Tube pa conversion - volume level

[trobbins]

Have you been able to set up the amp stages ok - the output valves are all passing similar bias current (this will need added cathode sense resistors) - the idle operating voltages are all to be expected (especially the input stage 12AX7's which are grid-leak biased)?

Gain through the stages is knocked down a bit along the way. Eg. if the 470k mixer resistors, and the reverb input mixer resistors, are disconnected for un-used channels then the loading is reduced on the input stage.

The main amp uses global feedback, and a lot of feedback at the input 12AU7. You could disconnect the 10k global feedback circuit, and bypass decouple the 5k6 cathode bias on the input 12AU7, to add a fair bit of gain - which for guitar should not be a disadvantage I suggest.

I strongly suggest you try and organise a dummy resistor load, and set up a computor with a a signal divider to the soundcard input, and spend some time getting a free spectrum analyser application working to get to see if your noise/hum is ok, and a clean output signal is achieved with a clean low level input. You can use an old mp3 player to output a clean sine wave to the amp input. Imho, this is the best and most informative way to check how everything is working signal wise wihin the amp.

I think you will want to modify one input channel to normal cathode bias, as a comparison against the grid-leak bias configuration - especially for guitar input level. The 22k input resistance is fine, as long as you use the switched input so that the signal is not attenuated by 50%.

[gingertube]

THe suppressor tied to the bias supply instead of to the cathode is something IFAK Pete Traynor introduced. Garnet being a fellow Canadian probably copied from Pete. I know that Kevin O'Connor (another Canadian) is a big fan.
He states that it does lower the anode current in a bias failure situation by 30% or so. From that I would conclude that it may also have some audible compression effect.
Cheers,
Ian