short bursts of hi freq squeal?

[mike_mccue]

Hi,

I just bought a used 1974 Vibrolux Reverb 2 weeks ago and it is pretty clean in physical appearance. It came with an assortment of made in USA tubes but they aren't factory original. It does have original speakers and transformers.

I haven't pulled the chassis but rather have been playing it and letting it stay powered up for a few hours each day.

When I first got it it was a bit noisy... a crackling noise would coma and go.... playing it seems to have helped as now it will remain dead quiet for several hours at a time.

I assume this indicates that the old electrolytic caps are happier being powered up and used rather than sitting unused.

One thing that I am noticing is an occasional short burst of a high frequency squeal... maybe 2 or 3 seconds long and then it subsides.


I look forward to going through the whole amp and I'm willing to do a cap job on the power supply and perform maintenance where ever necessary.

I haven't pulled any tubes to try to isolate a stage.

I thought I'd kick start the work by asking if the symptom of the squeal might suggest a particluar place to look at first.

2 of the tubes that are spec'd as 7025 are made in USA 12AX7A... when I tap them I don't hear any bad micro-phonics... but I thought I should mention that they are not 7025s.


Anyways, you all have helped me a bunch in the past so I thought I'd check in before I start with this project.


Thanks.

best regards,
mike

[52 Bill]

My first reaction, is to clean all contact points and pots. All jacks, all tube sockets, etc.

[pdf64]

'One thing that I am noticing is an occasional short burst of a high frequency squeal'
May be arcing in a plate resistor.
If you want to ensure lowest noise and best chance of trouble-free service, then replace every resistor and electrolytic cap. Regard them as consumables, as carbon comps drift (up) in value and / or get noisy, and electrolytics develop increasing levels of ESR and dc leakage.
Don't worry about any distinction between 7025 / 12AX7. As long as they have good gain, low noise, hum and microphonics then they're fine. Pete.

[pdf64]

'My first reaction, is to clean all contact points and pots. All jacks, all tube sockets, etc.'
Good stuff, but ensure that a non lubricating cleaner is used for tube sockets.
Also retension the tube socket contacts, see Billm's page on this -
Billm Audio » Re-tension, resolder your tube sockets
Pete.

[mike_mccue]

Thanks!