Mesa Boogie 50 Caliber + red plating

[rkohansk]

My 50 cal amp red plated Saturday. Found a set of schematics online and began to check the voltages on the amp and noticed that pin 5 of one of the power tube did not have power on it like the other did. I traced it back to the .047 cap. Voltage on one side and not the other. so if you look at the schematics off of the 6l6gc pin 5
after the 220k and where the .047 and the 150k all meet there is now voltage. Neighbor had a new cap same value so I put it in and have the same problem.
???

[Jazz P Bass]

Mesa Boogie 50 Cal

Pin 5 of a 6L6 is your #1 grid.
You should have -46 Vdc on the pin.
Pull all the output tubes .
Check each 6L6 pin 5 socket.
The -Vdc bias comes from point D on the schematic.
So either the 150 K or the 220K resistor is open.
Turn off the amp, drain your B+ with a 1K 2 watt resistor to ground.
Now measure the 150K & 220K.
One of them is open circuit.
By the way the .047 capacitor Blocks Vdc from the previous plate.
Bad call.
Schematic link:http://www.freeinfosociety.com/elect...iew.php?id=503

[chipprogr]

that tells me that the 220k resistor is bad, or tube is shorted. Id swap the output tubes and see if your redplate issue stays with a specific power tube, or stays with the socket. If its the same tube that red plated before, your power tube has shorted internally and they both need replaced. If the redplate issue stays with the same socket, then its not the cap- its the 220k resistor- you DID say there was voltage on the cap side of that resistor.

[clydey]

ditto with jazz p bass
but if the resistors are ok check for continuity on the d to the 150k /other side 150k to the 220k/ other side of 220k to pin 5 grid

[Jazz P Bass]

Continuity check

Thanks for the clarification on the continuity check.
I had assumed that would be done.
It is always best to spell it out.

[clydey]

yes still waiting on his good news
but some times its hard to see the wood for the trees

[rkohansk]

Problem solved.

The cap that read the same voltage on both side was the issue.
It was allowing 260 Vdc to go to pin 5 of the power tube.
I replace both .047 cap and the voltage on pin 5 of both power tube
was -37 Vdc.
Amp is work fine.

[chipprogr]

The cap that read the same voltage on both side was the issue.

Good, glad your fixed, guess you changed out the "good" capacitor the 1st time. We didnt have any data from you on the other one or we could have saved you some time. Take care-Eric

[Jazz P Bass]

Mesa Boogie 50 Cal

Way to go!
That was some good trouble shooting on your part.
Glad that you got it.