5F2A converted into a 2 channel amp?

I have a stock 5F2A:

https://taweber.powweb.com/store/5f2a_schem.jpg

And, I would like to insert another 12ax7 into the amp to act as the second channel. Actually, the 12ax7 preamp circuit I want to insert is already housed in a stomp box with volume and drive controls. The 2 channels would be controlled and seperated with a standard A/B box.

Would it be feasible to just solder in an input jack directly to the grid of the 6v6, then plug my tube stomp box into this jack? I'm worried mostly about impedance problems. Opinions appreciated.

Not exactly. What preamp circuit is in the stompbox?

Taking one channel and feeding it's plate output to the next's grid is cascading. This is exactly what you would be doing by feeding the stompbox preamp to the input jack (the signal would go through the 68k input grid resistors as well). The 5F2A already has limited headroom and doing this would be much more gainy than the stock preamp.

To make it a truly seperate channel, you'd have to bypass the usual input jack and insert the new preamp (AKA the stompbox) into the amp's circuit after the amp's preamp.

You might be able to convert one of those shorting input jacks into a "preamp input" that injects directly into the 6V6's pin #5, but it would also depend on the stompbox preamp's circuit. You may need to pad the preamp in jack with a resistor or make some other tweaks (maybe have it switchable). You might also be able convert an unused speaker jack if the chassis has one.

1:Break the connection between the wiper of the volume control and pin no.7 of the preamp tube.2:Connect a 470 kohm resistor between the wiper and pin no.7. 3:solder another 470 kohm resistor to pin no.7. 4:The other end of this resistor is now the point where the output of your box should go.Of course you also have to connect the ground on your box to the amplifier ground.You could use jack no.1 as input by removing R2 and disconnecting the link to jack 2.If I am unclear please tell me.Limbe

You might be able to convert one of those shorting input jacks into a "preamp input" that injects directly into the 6V6's pin #5, but it would also depend on the stompbox preamp's circuit. You may need to pad the preamp in jack with a resistor or make some other tweaks (maybe have it switchable). You might also be able convert an unused speaker jack if the chassis has one.

hmmm...a preamp input. I've seen those on the back of amps before, but never knew what they were for. It sounds like what I need. But why a self-grounding jack? Wouldn't that ground out the signal at the power tube grid when I don't have anything plugged in there? I may want to use the amp as stock occassionally. I guess I never really understood why guitar input jacks are self-grounding.

1:Break the connection between the wiper of the volume control and pin no.7 of the preamp tube.2:Connect a 470 kohm resistor between the wiper and pin no.7. 3:solder another 470 kohm resistor to pin no.7. 4:The other end of this resistor is now the point where the output of your box should go.Of course you also have to connect the ground on your box to the amplifier ground.You could use jack no.1 as input by removing R2 and disconnecting the link to jack 2.If I am unclear please tell me.Limbe

So your thinking is to use 1/2 of the current preamp tube. The new signal would be introduced at pin 7 of the 12ax7, allowing me to use the current volume control on the amp. Would that still allow me to switch between the two channels while playing?

Yes.Using your A/B Box connect output A to jack 2 on the amp.This will be your normal channel.Adjust the volume for this channel on the amp.Connect output B to the input of your stompbox.Connect the output of your stompbox to jack 1.Since jack 1 now goes to pin 7 via a 470k resistor this will be your "hot" channel.The volume for this channel is on your stompbox.Limbe

Yes.Using your A/B Box connect output A to jack 2 on the amp.This will be your normal channel.Adjust the volume for this channel on the amp.Connect output B to the input of your stompbox.Connect the output of your stompbox to jack 1.Since jack 1 now goes to pin 7 via a 470k resistor this will be your "hot" channel.The volume for this channel is on your stompbox.Limbe

That makes sense. But what do the 470k resistors do in the circuit? Are they there just to keep out spurious high frequency?

They are there to prevent the two channels from interfering with each other.Otherwise,for example, if you were using your "hot channel"with the amp´s volume control turned to zero the signal from your stompbox("hot" channel) would be shunted to ground too.I believe it is called "passive mixing".Limbe