no Twin no Reverb - noise - help please

[aquitania]

Hi everyone,

I built a twin reverb, with no tremolo and no reverb... (so it has no name now haha)

Small problem. It makes a funny sound after it's been on for a little (about 5-10 minutes), this has only started recently.

Has anyone heard this before?

https://www.dropbox.com/s/ltif8w69b096cq1/twin%20reverb%20problem.wav?dl=0

It's loudest when the treble is all the way up and completely silent when treble is all the way down.

Anyone had a similar problem and solution!

Thanks so much,

Dan

[eleventeen]

Sounds most to me like bad old plate resistors....the 100Ks that feed the preamp tube plates. If so...that's the cheapest thing you can do for the most improvement to a Fender (or other) tube amp.

[DummyLoad]

or a bad tube.


--pete

[aquitania]

so I was wrong about the treble know, there's still noise when it's all the way down (it's just quieter).

Sounds most to me like bad old plate resistors....the 100Ks that feed the preamp tube plates. If so...that's the cheapest thing you can do for the most improvement to a Fender (or other) tube amp.

hmmm, changing the plate resistors is an easy fix, but they're metal film and there isn't much reason for them to die, resistors are pretty hardy, they've only been in there for about a year. What's you're rational? I'm not questioning you of course, but if I knew why you say this it might help me!

or a bad tube.


--pete

I'm still getting signal through the amp though.. I guess a tube could go microphonic all of a sudden.. but I pulled the 6l6s in pairs, so unless 1 in each pair is bad I don't think they are the problem. I also replaced the 12AT7 phase inverter and 12AX7 preamp tubes with known good 12AX7s. I'm pretty confident the tubes are fine.

I'm thinking that the problem lies somewhere near the input of the amp because I can EQ the noise..

I might try replace the 100k anode resistor on the preamp tube like eleventeen suggested this afternoon.

Any other suggestions?

Thanks for the help so far!

Dan

[shooter]

try bypassing your tone stack, cap couple and use a fixed "vol" resistor grid to ground between the stages that were coupled by the tone stack

[eleventeen]

"hmmm, changing the plate resistors is an easy fix, but they're metal film"


Didn't know that; I assumed you might have used carbon comps or that they were old. The sound recorded in your sample is in my experience VERY much like that produced by old CCs. Plus...the effect occurring only after modest warmup suggested thermal noise. If they are indeed MF, that's probably not the culprit.