Re: Preamp causes hum even when off

On Feb 29, 3:17 pm, Peter Wieck <p...@xxxxxxx> wrote:
On Feb 29, 2:50 pm, t...@xxxxxxxxxxx wrote:

(Please try to ignore my sometimes-embarrassing contributions to that
thread.)

Tom:

Forgiven... of course.

Tell me, is that a horked PAT-4 (Dynaco) case?

A couple of questions, although I am leaning with Phatty so far:

a) Hum is same with top on as top off?
b) Does the hum follow the volume or is it separate and always at the
same level?
c) If you short the inputs to the pre-amp, does the amp still hum?
d) Is there AC or DC on the tube filaments (heaters)?

One thing you might try: get a small screwdriver with a good plastic
handle (well-insulated). While the pre-amp is connected and humming,
but *NOT* plugged in, try touching the various shields and signal
solder points with the tip of the screwdriver. If one or more points
increases or decreases the hum, please let us know - as that will be
your point-of-departure for a search for the cause.

Please let us know.

Peter Wieck
Wyncote, PA

Thanks, Peter!

I hope I mentioned that it's not my preamp, and I am just a go-
between, trying to help out, while the main discussion continues on
the linked-to diyaudio.com thread..

Answers to your questions:

I am not sure of the origin of the case. Sorry.

a) The top on/off issue was recently brought up. I think that the
answer was that it was the same, either way.

b) I am not sure if the volume control affects the hum level. I think
I read that it does not.

c) With inputs shorted, hum is unchanged.

d) Tube filaments have SS-rectified and smoothed DC, except for the
12X4 rectifier tube for the B+, which has AC on filament, from the
same source as the discrete SS diode bridge's AC.

Thanks for the tip about the screwdriver. If you don't object, I
would like to post that in the diyaudio thread (including proper
credit to you, of course).

Thanks again, Peter.

- Tom Gootee
.

Relevant Pages

  • Re: Preamp causes hum even when off
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  • Re: Hum has me stumped
    ... The real story behind the most common source of hum in unbalanced audio ... systems (common impedance coupling and inter-chassis current) and how to ... The "ground loop" explanation may lead one to eliminate the hum, ... Turn off and unplug pre-amp = hum ...
    (rec.audio.tech)
  • Re: Hum has me stumped
    ... The real story behind the most common source of hum in unbalanced audio ... systems (common impedance coupling and inter-chassis current) and how to ... The "ground loop" explanation may lead one to eliminate the hum, ... Turn off and unplug pre-amp = hum ...
    (rec.audio.tech)