Another radio->Gtr amp mod question

[rafaelblock]

Here's another mod I'm working on: an all octal output section from an old radio. I'm converting for use as a guitar amp and wonder

1] anything look out of sorts? I'm thinking of losing the caps across the output xformer but what else?

2] can I just hook this puppy up with a volume control into my favorite preamp?

Any other suggestions would be appreciated.

Thanks in advance.

[Miles Prower]

^^^^

I would try it both with and without those capacitors at the primary of the OPT. More importantly, I would get rid of the feedback loops. For a guitar amp, these will make for a nastier clip behaviour that probably won't be desireable.

I would also redesign the 6SN7 input stage as an LTP phase splitter/preamp. That'll certainly be an improvement, and, yes, it will work just fine with your preamp.

That's what I'd do.

[rafaelblock]

Originally posted by Miles Prower
^^^^

I would also redesign the 6SN7 input stage as an LTP phase splitter/preamp. That'll certainly be an improvement, and, yes, it will work just fine with your preamp.

That's what I'd do.

Any links for design ideas to utilize the 6SN7 as a PS/Pre? I would assume it would be similar to using a 12AX7 or such, but the component values would be way off [?] I've sniffed around a bit but cant' find anything readily.

[Miles Prower]

Originally posted by rafaelblock
Any links for design ideas to utilize the 6SN7 as a PS/Pre? I would assume it would be similar to using a 12AX7 or such, but the component values would be way off [?] I've sniffed around a bit but cant' find anything readily.

Here's an example loadline, using the original 47K plate resistors. This gets you a THD estimate of about 3%, which isn't half bad. Since these are 6SN7s, most of that harmonic distortion is going to be h2, and the LTP, being a balanced topology, will null it out. Since the graphical estimates tend to run high anyway, the sonic performance is going to be pretty good there. After all, that's what 6SN7s are known for.

This new LTP input stage should have active tail loading from a CCS made from cascoded BJTs. This can either run off its own negative rail, or you can elevate the grids of the 6SN7s a bit to make headroom for the CCS.