18 watt Marshall clone went quiet

[ernest]

18 watt Marshall clone with a bass/mid/treble/Vol/MV channel 1 and a Tone/Vol channel 2. The Channel 1 started playing at low volume all of the sudden. I can still turn up the Vol/MV and get sound but it is way lower volume than normal. I plug into Channel 2 Tone/Vol, and all of the volume is normal and loud. I've swapped out the pre amp 12ax7 with others scavenged tubes I had lying around. I didn't have any other el84 tubes or ez81 rectifier tube to swap out. I have them ordered but I wouldn't think that would be the issue if one channel was fine and the others volume is low.

Here are my plate voltages from the pre amp tubes.  I was advised to read these to see what gain stages are different.  Honestly I don't know what this is telling me or if this is the problem.  I should be getting all new tubes in the mail this wknd but if it's related to a capacitor then I really only know enough to basically remove each one and test it on a meter.

V1 - pin1-148.9v, pin 6-149.6v

V2 - pin1-189.6v, pin 6-191.6v

V3 - pin1-134.0v, pin 6-238.6v

[SILVERGUN]

Are you using 2 separate input jacks?
If so, how are they wired?...picture?...schematic?

I had this problem on a 2 input jack JCM800 and it turned out to be one of the shorting connections wasn't making contact on the jack that wasn't being used.

[92Volts]

Is this the schematic you're building from, more or less?
http://el34world.com/charts/Schematics/files/Marshall/Marshall_18watt_schem.pdf

If V3, pin 6 is the plate of the second channel input triode (like that schematic), the voltage is too high. Based on the larger (220k) plate resistor and shared cathode resistor with the rest of V3, I'd expect the voltage to be slightly lower than V3, pin 1. Not 104v higher.

It would also help to know your supply voltages. If almost no voltage is dropped across the plate resistor, the tube's drawing little current and something is seriously wrong. If the supply feeding that triode is well over 300v, that's slightly less wrong.

If there's a loose/lost connection from input jack to the input triode grid, you'd lose bias reference and signal, so that fits the bill.

Otherwise my blind guess is "it's a loose connection somewhere"... start checking/re-doing solder joints. A loose coupling cap would allow normal biasing and voltages but cause you to lose almost all the signal.

You're right that the problem being in channel 2 only means EL84s and EZ81 are not suspects here.

[ernest]

One input per channel.  I bought this amp, didn't wire it.  Just built the cab.  I couldn't find a schematic online that matched all of the same value caps that were used and have not drawn out a layout yet. 

[92Volts]

Try Chopsticking. Take a NON-CONDUCTIVE stick (like a wood chopstick) and poke around while you're playing through it (music/test tone would be easier than playing a guitar while doing this). If the issue comes/goes when you poke at a certain wire/component, you've found the problem. I'll bet it's a bad solder joint, but could be a failing component.

A capacitance meter will rarely show if a cap leaks DC current or goes open/short intermittently. Those are bigger issues than an out-of-spec capacitance value. Closer inspection of voltages in the circuit or just going ahead and replacing the caps could tell you.

I assume when you say V3 you mean the one closest to the powertubes, which is labeled V2 on the schematic I linked. That would be the phase inverter. There's obviously voltage imbalance between the two halves but I'm not sure how normal (or not) that is.

Hit all the pots with electrical contact cleaner, DeOxit or whatever you can find near you.

Check (or just replace) whichever cap is between the phase inverter and channel 2 volume knob. C18 on the schematic I linked.

[ernest]

Thanks

[SILVERGUN]

Are you using 2 separate input jacks?
If so, how are they wired?...picture?...schematic?

I had this problem on a 2 input jack JCM800 and it turned out to be one of the shorting connections wasn't making contact on the jack that wasn't being used.

Try to troubleshoot this:
-Plug your guitar (or a test signal) into the Channel 1 input (the one that isn't working correctly)
-Using 92Volts' chopstick method, press on the tip connection of the Channel 2 input jack (the one you're not plugged into) so that it shorts the little tab that would normally contact the tip of the cord that gets plugged in. ("Switch Closed" in red box on attached pic)

**BE CAREFUL!! - NEVER PLAY YOUR GUITAR WHILE TROUBLESHOOTING!! - IN THS CASE JUST LET IT SIT AND DRONE A NOTE THAT YOU'LL BE ABLE TO HEAR IF THIS METHOD WORKS - IF YOU DON'T HAVE A TEST SIGNAL TO INJECT