i need help! mesaboogie 2020 poweramp redplating

[akgun]

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.

ok, this means less work for me. i dont know much about the technical side of tube amps but what if i accidentally turn the trimmer to one end where the value is zero ohms?

[martin manning]

Zero resistance would the most conservative (most negative) setting.

[akgun]

installed 47K trimmers, dialed them around 14K and the redplating stopped. the readings didnt alter much, they are still above the specs for EL84's but no visible redplating.
next question is, when i swap the JJ 12ax7 PI's with Mullard 4024 tubes, V5 and V6 start to redplate again. voltage readings for Mullard and JJ's are very different. also mullards made the EL84 pin#2 voltage increase to -8V. is there such a strong relation between the PI tube specs and redplating?

[thejaf]

Maybe I'm looking at it too simply, I keep thinking that if it worked for 13 years and now all of the sudden it's redplating every power tube you try, then something else is going on other than bias voltage (which you measured to be within specs).

For the cost of a postage stamp you can replace the 220K grid stoppers and see if that does anything.

[martin manning]

Given the age, it would be a good idea to replace the electrolytic caps in the bias supply.

[akgun]

i will replace the caps but it is hard for me to find them here in Turkey. i need to order them via internet. do i also need to change the big filter caps?

[martin manning]

CV4024 is a 12AT7 type, and its characteristics are different. In this circuit, the 12AX7 PI tube current affects the power tube bias point, so you need to keep a 12AX7 there. I think the trimmer is working for you, so I would use that to set a bias point with a 12AX7 installed in the PI socket.

Regarding the electrolytic caps, 13 years is old, but not too old. I suggested changing the bias supply caps because of the red plating you are now experiencing (which you had not seen in the past). If those caps are deteriorated that could be a cause. I think I would replace both the bias supply caps and power supply filters. Being a rack-mount unit, it may have run in a hot environment (?), which would shorten the life of the electrolytic caps.

[akgun]

another problem started before solving the previous one. i started hearing a "pop" sound from channel B. i swapped the tubes of that channel with channel A. the "pop" sound started at channelA. i have only old tubes available so i swapped the tubes with an old set but one tube made an arc inside if i didnt see it wrong. there was also a nasty sound coming from the amp. i shut the amp and changed the tubes back again. this time i noticed that both volume pots are disabled. even they are both set at zero i can hear the guitar sound. the sound level doesnt change if i turn the volume up to 10. any idea about this situation?

[Structo]

All bets are off until you can get working tubes (new) in the amp.

[akgun]

the volume pot problem is solved. it was a bad solder joint on the ground leg of the pot. buying tubes, caps etc are time consuming in Turkey so i will wait for a couple of weeks until they arrive. thanks for the support. i hope to have your support back again when the parts arrive.