Static sound coming from my PR40

[carioca100]

I have the PR40 hooked up to my C3 and I noticed a few months ago some intermittent static coming out of it. The description would be similar to driving down the road listening to an AM radio station and in the distance is a thunderstorm. You get the occasional pop and static in the speakers as the storm rumbles on and approaches This static/pop has now gotten so bad it is almost constant. The PR 40 was rebuilt about 3 years ago with all new caps and so forth in the inside chassis. Speakers are in very good shape as well as all of the wiring. I removed and lightly sanded with #2000 sand paper the tubes metal male prongs that go into the sockets. I then lightly sprayed deoxit on the prongs prior to re seating in the sockets. this did not help at all. i'm wondering if I have some bad vacuum tubes or something else?
i use the PR40 for some bass and have a Leslie 122 hooked up to it also. The 122 is fine.


thanks

[David Anderson]

It is possible for tubes to develop internal noise. Since this is a 3-channel amp with 14 tubes, you might try to figure out which channel the noise is coming from or if switching reverb on/off has any effect. Did you replace the coax cable from the necklace reverb to the amp during the rebuild?

Internal tube noise can be from a number of different things: heater/cathode leakage, bad welds, or other inter-element shorts.

[Jyvoipabo]

i'm wondering if I have some bad vacuum tubes or something else?

Hello,

You wonder very good.
I got the same problem and i found a preamp tube with Gas TEST = KO on my tube tester.

If you have no tube tester, try to identify the noisy channel by removing some tubes.

Remove V9 12BH7 disable all the reverbs.
Without V9 remove the first tube of each channel one by one.
Removing V1 12AU7 disable Treble.
V10 12AX7 disable Treble Reverb.
V4 12AX7 disable Bass Reverb.
V5 and V6 disable Bass.

If the thunderstorm is still there remove The 6BQ5 by pairs.

JP

[David Anderson]

You can also gently tap on the tubes with the amp on. This will sometimes make a misbehaving tube make even more noise and indicate which one it is. Check the 6AU6s, particularly. They have a habit of going noisy. I'd look at 12AX7s next.

But then, I've seen a number of PR-40s where the output tubes were worn down to the point of failure. The Amperex originals do last a long time, but not forever.

[carioca100]

Thanks guys.

David, I did not replace the coax cable from the necklace reverb to the amp. I did un plug that cable and notice a reduced static/pop but not eliminate it. I'll have to replace that cable. is that difficult to find?

i will also check the tubes as you two suggest and report back on my findings.

thanks very much.

[geoelectro]

I usually get a patch cord with the RCA plug on both ends, cut one end off and use that. Also, there is a transistor in the reverb recovery circuit with a couple of suspect 1oo/3 caps. (100uf/3V)
You can turn the reverb knobs to off and remove the reverb circuit from the audio. That may help isolate the source of the noise.

Geo

[David Anderson]

Good point mentioning the reverb recovery transistor. 1960s transistors can certainly go noisy. That's why I mentioned switching reverb on/off above.

You could also simply short out the speakers to see which speaker(s) the noise is coming from. Don't do this with a solid-state amp, but with tube amps, it's fine.

[carioca100]

I turned the reverb knobs to off and that helped out a lot, reduced the pops/static by 75% but still have some. I haven't had time to delve into the tubes but since the reverb is off and indicates a problem there what should I do next with that?


thanks

[David Anderson]

I would first replace the reverb coaxial cable. If that didn't help, I would rebuild the entire reverb recovery circuit, the whole thing, all resistors, capacitors, and the transistor, using modern metal film resistors.