The Mighty Terror!! (50W OTT inspired amp)

[Hardtailed]

Hopefully you guys can help me refine my planned circuit before I start building. (see attachment)

This is basically the Orange TT preamp coupled with a 5F6A power amp.
I'm looking for a 50W-ish version of this marvel with a solid low-end so a pair of 6L6 in push-pull with higher voltage and fixed bias.

What I have changed:
- Dual ganged pot for gain has been replaced for separate controls for each gain stage
- Cathode resistors for preamp have been changed to 820ohm (as used by the 5F6A preamp, I'm guessing because it runs more voltage).
- A pair of caps have been added on each side of PPIMV and tone control since I'm not going for cathode bias.

(power supply has not been included for now, will probably go with typical 5F6A design, but with solid state rectifying and maybe a bit more filtering, I want a tight front end)

My main questions:
- The OTT doesn't use NFB, the 5F6A does. Is it a big deal if I go without, considering it is part of the power amp I'm modelling this on? I understand that a PPIMV doesn't necessarily play very well with NFB (since we are taking a signal at the end of the chain and injecting back before the PI)

- The 5F6A has a 47pF capacitor joining the two cathodes of the PI, the OTT doesn't (it's a straight path). What does that cap do? (It is included on my schematic)

- The PI plate resistors are both 100K on the OTT, but on the 5F6A one is 82K. That puzzle me a bit...

- The 470pF bypassing the second plate resistor in the preamp, I have a feeling this part is completely useless, am I right?


I expect some fine tuning of the cap values will be required.

Thanks everyone!

[Steve Conner]

I don't know the answers to any of your other questions, but I do know the 47pF capacitor is to keep the NFB loop stable. The Tiny Terror doesn't have it because it doesn't have NFB.

Using a post-PI MV with NFB isn't necessarily bad. It will lower the amount of NFB as you turn the MV down, giving you a bass boost and a presence boost which helps to compensate those Fletcher-Munson loudness curves.

[Hardtailed]

I don't know the answers to any of your other questions, but I do know the 47pF capacitor is to keep the NFB loop stable. The Tiny Terror doesn't have it because it doesn't have NFB.

Thank you for the information, now that make perfect sense.
I figured it was some kind of HF rollout, but I guess it serves to prevent oscillation at the same time.

By the way, my mistake, the OTT doesn't just short the two plates, they are only connected by way of being on the same power line (but they have their own plate resistance so it's not a straight connection).
So basically, that capacitor could be ignored, but I may as well leave it there, especially if I intend on experimenting with NFB.

Thank you for the information!