Help with noise problem after changing tubes

jzucker

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Just got a 50w holland lil jimi. It sounds great. It came with some pretty bad EH tubes so I retubed it with 2 Tungsol reissue AX7, an RCA AT7 driving the reverb, a jan/phillips AT7 in the phase inverter and a pair of Tungsol reissue 6L6 tubes. I left the GE GZ34. Biased it at 28ma.

After 2-3 minute warmup on standby, i get the following noise for about 20-25 seconds after flipping off standby. Obviously, I can swap the tubes out one by one but any thoughts on this noise? It wasn't making this noise with the original tubes. Once warmed up, the amp sounds better and much nicer than the original tubes.

 

jzucker

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Do any of the controls affect the noise?
I haven't tried every knob but turning the reverb knobs (it's got dwell and mix) and the presence knob to zero does not effect it. It quiets down pretty quickly so i haven't had a chance to do much interactive testing with the other knobs
 

HotBluePlates

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... 2 Tungsol reissue AX7 ... a jan/phillips AT7 in the phase inverter ...
Do any of the controls affect the noise?
Assuming this is a Marshall/Bassman type circuit, would the noise be arcing at the Tung Sol-ski cathode follower?

To check this, turn the amp off, turn the Volume control all the way off, turn the amp on. If you still get the noise on startup, it's after the Volume control and possibly at a cathode follower stage.

You might even test the notion afterward by swapping in one of the old 12AX7s for the 12AX7 closest to your 12AT7 phase inverter tube.
 

jzucker

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Assuming this is a Marshall/Bassman type circuit, would the noise be arcing at the Tung Sol-ski cathode follower?

To check this, turn the amp off, turn the Volume control all the way off, turn the amp on. If you still get the noise on startup, it's after the Volume control and possibly at a cathode follower stage.

You might even test the notion afterward by swapping in one of the old 12AX7s for the 12AX7 closest to your 12AT7 phase inverter tube.
i was under the impression that it was more like an AB763 circuit. I don't have a schem but someone did a layout diagram.
 

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jzucker

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Sounds like a bad preamp tube did you recently buy those nos preamp tubes?
no, i bought them about a year ago, used them for a couple months and put them away. It's possible it was bad from the beginning. But it's interesting that after 60 seconds it the whole thing settles in.
 

HotBluePlates

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... Thoughts?
Are any of the tube sockets loosely holding the tubes?

If yes, it could be loose contact between pin & socket-contact. If no, the noise sounds like arcing in one of the tubes.

Since you had no problems before putting the new tubes in the amp, odds are the problem is a defective new tube.
 

Amp_Addicted

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Did you clean the sockets? I am not a tech, but the other day I noticed a drop in volume that seemed suspicious on a Deluxe Reverb. I started pushing on the all of the preamp tubes, and noticed the phase inverter, reverb recovery tube & V2 12AX7(1st gain stage of the reverb channel) were all noisy when I pushed them in the socket. I swapped the normal & reverb channel 12AX7 tubes to verify if they were working. Both sounded fine in the opposite sockets, so I pulled out my contact cleaner & sprayed all of the preamp sockets. The amp now sounds great again. Tubes generate heat. This heat also seems to attract dust particles that get lodged into tube sockets. This can often cause poor electrical contact. A tech friend pointed out that many amp repairs come down to a dirty socket: did you clean the sockets before swapping out tubes?
 

jzucker

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doesn't seem a likely candidate for a socket problem. The amp's only 15-16 years old. Not like a 50 year old deluxe reverb. Regarding a loose contact, if that were the case, wouldn't it be making noise continuously and not just for the first 20 seconds after coming off standby? And what about the fact that the noise is not there if I just flip both switches on allowing the rectifier tube to be the gentle standby?
 

SnidelyWhiplash

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I'd still clean the tube sockets with contact cleaner with no lube. Couldn't hurt. 15/16 yrs would still make it a candidate for a little routine maintenance. :cool:
 

jzucker

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I'd still clean the tube sockets with contact cleaner with no lube. Couldn't hurt. 15/16 yrs would still make it a candidate for a little routine maintenance. :cool:
good idea agreed - but doesn't explain why bypassing the standby causes the problem to go away. I'm looking for more cause and effect!
 

HotBluePlates

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doesn't seem a likely candidate for a socket problem. The amp's only 15-16 years old. ... Regarding a loose contact, if that were the case, wouldn't it be making noise continuously and not just for the first 20 seconds after coming off standby? ...
I asked about loose sockets because who knows how long a given socket might take to develop sprung contacts. But loose contacts (jacks, tube sockets, cable-breaks, or poor solder joints) make this same type of noise. The troubleshooting challenge is systematically eliminating sections of the circuit that don't have a problem.

... doesn't explain why bypassing the standby causes the problem to go away. I'm looking for more cause and effect!
Let's assume the Standby switch is the key bit of info. One problem all Fender-style standby switches suffer (eventually) is bounce/arcing of the switch contacts. The problem with this theory is the noise shouldn't have started solely because of new tubes (unless the rectifier tube is arcing and not the switch. See below.

 




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