Push pull designs.

[audiousername]

barretter, I think Ian means the RC network between the anodes of V1, i.e. C5 and R21

 

[barretter]

Quite right! I missed that!

 

[Jax]

Originally posted by gingertube
Jan,
A nice simple circuit - simple circuits have simple problems - simple problems have simple solutions - therefore I like it.

Thank you. Simplicity was one of the goals.

In a light hearted sense - here is your design review:
1) I don't like the resistor capacitor across the two anodes of the fist differential stage - separate series resitor and capacitor network across each anode load is reputed to better.

Interesting, could you explain this further? My version should be the small signal equivalent of what you are suggesting. Would it be that the balance can't be perfect in the real world?

2) Should you ever contemplate using anything other than EL34 then the R8,P2,R12 (and the similar network on the other side) needs modification to reduce total grid circuit resistance. This combination is giving about 250K Ohms (at DC) from grid to 0V. For KT88 or similar you want to get this down to at least half - that is not more than say 120K - the driver stage will cope with this OK.

I am aware of this, the bias net is for EL34 only, at least NOS. I will lower the resistance to around 100k anyway, it does not hurt.

3) Unless you are running pure Class A - which I don't think you are, you should probably bypass R16 with say a 470UF to 2200uF high quality electrolytic (a Blackgate or Nichicon Muse etc.)

They are running in B1 or low AB1 with some 35-50mA cathode current per valve (exact bias to be determined later, maybe I'll push them into full AB1 at 65mA). Another goal with this design was to get rid of as many LF poles as possible. I may have removed one too many here 😱 , thanks for reminding me, with the valves in AB1 it is required. I haven't played the amp at higer volume yet, I guess I would have heard the distortion when the valves start to cut off.

4) C7 and C8 could benefit from a 100nF film cap across them. Very occassionally this may introduce slight ringing - if so put 1 Ohm in series with the film caps. (Polyester would be fine - no need to go to polypropylenes).

I'll try this.

Otherwise I give you a 9 out of 10. Congratulations.
Cheers,
Ian

Thank you, I'll try to improve 😉

I should add that this is a high gain high feedback design (at least for valves), running without the compensations is not recommended. With the values shown I have a phase margin of 70 degrees at 21 dB feedback. Good output irons is probably called for, my prototype is using Tango FW100-3.5 with 6 ohm speakers on the 4 ohm windings.

Also, making the current sinks adjustable is recommended (R102 replaced with 360 ohm + 100 ohm trimmer), I have modified them after I did the schematic.
R7 is currently two 33k 3W metal films in parallel.
The diodes are only needed if you don't have delayed HT.