Mesa Electradyne-No Sound

[Perkinsman]

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.

There is clearly a problem if the Q3 gate is supposed to be receiving ~12v. I clipped Q3 mid leg & measure 0v. Each of the other legs are receiving -11.8v, as do both sides of the SA22 diode.

I did replace V1-V5 already.

[Gregg]

Each of the other legs are receiving -11.8v, as do both sides of the SA22 diode.

That's not possible unless:
1/ you have broken ground
2/ your measurements are not correct (for example not with respect to ground)

[Perkinsman]

That's not possible unless:
1/ you have broken ground
2/ your measurements are not correct (for example not with respect to ground)

Thank you for calling me out on that. I double checked & discovered what I thought was a good ground for my probe, wasn’t...I remeasured Q3 & Q6 gates at -11.8vdc...It appears both fets are working correctly. So it now has me wondering whether the power tubes (originals from 2008) are the source of the low volume.

[vintagekiki]

So it now has me wondering whether the power tubes (originals from 2008) are the source of the low volume.

Power tubes are 12 years old, not designed to last forever as transistors.
It's high time to rejoice your Mesa Electradyne for birthday, and buy him a gift (power tubes)

1)
Amplifier power P=R I² (W) decreases with the square emission the plate current (mA).

Ia (mA)	P (W)
100%	100%
70%	50%
50%	25%

https://mesaboogie.zendesk.com/hc/en-us/articles/217178237-How-long-will-tubes-last-
How long will tubes last

If you play at moderate levels, 10-15 hours a week...
Preamp Tubes are generally at their best 2 - 3 years.
Power Tubes are generally at their best 1 - 1.5 years.
Rectifier Tubes are generally at their best 3 - 5+ years.

[Jazz P Bass]

"So it now has me wondering whether the power tubes (originals from 2008) are the source of the low volume."
Measure the Volts ac signal at the power tube grids right before clipping.
You should see about a 50Vac signal for a 50 watt amp.
Double that for a 100 watt.

[Helmholtz]

You should see about a 50Vac signal for a 50 watt amp.
Double that for a 100 watt.

Sorry, no.

4 power tubes don't need more grid drive than 2 tubes.

Class AB1 amps must deliver full power just before the peak grid signal at each tube reaches the bias voltage value. Higher signal will be clipped by grid conduction.
So if we have -44V bias, a grid signal of 31Vrms = 88Vpp is sufficient for full output, independent of number of tubes.

BTW, doubling grid drive within an amp's linear range (say from 10Vrms to 20Vrms) increases output power by a factor 4.

[vintagekiki]

4 power tubes don't need more grid drive than 2 tubes ...
... grid signal of 31Vrms = 88Vpp is sufficient for full output, independent of number of tubes.

[Perkinsman]

Not being an engineer, are you saying the bias V should be measured at -44vdc & the grid should be 31v on each power tube?

[Helmholtz]

Not being an engineer, are you saying the bias V should be measured at -44vdc & the grid should be 31v on each power tube?

I took the -44V bias value from the Mesa schematic (just for example). Please always specify if you mean DCV, V (AC)rms, Vp (AC) or Vpp (AC).
The 31V in my example is ACV rms.

With a bias voltage of -44VDC, a grid signal of 31Vrms should deliver full output.

A DMM measures Vrms, a scope shows Vpp = 2xVp. Vpp = 2.8 x Vrms. Some digital scopes also calculate/display Vrms.

[Perkinsman]

I just received & installed 4 new 6L6 tubes. Everything seems to be working just fine. I’m just surprised that the volume on a Mesa doesn’t get too loud until the Master & Volume is turned up to 3/4...but all channels now work! Thanks guys for your help!