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Need Help Troubleshooting

Posted: Tue Nov 16, 2010 11:10 pm
by lgehrig4
I'm building my first amp (JTM45). I sourced the parts from a builder and the transformers are ******. I've been following the Metroamp build instructions and I just finished the amp today.

The voltage measurement were all pretty close to those in the instructions, but when I installed the power tubes and tried to bias, the V5 tube was much lower (about 15-20mA) than V4. I know that they tubes matched much close, but for the hell of it I tried two other pairs and got the same thing.

Up to that point I was biasing according to the instructions so I figured I try my Amp Head bias probe. Same thing. With these I could measure the plate voltage and V4's was .437vdc while V5's was .022vdc.

Like I said, this is my first build. What should I check first? :what:

Re: Need Help Troubleshooting

Posted: Wed Nov 17, 2010 6:39 pm
by Bierschinken
Hi,

check the solder connections and the value of the resistors in the biaspart/on the socket.
Do you measure the same Ua on all tubes?

Cheers,
- Swen

Re: Need Help Troubleshooting

Posted: Wed Nov 17, 2010 10:39 pm
by lgehrig4
The resistors are 2W 1ohm and I tested them. I think I found the problem, but I don't know what the cause could be. All of my voltages are pretty much spot on with the exception of pin 3 of V5. The readon is -5.65, whereas the reading on pin3 of V4 is 458.

Man, I hope it's not the PT :(

Any suggestions?

Re: Need Help Troubleshooting

Posted: Thu Nov 18, 2010 2:30 am
by robert
Hi,

check the primaries of the OT with your DMM for continuity.
Could it be possible that you have soldered the CT of the OT primary to Pin 3?

Regards

Robert