10W Push-Pull Class A

[Mike K]

I'll keep the title pretty general because there's much that can be done here.

This amp changed it all for me. That amp, unto itself, is probably pretty unspectacular but what it did do was start me on a quest for designing 10W Class A PP amps. Maybe it's a stupid idea, but I wanted something that was 1) super responsive 2) didn't have ear splitting volume for playing in my small house 3) could be adapted to different tonal variations.

Up until now, this has mainly been in the form of single-ended amps. But I have to be honest, even my best ever single-ended amp left something to be desired. They just don't have the same harmonic content (yes I can prove that even on my crummy old analog scope). I never much liked the sound of PP triodes and small signal pentodes just overload too quickly IMO.

So when I set out on rectifying this fever dream, I was also inspired by the JJ EL844. I figured that I could get around 7W in Class A PP with these, just a tad over what my low powered SE amps were. Oh man, that was the dream. Well, I'll be quite honest. That turned out to be fizzy $hit. The JJ EL844, IMO, doesn't sound great. It also doesn't seem to have any less apparent volume. In my tests, it just sounded like a crummy EL84 amp. I scrapped them. But I didn't give up...

I actually went through many variations of preamp and power amp setup but I finally came to this:

You might recognize it has some resemblance to a top boost AC30 and an AC15.

I wanted to have the minimum controls - I settled on volume, treble and bass (and it works!) and just play with my guitar volume and tone. The bright switch I found necessary because it's too much with gain and not quite as chimey as I'd like at cleaner volume settings, especially with neck pickups. I suppose I could have put a treble cut on the PA like a VOX and left the bright cap hardwired, but with my guitars, this actually works fine.

So with my strat, this thing doesn't fizz out. You can play it dimed and it's just all out. It might fizz with a high output 'bucker, but I don't care. I built this to play with that guitar, and it works.

I messed around a lot with the exact tone stack and bias of the PI. As you can see, it's not a std Vox setup. Not having the cathode follower after the second stage changes the tone control characteristics and these stacks are super sensitive.

I also started with a tube recto (no idea why, this thing is true Class A, but I tried it). I also had a different PT initially. I finally got that dialed and got my voltages right on with no tube recto. The result is it's fairly tight for an amp with no NFB. There's tons of bass on tap with this tone control so I really roll it off when I put the volume up. Treble can go anywhere but it's SUPER bright with my strat - I tend to keep a bit below middle of rotation with the volume up. If I go to position 2, I need to add quite a bit and roll off the bass more, but it still has a good useable tone. I'm generally a bridge player though, and my tone control is wired to work with it.

For such a simple result, and minimalist amp, it was a LOT of work. I worked on and off on this thing for 3 years (at least) and finally am really happy with the tone.


I have another 10W version that is 6V6s and a plexi type preamp. That one screams too - a little more midrangey and can fizz out a bit easier. I have a different PT to try to dial in the voltages and I may add a little NFB on that. It has just a bit more gain that this one, and it's enough to overpower the PA I think.

Overall the 6V6 version sounds best with a g12m greenback and the EL84 version with a g12 Alnico blue, but one might expect that based on the preamps.

[Helmholtz]

A shared cathode resistor of 50R with 2xEL84s looks quite low even for pure class A.
What voltage does it drop?.

[Mike K]

Va-k = 218V at idle, Ig2 = 8mA x2, Vk = 6.15 V, Ik = 123mA. Ia = 53.5 mA, Pa = 11.7W.

Sorry voltages on the schematic were design voltages. Actual B+ is 238V and OT has a fair bit of DCR.


In retrospect I believe the voltages I was shooting for were the ones listed but I'd need a bit more winding on the PT. I pulled this one off another amp and it got me in the park but if I was going to start from scratch I'd bump them up and I believe I had initially calculated a bias resistor somewhere between 100-75 ohm. I think I had started at 100 and kept lowering until I got the power dissipation I wanted. Even though it's high current, the dissipation is low (5W is not necessary, but it's what I had). If I were to do it again I'd probably just use a pot there to trim it and use a bit higher voltage PT than I need and trim the voltages with a dead node out front.

[Chuck H]

And LT spice test I JUST did shows a dissipation of just under 11W per tube with 218 working volts (anode to cathode) and a shared 50R cathode resistor. Just a little over 11W at full clip.

[Mike K]

It's not a bad volume to play at. One might think these low voltages would equate to a dark tone, but I find it quite bright. Mostly due to the preamp.

Compared to a Vox AC15, it sounds quite a bit different. For my purposes, much better. For someone who wants to use pedals, likely a stock AC15 would be better.

[Helmholtz]

Va-k = 218V at idle, Ig2 = 8mA x2, Vk = 6.15 V, Ik = 123mA. Ia = 53.5 mA, Pa = 11.7W.

Sorry voltages on the schematic were design voltages. Actual B+ is 238V and OT has a fair bit of DCR.


In retrospect I believe the voltages I was shooting for were the ones listed but I'd need a bit more winding on the PT. I pulled this one off another amp and it got me in the park but if I was going to start from scratch I'd bump them up and I believe I had initially calculated a bias resistor somewhere between 100-75 ohm. I think I had started at 100 and kept lowering until I got the power dissipation I wanted. Even though it's high current, the dissipation is low (5W is not necessary, but it's what I had). If I were to do it again I'd probably just use a pot there to trim it and use a bit higher voltage PT than I need and trim the voltages with a dead node out front.

Yes, the lower B+ makes difference and values look ok now.
Plate current still a bit on the high side for center- bias.
Expect around 7W before clipping at output.

With low B+ center-bias often results in a plate dissipation below the limit (also depends on OT impedance).
Running hotter than center-bias does not increase output.

[Mike K]

Yes, the lower B+ makes difference and values look ok now.
Plate current still a bit on the high side for center- bias.
Expect around 7W before clipping at output.

With low B+ center-bias often results in a plate dissipation below the limit (also depends on OT impedance).
Running hotter than center-bias does not increase output.

I wasn't all that concerned about actual output but how it might affect tone. I'd like to revisit this some day and get it right on the limit for Class A and compare. My guess is it'll just have a touch more headroom, which isn't a concern for my intended use with this amp.

All-in-all, as I alluded to above, I initially intended to use the EL844 which is a lower current, lower power EL84, which put my voltages more at the limit of Class A. I then scrapped those because of tone, and found even at less-than-ideal Class A operation, the EL84 sounds much, much better. I suspect it just handles grid current much better when pushed to grid-current limited distortion.

This was mainly a project of tinkering (as I said, 3 years on and off messing around). I ended up learning about this particular OT, bought one with the intention of building that GA-5 for someone else but wound up just doing my own thing and coming to this in a round-about way. It's by no means perfect in terms of design. But in terms of how it handles a guitar, it's the best it's been and one of my favorite amps I've played.