I don't see anything in there. A higher resolution meter and a 60Hz tone will be required to figure out which half of the amp to look at I think.
Hum Pot adjustments [resolved]
- Thread starter ArtnShit
- Start date
Ok sounds good! I am happy to start running a test after work tomorrow and I should have everything by then. Is the tone an audio file? When you are able to, let me know the first steps so I can report back once I have the opportunity.
Upon further inspection, it looks as if A5 and A4 were potentially touching, would that cause the problem? All of those terminals move around a bunch so not sure if it was leaning up against it.
Upon further inspection, it looks as if A5 and A4 were potentially touching, would that cause the problem? All of those terminals move around a bunch so not sure if it was leaning up against it.
If A5 and A4 touch, that will also throw off the DC voltages, but certainly I would do some trimming if they look close.
The first test you can do with the amp off, which is to measure the AC voltage between the center post of the 9 pin socket and ground (you can use the shell of the RCA jack) with the trim pot all the way up.
I typically use a free tone generator app on an old phone to get the 60Hz file. You can also use a laptop with a headphone jack and grab a tone generator webpage. Your DVM will read 60Hz AC, and typically I'd expect about 0.3V of signal to be available.
The first test you can do with the amp off, which is to measure the AC voltage between the center post of the 9 pin socket and ground (you can use the shell of the RCA jack) with the trim pot all the way up.
I typically use a free tone generator app on an old phone to get the 60Hz file. You can also use a laptop with a headphone jack and grab a tone generator webpage. Your DVM will read 60Hz AC, and typically I'd expect about 0.3V of signal to be available.
Yes, the meter will measure the 60 Hz tone as AC voltage.
For the first test, the tubes don't need to be in and the amp can be off.
For the first test, the tubes don't need to be in and the amp can be off.
OK, now also check the voltage at the center pin of the RCA jack. 200mV is certainly enough to work with!
Assuming you have about 200mV of signal at the center pin of the RCA jack, the next step is to fire up the amp with the tubes in it and check the AC voltage at pin 3 of the 300B tube socket. A normal working amp with the trim pot turned all the way up and 200mV of signal at the input is going to give something like 5-6V of AC signal at pin 3.
If the AC voltage appears at pin 3, you will then want to measure the AC voltage across terminals 5 and 10 of the output transformer (these have the red/white twisted pair of wires connected to them). If you have 5-6V at pin 3 of the 300B socket, you'd expect to see 3-4 times that across the output transformer. You have to move the black probe because the output transformer primary is sitting at about 70V DC, and your meter may not see the AC voltage through that.
The black probe can stay on the RCA ground until you measure the last AC voltage at the output transformer.
Now measure the AC voltage across the speaker jacks.
1.133V AC is audible signal coming out of the speaker jacks. This confirms that the amp is working. I would hook it back up and have a listen.
