Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gtrbryan on October 25, 2016, 09:06:35 pm

Title: GA-9 5C2 Combo - switching question - best way to switch between preamps
Post by: gtrbryan on October 25, 2016, 09:06:35 pm

Several years ago, I planned a build where I would have two channels.  One would be based on the Gibson GA-9 preamp and the other based on the Fender 5C1.  I had anticipated to use a rotary switch to go from channel to channel into a single 6v6. 

After moving on to other projects I came back to this, and decided to use a 6SC7 and make the second channel the 5C2 preamp.  My rotary switch is 4 pole, three position (4P3T) and I am thinking of wiring it as attached to give the option of putting a 6SC7 triode after the 6SJ7.

My question is this:  In any configuration like this, when one channel is switched "out" there will be a tube with active heaters and voltage on the plate.  The input would not be grounded.  However, it would be outside of the signal chain.  This is different than is usually done.  Is this a potential noise issue?  Any thoughts of a better way to do it.  Schematic attached.  Thanks as always.

Title: Re: GA-9 5C2 Combo - switching question - best way to switch between preamps
Post by: PRR on October 25, 2016, 11:11:55 pm

I can't follow your switching for-sure.

Plate circuits of both 6SC7 stages sure are incorrectly drawn.

Combined cathode circuits of sequential 6SC7 stages requires HUGE cathode cap or it will oscillate. That's why it was almost never done. EDIT- yes, 5C2 does do it. But with that heavy 250K:75K divider between stages. Even then, I think when the 25u cathode cap drifts a little it will get wonky.