Question about voltages on Dual Showman

[Paul G.]

We're lifting the shelter-in-place in my state and soon it will be time to deliver an AB763 Dual Showman I did a recap and general maintenance on.

The amp sounds fine and works as it should but...I felt the vibrato channel was a little quieter than I thought it should be, maybe a little more than half the volume as the normal channel. Owner didn't complain but I thought I'd just check all of my voltages. Every one was pretty much dead on, except one.

On the first plate of the vibrato tube I should be seeing 260Vdc. With the shorting plug out I see 445, same as the other side of the 220K plate resistor. I figure, ok trem is off so no current being drawn, so no voltage drop. But when I put the shorting plug in, even with the speed and intensity pots all the way down the voltage fluctuates, not really oscillating (although it just may be too fast for my meter to keep up with), but not steady. Now the channel and vibrato are operating properly, reacting to all the controls. The volume does not change with the shorting plug in or out.

1. Is this normal? I don't think so. 2. I checked all the resistors (in circuit) and they're fine. I traced everything and the wiring is correct. Bias voltage which charges the oscillator is fine at -50.

Showman-AB763-schematic.pdf

[Roe]

check heater wiring. reflow soldier joints

[martin manning]

The intensity control doesn't have anything to do with the oscillator, but it does load the output of the vib channel when it's turned up, by paralleling the LDR with the 50k pot. Also, with the amp out of its box the LDR's resistance may be lowered due to ambient light leaking in. When speed is all the way down, the oscillator will still run, maybe 3-4 Hz, so voltage will fluctuate at the plate of the oscillator tube, but there will be no tremolo effect if the intensity is turned all the way down.

[jjman]

Scheme shows the plug in and switched on. We know it's oscillating so Fender must have shown an average figure for the voltage. Same behavior on my Fender.

[Stevem]

The volume of the vibrato channel should be the same as the normal channel if the preamp tubes number 1, 2 and 4 test out good, and also if all of those tubes plate load resistors a, Cathode resistors and Cathode bypass caps are good also.

Since tubes 1,2 and 4 are all 12AX7s swap them around and make note of what happens between both channels.

A failing Cathode bypass with high ESR can knock a good amount of gain out of a channel, and yes I have had new caps with high ESR which is why I test all of the new caps and filters I buy before they go in my parts draw, no less in a amp under repair or mod!

Note how on these and many other amps Fender the vibrato Channels recovery ( second gain stage) gain stage shares the Cathode resistor with the second gain stage of the normal channel( 820 ohms vs 1500 ohm ) this can be a problem so I add another 1500 ohm resistor and 25 uf cap to the circuit to bias the recovery gain stage of the vibrato channel on its own.
That 820 Cathode resistor really needs to be 1/4 watt bigger then it is, so just stepping up to a 1 watter is a good idea.

Another thing to check are the two 220K mixer resistors that are before the input cap to the phase inverter.
These can change value and make one channel lower in volume then the other.
You can simple swap the input wires around to test out if there's any change in volume.

[Paul G.]

I've got a new set of 6L6s coming in on Wednesday, I'll get the amp back on the bench and check those things. This amp came in as a barely running mess with original filter caps, bias cap, burnt out screen resistors, worn out octal sockets and an output tube with intermittent shorts. The bypass caps were all replaced (poorly) with one put in backwards. Client would be happy with a working amp, I'm the one looking to make it as perfect as can be.