How to Determine Feedback Resistor Value

[mwelch55]

I am building a low-power push pull amp based on the 12BH7 dual triode as an output tube.  I am using an output transformer with 22K primary and 4,8,16 ohm secondary.  I have a B+ of about 258 volts and I am using a LTP Phase Inverter based on the JCM800 design.  My question is how do I determine the feedback resistor value?

[PRR]

Welcome.

You may not *need* NFB for a guitar amp using triodes. Pentodes (EL34 6L6) have very-high output impedance, which may "want" to be reduced for best loudspeaker response. The Fender tradition actually reduces Zout to roughly what a triode would give (Zout ~~~ Zload). Marshall tapped NFB on the early 5F6-inspired amps for a somewhat lower Zout.

Prepare the build with required turrets/lugs and the to-ground NFB resistor. Have a place for the from-speaker NFB resistor but don't install it. Smoke-test. Play. With all different speakers (or at least ones likely to be used). Pentodes can be "raw" sounding, triodes less so. Pentodes on most guitar speakers will have a heavy bottom-boom and ice-pick highs from speaker impedance not being damped. Now try the stock Marshall from-speaker NFB resistor. Better or worse? Try half and a quarter that value. If it's getting more raw or boomy, or outright howls, reverse the plate leads.

If you go way lower than Marshall's value the amp will get "hi-fi" (too polite) and then get unstable (subsonic or supersonic howls). I think before that happens you will regret the large loss of gain. I would expect Marshall's value to be a good ballpark, but more or less depending on speaker, genre, and venue. (A bedroom is not a crowded club nor a giant arena.)

[mwelch55]

Thanks for the reply PRR.  I was thinking of trying what you suggested, but wanted to run it by someone first to see what my options are.  The preamp will be based on JCM800 with minor tweaks.  I was hoping to get about 5 watts output.  This is my third amp to build.