mystery in Dual Showman - where is the wire?

[Groover]

I have a SF Dual Showman Reverb which has been modified to have Rev & Vibrato [sic] in both channels. This work was done before I had educated myself much about amps.

I was looking at it today and I cannot see how the "Normal" channel output is getting connected to anything. The .047 coupling cap is there, but the other lead of the coupling cap seems to be a dead end. It does not have a wire that I would expect to see leading from the "downstream" end of the coupling cap to another point in the circuit. Tests with audio input confirm that signal from this channel is in fact getting to the output.

This is a '70s amp, so more of the wiring is above the eyelet board than was the case with the BF amps. I have attached a detail of an earlier Twin layout. The wire I am talking about would normally connect to the points marked [X] I understand that the usual way (or at least Dan Torres's way) of doing this mod is to remove said wire from point [X] at the 220k mixing resistors before the PI, and connect it at the point marked [Z] where the signal goes to the reverb recovery gain stage.

I have checked with a VOM for continuity in case there was a wire running under the eyelet board, but I didn't find any point on the board which connected to point [X] on the preamp side of the board. (I didn't check every solder pad on the whole board, just the places I though it might be.)

So any ideas how the output of the Normal channel is getting passed forward/mixed with the Vibrato channel?

[rf7]

The point marked X on the right side by V1 connects to the other point marked X on the left side of the diagram by V6. I'd look at the schematic to see exactly where it connects to in the circuit.

[Groover]

Yes, I know. Those points labeled with letters are meant to be connected by wires.

Except in my amp there is no wire connecting [X]v1b to anything as far as I can see, and it is still getting to the output. Perhaps my post was unclear on this point.

[Enzo]

Are ther any ADDED caps anywhere esle? A cap completes a circuit but won't reveal itself to continuity testing.

[Bob M.]

John,

I'd bet the plate of V1 (pin 6) is connected to the plate of V2 (pin 6) by a short wire. This is one of the main ways to put the amp's effects into both channels. Of course, without benching the amp, it's pretty hard to diagnose but that's my guess.

Bob M.

[Groover]

Well Bob M. you were one of the last techs to be inside the amp, so I'll bet you are right. It didn't occur to me to look there.

I would have thought that directly connecting the plates of two tubes would cause undesired interactions or other problems.

[Groover]

But no, I just looked at the amp and V1 pin 6 is not connected to V2 pin 6.

Of course the cathodes (pin 8) of V1 and V2 are tied together, but I believe that is normal for Fenders.

[Bob M.]

It is normal (and stock - a Leo parts-count-lowering manouver) to tie the cathodes of V1 and V2 together at pin 8. Fender then halved the cathode resistor (almost).

As I said, tying the plates of V1 and V2 together via pin 6 of each tube is one of the usual ways of adding effects to both channels. If one does this, you should also disconnect the coupling cap at pin 6 (which is now redundant) or the wire coming after that coupling cap to the summing resistors just before the PI, or hopefully, both.

Another way of integrating the reverb into the normal channel is to jumper the coupling cap out for V1 (normally a .047uf) to just before the reverb circuit. Look for a wire leading to the reverb drive circuit input cap (500pf) or somewhere around that 3.3 meg/10pf array. There's a couple of places the wire can go in this configuration to make it work. Once again, the wire from the V1 coupling cap out to the summing resistors is removed.

John, bench the amp, make copious measurements, study the circuit carefully and I'm sure you'll find where and how the reverb is jumpered to the normal channel. You might start with the reverb drive input cap (500pf) and work backwards.

Sometimes, certain techs use a resistor, say somewhere around 10K, because they feel there's no need to use a shielded wire when doing this. So, I'd focus more on location rather than just a jumper wire. That could be the answer to this puzzle.

Bob M.

[Groover]

Found it!!

Some sneaky tech had added a wire under the eyelet board jumpering the plates V1b and V2b where they connect to the plate resistors. I wonder if that choice was based on keeping it concealed/proprietary or for noise/stability reasons? I may find out, since I moved the wire to above the eyelet board.

Serendipitously I found a B+ wire to the V2 plate resistors that was about to break at the solder pad, so I fixed that.

[Bruce / Mission Amps]

Some sneaky tech had added a wire under the eyelet board jumpering the plates V1b and V2b where they connect to the plate resistors. I wonder if that choice was based on keeping it concealed/proprietary or for noise/stability reasons? I may find out, since I moved the wire to above the eyelet board.

Serendipitously I found a B+ wire to the V2 plate resistors that was about to break at the solder pad, so I fixed that.

Might have been me.. I'v been doing that mod for like what, 20-25 years and hiding the jumper wire under the board so the little twerps in the amp hood couldn't find out what I was doing.

[Groover]

Sorry Bruce. Didn't mean to blow your cover. Can we delete this thread somehow?

I will have to try and figure out what happens when you tie the two plates together when i get my replacement copy of Merlin's book!
The fact that it puts the plate resistors in parallel makes it interesting. Hmmm cathodes are already paralleled. The dual triode as parallel mixing stage. Interesting.

[Bob M.]

I used to do the same thing as Bruce but nowadays this is such a common way to put effects into the normal channel that I don't bother with any heroic concealment measures. As I increase my knowledge and develop more and more circuits and ideas, I feel less inclined to hide anything I might do to an amp. I think it's great how many people share so much of their knowledge and experience for the benefit of others, especially right here!

Bob M.