, the Russian 6B4G. These are Svetlana, and great quality and build. This was modelled on the RCA 2a3. These are the tubes I use in my SE amp in preference to 300b. Expect to pay around £30 each from Russia or the Ukraine. I bought a dozen or so for all my projects. They also sound marvellous as a preamp tube, so that's an added benefit.

Second choice would be EL38. I have plenty if you PM me - I'm in London. I spent all of 2020 building prototype SE amps with pentode tubes in triode, and EL38 was easily the best sounding. I have a few different types including some rare ones. These have a top cap if that's OK.

Third choice, again from my 2020 experiments, was the EL12 which was second only to the EL38. Hot tip is the EL12n which has no top cap and is available new from a few European dealers. Again, I have stocks of unused ones including a matched quad. These are good value tubes. I also have several of the regular EL12 and EL12 Spez.

Since the EL38 and EL12 have a gain of around 15 in triode, this simplifies the driver stage a lot. You can use a simple 6SN7 for instance. In fact you can use a DHT stage like a 26, 4P1L, 2P29L or even 46, 47 or 10Y. That's what I was doing since I have stocks of all these. Sounded great! What I called the "inverted DHT amp" with the DHT on the input where actually it has more influence on the sound.

My final choice was to use 6C4C outputs, but the EL38 or EL12 with DHT input was running it pretty close.

I
 
Here are the triode curves for EL38, EL12n and EL12. Very good sounding tubes, these. The EL12 was much used and valued in Europe. I slightly prefer the EL38 but it's close.
 

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EL38 is around £20 each used for Mullards. I did a big shootout and it sounded better than the EL34s I had in triode. How cheap do you want to go? There's always PL38 of course - that's cheaper.

4P1L are around £12-16 for a quad but you need DC heating there, preferably Rod Coleman regs. Good sound, though. Forget 6P21 - I had some and they don't sound good, plus they don't last.

If you come down a bit in output the choice widens to tubes like EL84, SV83, EL33, EL41, KT61 and KT81, EL11 etc. Those are all good in triode. I've tried them all. Good as drivers also.
 
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DC heating for 4P1L's? Why? Of course I know it's a directly heated, filamentary cathode type, but won't a center tapped 4 Vac winding suffice?

Best regards!
Can't tell you - I never tried it! I always use Rod's regs. Mostly in filament bias with the 4P1L but that's not necessary if you use good bypass caps like DC Link ones.
 
You mention PL36, which has the advantage of a higher g2 spec than many of the American sweeps which, as you mention, are not economical to obtain in the UK. Many have built successful amps with triode connected EL36/PL36. You also mention 12E1 which has the potential to offer the power you desire, although if I had a stash of these, I would be looking at plate to grid feedback since they have so much gm to trade. I think it will take many 4P1L in parallel to make 15-20 watts, at least if you want class A.
 
OP explicitly said that he doesn't want triodized pentodes or beam tubes. Anyway, if I were him, I'd follow the already given hints, as some (most?) triode tied pentodes offer far superior plate curves in terms of linearity than many triodes.

Regarding PL36: This, and it's 6.3 V complement EL36, meanwhile have become scarce. So we'd better look for the still cheap Russian 6P31S. Attention: Do not look for 6P13S's, which in contrary to some vendors' claims is not an equivalent!

Best regards!
 

kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
Perhaps my Modular Amplifier 1 design would suite your purposes?

EDIT: - You don't want triode connected tubes which limits you severely (Seriously, triode connected sweep tubes kick royal booty!). I have 6AS7G/6N13S ready to go but if you're interested, tell me which tube you want to use and I'll design the output board for it.

https://www.diyaudio.com/community/threads/ma-1-modular-amplifier.375206/
 
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stenak

Member
2009-11-08 4:17 pm
I can recommend The Tubelab el84 pp with el84 and ecc81. I think it is my third version of el84pp, The first was Morgan Jones "Bevois Valley"?, and the second was a copy of Fischer or Dynaco?, I really do not remember!? I did not liked the sound of them. Flat, hard and dead-ish...The Tubelab plays MUSIC, warm, detailed, with a lot of punch. I love this amp. I use the russian 6p14p and some JJ ecc81. With a pair of Klipsch, it is next to heaven. Easy to build. Point to point soldering, and with Hammond 1650 outputs... Thanks to Mr. Tubelab for a great construction.
 

kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
Mr Tubelab (George) is indeed a great constructor... However EL84 isn't a "real**" triode... OP wants ONLY triodes apparently... Even if it was, a triode connected el84 amp can't push 15-20W. :(

** Electrically, if you connect screen to plate you've made a triode. In a "black box" scenario where you have a 3 pin device, the 3 element triode and the 6 element pentode (indirectly heated so sixth element because of capacitive coupling) that is electrically tied as a 3 pin device, you couldn't tell which one was which - just that one performed better. That's why I'd use a 6AV5 tied as triode instead of a 6B4G - they are basically the same electrically, but one (6AV5) is FAR easier to use - indirect heating. George wrote about it on his site...

http://tubelab.com/articles/tube-testing/6av5-sweep-tube/

His work inspired some of my designs.
 
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Re EL38, after a quick look a a uk supplier has NOS £45 each, whereas they have 6B4G for £35. I haven't searched extensively yet. Re PL36 I fell in love with that tube 30+ years ago. I am going to try some in P Millets engineer's amp type circuit, build in progress, which will scratch that itch.
I don't mind using triode wired pentodes, my list el34, 84 6L6 etc.... PL36s EL500/4/9, looking for adventure on the road less traveled type of thing.
PL83 puts that tube back in the raconing though at £20 :) I Have all the parts to do an SE EL12n amp, still looks like a nice tube. RE 6P21s, duly noted. pitty, could have been an interesting quirky little amp.
 
As another Brit I would just make the point that US or Canadian prices don't make much sense for the UK. By the time you pay shipping and customs charges the prices are considerably higher, so the raw US prices are pretty useless really. You then have to work out which tubes are commonly available in the UK (some just aren't) and what they cost over here. For instance the 6AV5G is pretty rare here and the few on ebay are around £25 each. So buying from over here the 6C4C is a much better deal - around the same price and a much better tube.

Shipping, customs charges and availability are facts of life, I'm afraid. As a Brit I buy little or nothing from the USA as a result. Back in the day I fortunately stocked up on DHTs like 26, 01A, 10Y, 46 and the like but I wouldn't buy them today and you rarely see these on sale in the UK anyway. European tubes are more interesting here, some with old style bases almost unknown in the USA. There is an ocean between us!
 

15-20 watt Triode push-pull? um not easy. 6c33 could do that, but other contenders (broadcast triodes) will need quite high B+. The suggestions with 2a3, 6B4G, 6C4C, 300b, etc... won't get you 15-20 watts in Push-Pull. Nope, that is not going to happen.


Suggestion of EL38 in Pentode or Ultra-Linear? This idea looks worth trying. Won't be Triode though. But if you are lucky to find an appropriate OPT then it might get your 15 watts. Maybe more if you go for screen drive or something like that...

Ooops. I just saw 'low cost' as a requirement too... Do mind using sand in the power supply instead of big heavy expensive chokes?