Echo North
Well-known member
Hi all,
So I'm looking at a guitar amp for someone. It's an old Peavy "The Classic".
The schematic (possibly a different revision) is located here:
http://schematicheaven.com/newamps/peavey_classic_b.pdf
This amp has 2 6L6GC power amp tubes...the pre is solid state. I believe it is suffering from over heating.
Here is the breakdown of things I can confirm:
Issue:
The amp works fine for about 1 hour the fizzles and farts out. The amp needs to rest for a while before it works again. (this is why I am guessing overheating)
Observations:
1. The tubes do not seem to be glowing excessively, although they are quite hot. The problem with these observations is that my experience with tubes is minimal.
2. In both tubes there are two metal circles that come out of the middle...the glass near these metal circles (heatsinks, spacers?) is all black. Both tubes have two big solid black spots on the glass.
3. One of the tubes rattles a little...all the inner connections look fine...there are a couple of small pieces of what looks to be glass rattling around in there.
Measurements (using Fluke DMM):
1. Looking at the schemetic, it indicates a voltage of -55V to the wiper of the bias. My reading is -53.5V. Seems reasonable?
2. The side of the bias trimmer where "bias" is written indicates a reading of -77V. Here is where the amp differs from the schematic. There is a 1k resistor in series before the cap/diode/power transformer. The voltage at the trimmer is -76V. Seems ok?
3. The other trimmer lead indicates a voltage of -25V. However on my unit that lead connects directly to ground (no 22k resistor). Soooo.... I get a reading of 0V (measuring ground against ground)
Any advice? Tips? Clues? Or anything you would like me to try.
Also, a question about safety. I'm being very careful since I understand tube voltage/current is excessive. I understand a lot of injuries occur from getting zapped by caps still holding high voltage after the amp has been powered down. What is a safe amount of time to wait before I can know caps have discharged?
Thanks again!
Mike
So I'm looking at a guitar amp for someone. It's an old Peavy "The Classic".
The schematic (possibly a different revision) is located here:
http://schematicheaven.com/newamps/peavey_classic_b.pdf
This amp has 2 6L6GC power amp tubes...the pre is solid state. I believe it is suffering from over heating.
Here is the breakdown of things I can confirm:
Issue:
The amp works fine for about 1 hour the fizzles and farts out. The amp needs to rest for a while before it works again. (this is why I am guessing overheating)
Observations:
1. The tubes do not seem to be glowing excessively, although they are quite hot. The problem with these observations is that my experience with tubes is minimal.
2. In both tubes there are two metal circles that come out of the middle...the glass near these metal circles (heatsinks, spacers?) is all black. Both tubes have two big solid black spots on the glass.
3. One of the tubes rattles a little...all the inner connections look fine...there are a couple of small pieces of what looks to be glass rattling around in there.
Measurements (using Fluke DMM):
1. Looking at the schemetic, it indicates a voltage of -55V to the wiper of the bias. My reading is -53.5V. Seems reasonable?
2. The side of the bias trimmer where "bias" is written indicates a reading of -77V. Here is where the amp differs from the schematic. There is a 1k resistor in series before the cap/diode/power transformer. The voltage at the trimmer is -76V. Seems ok?
3. The other trimmer lead indicates a voltage of -25V. However on my unit that lead connects directly to ground (no 22k resistor). Soooo.... I get a reading of 0V (measuring ground against ground)
Any advice? Tips? Clues? Or anything you would like me to try.
Also, a question about safety. I'm being very careful since I understand tube voltage/current is excessive. I understand a lot of injuries occur from getting zapped by caps still holding high voltage after the amp has been powered down. What is a safe amount of time to wait before I can know caps have discharged?
Thanks again!
Mike