Diagnose this:

[Phil Allison]

**Hi,

serviced a Fender "Super 210" yesterday. All tube amp, 2 x 6L6s plus 2x10 inch speakers.

Fixed a few issues like open plate resistors in the PI and gave it a good clean up after being out of use for some time. Power was good, all controls working, no hum or excessive noise.

Slid chassis back into the cabinet and let sit for a while on "soak test".
Went into another room and soon could hear a steady hum.
Was it coming from the fridge, something outside or what ?

Just to clear the amp from suspicion, I toggled the standby switch. Hum stopped. Flick back on, hum gone but soon re-appears. Bad caps ? I'm doubting it.

Put my Keysight DMM probes onto the speaker wires and it read 58.55Hz - steady as a rock.

If I moved the amp to a new spot on the floor, hum stopped but soon restarted. Always at 58.55Hz.

Who knows what this is?


...... Phil

[Phil Allison]

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

>
> Who knows what this is?
>

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

All were set to zero at the time.

..... Phil

[John Robertson]

> ...... 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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[Phil Allison]

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

> >>
> >> Who knows what this is?
> >>
> >
> > ** Forgot to mention, control settings had no effect.
> >
> > All were set to zero at the time.
> >
>

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

** Not tried.

> 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?

** Yep - hum restarted about a minute later.

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

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

..... Phil

[John Robertson]

> ...... 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...

[Phil Allison]

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

>
> 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.
>
>

** 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

[John Robertson]

> ...... 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.

[guijarrosoy]

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 was present?
3. What was the voltage at SPEAKER LEADS or output connector when the standby 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 noises , for expected Signal to noise ratio?

Please let us know. Thanks and regards.

Al.

[Phil Allison]

guijarrosoy wrote:

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

> Hi there...

> 1. Did the 58.55 Hz hum disappeared after swaping tubes?

** Absolutely.

> 2. What was the voltage at SPEAKER LEADS or output connector when the hum was present?

** 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

[Tim R]

On Sunday, November 22, 2020 at 6:42:30 PM UTC-5, palli...@gmail.com wrote:

> guijarrosoy wrote:
>
> Or a guitar amp, where most tubes end up that way over time.
>
>
>
> ..... Phil

That's considered a feature, not a bug.