Hum Pot adjustments [resolved]

[ArtnShit]

Is this meter sensitive enough to use? Borrowed from work but can grab a different one if need be. I will also have the RCA adapter tomorrow, so I can run the tests.

Another thought - I am also using a newly completed but never tested Beepre2 that I built. Is it more likely the problem is occurring in the Kaiju? The BeePre passed every voltage test on the first go.

I am also attaching some photos of the D-socket.

Let me know what you think, and thank you for walking through all of this with me.

[ArtnShit]

Forum is being weird about me posting pics, not sure why!

[Paul Birkeland]

Nope, that still has a 200V minimum scale.

[ArtnShit]

Files too big maybe! 

[ArtnShit]

One more, sorry about all the individual posts for the pics! Will grab a more sensitive meter tomorrow.

[Paul Birkeland]

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.

[ArtnShit]

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.

[Paul Birkeland]

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.

[ArtnShit]

Once I get the meter, I will measure this! Should all tubes be in place for these tests? Is the tone playing in this test or no?

[Paul Birkeland]

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. 

[ArtnShit]

Hey,

Just did the test, am reading .191V of AC going from the center pic of DSocket and using the RCA grounding jacket as the other point.

[Paul Birkeland]

OK, now also check the voltage at the center pin of the RCA jack.  200mV is certainly enough to work with!

[Paul Birkeland]

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.

[Paul Birkeland]

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.

[ArtnShit]

Getting .241V of AC from center post of RCA to Grounding jacket. I need to head back into work, but will be running the next test once I get home around 630 EST!

Thanks so much for your help!