No plate voltage on 12ax7
Moderators: pompeiisneaks, Colossal
No plate voltage on 12ax7
Did a quick check on a Peavey 6505+ last night to see why it sounded so bad. Found v4 tube with 359 volts at the plate resistor and zero voltage to ground from the plate side of the resistor. Looks like someone removed the blocking cap and resisitor from the pcb and soldered them together off of the board, also. Trying to figure out whats happening so i can get home at 2 am and get it right. On the Peavey schematic, it's r42 with r101 and c11 that have been removed from the pcb and soldered point to point. Whatever it is makes the clean channel sound like crap also. I have changed tubes 3 times, so i'm guessing its a short or open somewhere on the pcb, but that just doesn't sem likely since I didn't notice any burn or damaged areas. Any ides? Thanks.
Re: No plate voltage on 12ax7
Sounds like R42 is open. Can you lift the off board items and check the resistance across R42.
Check pin 3 (cathode for the R42 triode) for voltage. It should read 0 with no voltage at pin 1 (plate).
Good luck!
Dave O.
Check pin 3 (cathode for the R42 triode) for voltage. It should read 0 with no voltage at pin 1 (plate).
Good luck!
Dave O.
6505
My mistake, there was flux or some coating on the rsistor lead making the meter read 0 voltage. I can't find anything else wrong, other than a burned spot on the pcb near the DC heater diodes. I am reading 7.2 volts dc on the preamp tubes also, but that shouldn't be causing the amp to sound so compressed and harsh. I have never heard a 6505 but I,m sure Peavey didn't put out anything that sounds this bad. Gotta be something wrong here. I did pull a 6l6 out of each side and got the same sounding amp with either or both tubes in. both read 465 volts and 51 volts bias, but pulling one or the other doesn't seem to change the sound much.
Re: No plate voltage on 12ax7
If that is -51 VDC on grid 5, it is over biased.
I would think that at 460'ish volts on the 6L6 plates, you should be in the -40 to -35 VDC range.
Can you poke around in the bias circuit area? Knowing the voltages on both sides of R93 and R 108 could tell us a lot. Hopefully I am looking at the right schemo. Those are the first and last in-line resistors in the bias voltage "control" path. I suspect that one of the bleed resistors is open.
Cheers,
Dave O.
I would think that at 460'ish volts on the 6L6 plates, you should be in the -40 to -35 VDC range.
Can you poke around in the bias circuit area? Knowing the voltages on both sides of R93 and R 108 could tell us a lot. Hopefully I am looking at the right schemo. Those are the first and last in-line resistors in the bias voltage "control" path. I suspect that one of the bleed resistors is open.
Cheers,
Dave O.