Switching Jacks for various inputs

[freshtapescent]

I have a pair of Heathkit A9-C that I’d like to make available for double duty as 1. Line/level input with NO volume or tone controls, and 2. A guitar/bass amp with volume and tone controls including an extra triode gain stage.

The units need a lot of work, aren’t pretty, aren’t collectible, no UL taps, so I’m willing to do some rewiring and mods to get them to be as usable as possible for me. I was hoping to be able to poke the line level input in as a 1/4” shorting jack located downstream of the extra gain stage and tone control that would gracefully (read: not unload, melt or shock anything) disconnect the preamp & tone sections for using as a straight line amp. If I plugged into the ‘instrument’ input, I’d have higher gain and tone controls feeding right into the line jack.

I can do it with regular toggle switches or the like, but the appeal of just plugging in to the right 1/4” input jack is very high. The assumption would be made that my preamp would have a volume knob, so If I poked the ‘line in’ jack right after the 0.1u input cap to V3/C 12au7, how much voltage would I need to drive the amp to full rated power?

Any insights?

 

[merlin]

how much voltage would I need to drive the amp to full rated power?

Hard to see anything on that tiny schem but assuming the 6L6s are biased to 15V or less, you'd need maybe 1Vrms at V3C for full power without global feedback. The feedback will increase this figure, so just eyeballing it I'd say 2Vrms.

 

[abbey road d enfer]

Any insights?

I've found a schemo with better resolution. Seeing NFB is taken from a supposed 32 ohm secondary and a 47k/820r network, I would reckon about 0.5-1V, which is consistent with typical HiFi levels.

 

[freshtapescent]

Thanks guys!