Cathode follower PA Driver in a cathode biased amp

[stephenl5]

I'm interested in adding cathode follower drivers on one of my existing guitar amps, mostly to see the effects on blocking, crossover, and bias excursion.

The donor amp has a hole for another tube and I have plenty of filament amps so I want to go with a triode instead of a mosfet source follower in this case.

The amp is push-pull, cathode biased and I haven't found any examples of a driver circuit used in one.

My question is, how do I correctly bias the CF to directly couple to the PA grids? What extra circuitry do I need to add?

I would probably use 6SN7, 12AX,T,U7 since I have those on hand.
_________________
Steve

 

[DF96]

You will probably need a negative supply rail and run the PA in effectively fixed bias mode, by adjusting the CF bias to get the right PA current.

 

[HollowState]

What DF96 said. Below is one scheme I've used that works well. Don't know what O/P tubes you're running so the values may have to be adjusted accordingly. Use a 6.3V filament transformer in reverse to get the negative source.

 

[stephenl5]

Thanks Hollow State.

So since I have cathode bias, I would just adjust your bias pots to bring the power tubes' grids to 0vdc at quiescence?

I'm using KT66's.

 

[GoatGuy]

What DF96 said. Below is one scheme I've used that works well. Don't know what O/P tubes you're running so the values may have to be adjusted accordingly. Use a 6.3V filament transformer in reverse to get the negative source.

My compliments to whomever authored that schematic. It may be a bit pedantic, but I still love crossing lines that have blank guards to imply “going over” on either side. And resistors that aren't pointless little rectangles, which has become “moderne”.

GoatGuy

 

[DF96]

I still prefer crossing lines to use a little hump-backed bridge. That is unambiguous, even if a part of a diagram gets missed when copying. And yes, I prefer zig-zag resistors. A little rectangle should be a rare component which doesn't have its own symbol.

 

[tomlang]

I used that Heathkit CF configuration above to design a 12AU7 CF for an EL34 amp. It works well in LTSpice but I have not breadboarded yet.

Use the variable fixed bias circuit of your choice to feed the R13, R16 junction.