Diagnose this:

Phil Allison wrote: =================

** Forgot to mention, control settings had no effect.

All were set to zero at the time.

..... Phil

What happens when you pull the phase splitter - is it a 12AX7?

Also when you moved the amp was it turned on at the time? In other words did you pick up the working, humming amp, and the hum stopped until it settled in the new position?

Any fluorescent lights nearby? Small bar fridges? Not too likely to affect your amp, but trying to think outside the Gibson's box.

John :-#)#

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John Robertson wrote: ======================

** Not tried.

** Yep - hum restarted about a minute later.

** The supply frequency here in Sydney is 50Hz. The sound was pure and very low.

..... Phil

Well, it is apparently some sort of multi-vibrator going on. A feedback loop perhaps especially as it increases in volume after being moved shuts it down - so sensitive to motion or mild shocks. One of the tubes could be slightly microphonic and the feedback loop circuit works like a tuned resonator.

Wild guesses, but what happens if you tap any of the tubes or the output transformer?

Schematics might help.

Any chance a capacitor is installed incorrectly leading to this resonate feedback loop? Another wild guess...

John :-#)#

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MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 
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        "Old pinballers never die, they just flip out."

John Robertson wrote: ==================

** Yep - 58.55Hz was the resonant frequency of the structure inside the plate of one of the 6L6s. They were JJ brand types in otherwise good condition.

So, mechanical feedback from the baffle to the *output tubes* !! Not easily self starting, but would slowly rise in level to quite audible over about a minute.

With over 2000 tube amps serviced by me, only come across that problem once before. Trick is, it can only happen when the speaker system resonance coincides with the tube's internal structure resonance.

As you may know, the whole issue is due to use of *fragile* mica supports inside power tubes that fit against the glass walls with dozens of soft pin points.

FYI : Some brands like Sovtek 6L6wxt now use small steel springs for the same job.

Problem solved by the damn Ruskies !!! ===============================

..... Phil

So, what do I win? There must be a prize...

John ;-#)#

PS, the Ruskies can do some things just fine it seems, even improving designs from time to time. Some interesting electronic devices come from there.

El domingo, 22 de noviembre de 2020 a las 14:29:17 UTC-3, John Robertson es

k s a

plate of

sily self starting,

once before.

s with the tube's

ts inside power tubes

job.

========

Hi there...

1. Did the 58.55 Hz hum disappeared after swaping tubes?
2. What was the voltage at SPEAKER LEADS or output connector when the hum w as present?
3. What was the voltage at SPEAKER LEADS or output connector when the stand by switch cut the hum out?
4. Did you have all volume/gain controls counter clockwise (Zero level)?
5. Have you checked the amplifier specifications for residual hum and noise s , for expected Signal to noise ratio?

Please let us know. Thanks and regards.

Al.

-------------------

** Absolutely.

** About 1V rms.

The problem was exactly as I stated - must be you have never seen a microphonic output tube.

Or a guitar amp, where most tubes end up that way over time.

..... Phil

That's considered a feature, not a bug.