Another pre-amp question - Teisco stereo pre-amp

[gui_tarzan]

Sorry to be a pain with this amp but there is so much I'm trying to learn about it. It has little output from channel 1. I'm posting the layout that I drew for just channel 1 because there has to be something missing. This is exactly how it's wired. I used arrows for the signal path as this is how I understand the guitar signal current flow through the components. I know some don't like this type of reasoning but it helps me.

In a nutshell, the guitar signal goes to half of V1, through the plate to the tone stack & volume, then into the other half of V1, then to the output jack. This doesn't make sense to me because the other channel has two gain tubes and a reverb section, four tubes in all. So the amplification of channel 1 is almost nil into the power amp. Small enough that there is almost no output to the speaker. If I plug into channel 2 at the same volume it's LOUD. As it should be.

So the question is, does it seem logical that this channel one only has one gain tube? If so, I have to figure out why there's almost no output.

[Helmholtz]

V1 contains 2 separate triodes, each wired as a gain stage.

Try to draw a real schematic from your layout. Would make it much easier to spot errors and follow the signal path.
Then measure supply, plate and cathode voltages to see if the tubes work properly.

[gui_tarzan]

That's what I'm doing but I ran into this oddity and it makes no sense. Without knowing what connections are actually there I can't know if it's right or not since there are no schematics that I've been able to find on this particular preamp. I've been looking for months. The only pictures I can find of it are on Japanese sites.

It almost seems like it's wired like a tube boost foot pedal with tone controls.

[glebert]

Do you have any info like a model number for this device? I am not sure if you are describing an actual stereo preamp, or the preamp section of a two channel guitar amp, or something else. If you don't have a model number than any picture you have would be better than nothing. Right now it feels like we are groping in the dark.

[gui_tarzan]

Yeah, it's a Teisco King stereo pre-amp in a head. No model number on the chassis whatsoever, just a serial number. I also included the entire chassis drawing I did.

[Helmholtz]

That's what I'm doing but I ran into this oddity and it makes no sense. Without knowing what connections are actually there I can't know if it's right or not since there are no schematics that I've been able to find on this particular preamp. I've been looking for months. The only pictures I can find of it are on Japanese sites.

Sorry, I don't understand.

What oddity?
You have 2 gain stages with a tone stack between. That's quite normal.

Your layout wiring contains all information needed to draw the schematic.
If you're not sure where a wire connects, use your Ohmmeter to find zero Ohm between the wire ends.

[gui_tarzan]

Sorry, I don't understand. What oddity? You have 2 gain stages with a tone stack between. That's quite normal.

What is odd to me is channel two has two gain tubes along with two tubes to handle the reverb. Why would channel one have only one tube and nothing else, and channel two have two just for gain? Maybe the issue is a bad component and not the single-tube amplification. I've looked at hundreds of different schematics over the years and have never seen this single gain tube in a 100w amp. That's why I asked.

[gui_tarzan]

Ok, you can all disregard this post. The question about the single tube gain was answered, and I have a bad connection in the circuit somewhere because it's as loud as channel two now after moving things around for drawing out the circuit.

[g1]

I was going to mention that a typical blackface Fender circuit uses only one 12AX7 in the normal channel, then into the PI from there.

[gui_tarzan]

Oh man, you're right. I guess I let the over-thinking get to me on this one. Now I feel stupid.