i need help! mesaboogie 2020 poweramp redplating

[akgun]

hi, i am the owner of the mesaboogie 2020 amp for more than 13 years. the problem is , all (4) tubes are redplating. here are some readings;
pin#7: 390V
pin#2: -11.2V
pin#3: 13mV
pin#4, pin#5: 3.5VAC
pin#9: 330V

i have used "this method" to measure the plate currents.
"The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side."
i have read it from,
http://www.aikenamps.com/Biasing.html

the readings are;
V3: 38.6mA
V4: 40.3mA
V5: 39.0mA
V6: 37.5mA
plate voltage at the time of the readings was 385V.

i noticed that when i pull the phase inverter the redplating stops. another way to stop redplating was to disconnect the 2M2 resistors between pin#2 and ground.
i am a newbie so i dont have any idea if sharing a schematic is forbiden or not but i am attaching the one i have . i am also not good at amp repairing so any help without being deeply into technical terms will be more clear. thanks in advance.
Akgun

[M Fowler]

But all voltages are good compared to your schematic, time to change power tubes.

pin#7: 390V
pin#2: -11.2V
pin#3: 13mV
pin#4, pin#5: 3.5VAC
pin#9: 330V

[akgun]

here is my tube swapping adventure;
i have changed the place of the redplating tube ( it was ch.A) and the same tube redplated again in its new socket.
i have changed the redplating tube with a new JJ EL84 from my “never touched” set. The new tube redplated in the same socket.
i have changed all the set with the new JJ EL84 set. All of the tubes were redplating.
i have changed an old set that i thought werent redplating. ch.B tubes redplated, ch.A had a slight redplating.
i also used the old tubes of Mesa for ch.B. again one of them redplated.
i did the measurements using the "new" tube set that redplated all.

[thejaf]

Did you swap the PI tube?

[Leo_Gnardo]

Why am I not surprised?

I'd alter the bias supply so you can dial it up to @ -14V.

And/or

Select output tubes with a low Pc (emission) figure. Typically I get JJ's from MagicParts (Ruby) with Pc from 16 to 22. You could ask Antique/CE or Mojo, I'm sure any of them could provide a low Pc set. Russian - Sovtek EH - also work well here.

[akgun]

yes, i swapped the PI's with mullard cv4024's. i think they made it worse. is it normal that when there is no PI inserted in the circuit, the redplating stops?

"I'd alter the bias supply so you can dial it up to @ -14V. " i have no idea how to do this?

[martin manning]

yes, i swapped the PI's with mullard cv4024's. i think they made it worse. is it normal that when there is no PI inserted in the circuit, the redplating stops?

Yes. The PI is sourcing current through the 120k resistor that is common to the output tube bias supply. When you pull the PI tube that few mA of current draw stops and the power tube bias voltage becomes more negative, which lowers the plate dissipation.

"I'd alter the bias supply so you can dial it up to @ -14V. " i have no idea how to do this?

You could try to dial the power tube bias down by decreasing the value of the 33k resistor at bias supply node E, but I don't know if there is enough room to work. You could clip a pot across it and give it a try. Otherwise you'd have to lower the two 330k's on the other side of node E.

That is a weird circuit. Anybody know what advantage it is supposed to have over a more conventional arrangement?

[Colossal]

that is a weird circuit. Anybody know what advantage it is supposed to have over a more conventional arrangement?

This is the so-called and Randall Smith patented Dynawatt circuit.

[martin manning]

Somehow I knew there would be a patent involved...

[Richie]

agree with what leo said, should be more like -14 or -15 volts. Also better use some mesa/branded tested tubes,which are more like 7189s or russian military type tubes and can handle the higher voltages. Just me, I think that's goofy to run them at 390plate volts and 330 screens. reg EL84 are not made for that.

[akgun]

You could try to dial the power tube bias down by decreasing the value of the 33k resistor at bias supply node E, but I don't know if there is enough room to work. You could clip a pot across it and give it a try. Otherwise you'd have to lower the two 330k's on the other side of node E.

those 330k's are actually 220K's on the amp. i found that there are owners making the amp adjustable bias by using trimmers and 15k resistors in series in place of those 33k's. i may try this but i actually want to find if there is a major problem causing the redplating.

anyway, i found that disconnecting the 2M2 resistors between pin#2 and ground stopped redplating. any comments on this?

[akgun]

Why am I not surprised?

I'd alter the bias supply so you can dial it up to @ -14V.

ok, it is getting clear now. i need to have a trimmer in place of the 33K's so that i will try to dial pin#2 voltage to -14V.

[martin manning]

You need to keep the 2M2, it's the only real ground reference the bias supply has.

[akgun]

do you think a 15K resistor and a 47K trimmer is suitable for this job? i will use a 15K to keep things safe in case that the trimmer fails.

[martin manning]

I don't think you can really protect yourself from a pot failure. I think I would just replace the 33k with the 47k trimmer wired as a variable resistor, i.e. with the wiper shorted to one end of the element.