Mesa Electradyne-No Sound

[Perkinsman]

Thanks so much for this. I measured Q3=83ohm(drain to source), -11.8v on each BUT 0 volts on the Gate. I measured 0v on Q6 gate as well. Does this mean they are both bad? If I order them, do I just order 2-J175’s?

[Jazz P Bass]

Find out why you do not have any gate voltage.
83 ohms D to S is normal.
A gate voltage will increase the resistance.

[Perkinsman]

Hmm, so are you saying that the fet is good as long as the resistance is normal?

[The Dude]

IMO, the easiest way to verify that the FET is the cause of the problem is to unsolder or clip the drain and/or source leg out of the circuit temporarily- effectively removing the FET from the circuit. All the FET does is short signal to ground. If you temporarily disconnect it and the amp suddenly works as it should, the FET is likely bad (although it could be a lack of switching at the gate). You don't need the FET for the amp to work.

[Perkinsman]

I clipped a leg & it didn’t increase the volume but that still doesn’t address why the gate voltage is 0.

There is a diode located physically right next to Q3. It beeps when I test it when it should show a v drop. Is this a suspect?

[The Dude]

By "beeps", I presume you mean it shows "0" resistance? If so, unsolder one end of it and check it out of circuit. Is there a circuit board designation for the diode?

[Perkinsman]

I’ll have to pull it to see it, they printed it underneath

[The Dude]

Oh, that's handy. I hate it when they do that!

[Perkinsman]

It may be by design but my Fluke 83 will beep instead of displaying OL half the time whenever I read a diode in circuit. So I pulled a leg & retested and it resulted in OL both ways.. The diode is SA-22-1 but is listed on the schematic as also SA22. Does SA designation & the symbol make it a Shotky diode? If so, is OL both ways the correct reading? I have some diodes here but not sure what value can it be replaced with? Sorry for all the questions.

[vintagekiki]

The diode is SA-22-1 but is listed on the schematic as also SA22. Does SA designation & the symbol make it a Shotky diode?

https://www.web-bcs.com/pdf/Gsi/SA/SA22.pdf
Suppressor Diode SA22. Datasheet

[The Dude]

If that diode is not shorted, it would not cause your symptoms. It's not a schottky diode, its a suppressor diode and I wouldn't suspect it as bad unless you measured a short.

[Gregg]

I measured Q3=83ohm(drain to source), -11.8v on each BUT 0 volts on the Gate.

There should be ~12V at FETs' gates otherwise the FET will be "closed" (low source to drain resistance effectively in mute state).
The TVS diode is there to protect the FET from transients and is not likely to get blown but it's not impossible.
Also note that TVS diodes may exhibit some capacitance in the nF range so they may affect tone as well.

[vintagekiki]

3.I decided to replace all V1-V5 since they’re all old original. I’m also replacing the power tubes with a new matched quartet of 6L6gc’s

And did you replace the tubes?

Just as a car does not test with old wheels, so amplifier not test with old tubes.

That’s a very good idea, because there is something else going on regarding the volume beside the tubes. Which Fet is typically the problem?

Description us what is something and how something it manifest?
Instead of an endless exchange of posts and vain measurements, it is much easier extract and Q6 and check - hear how Mesa Electradyne works without them.
When Q3 and Q6 is removed it is much easier to test their correctness.

note
Gate Q3 (J175) does not receive any command (hangs in the air) page 6

[Gregg]

note
Gate Q3 (J175) does not receive any command (hangs in the air) page 6

That must be a mistake otherwise it doesn't make sense.

[vintagekiki]

That must be a mistake otherwise it doesn't make sense.

This is not the only one mistake.

Voltages on schematics should be observe with reserve because something wrong with the values. Probably is error in writing value for voltage between page2 and page3.
page2 D+ = 366V page3 D+ = 348V
page2 E+ = 365V page3 E+ = 345V
Tube V3B on the anode has 367VDC, no anode resistance, that same anode connected has D+ = 348VDC.