Re: Selenium rectifier question

EricM wrote:
On Apr 24, 7:10 pm, "Arfa Daily" <arfa.da...@xxxxxxxxxxxx> wrote:
"EricM" <ew_m...@xxxxxxxxxxx> wrote in message

news:02dfaaf7-fc50-429b-b28b-a9990a805b56@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On Apr 22, 11:30 am, Heinz Schmitz <HeinzSchm...@xxxxxxx> wrote:
EricM wrote:
Forgot to mention that the 'buzzing' only starts after the relay
closes -
Well of course it does, because before that the amp isn't working at
all. A tube amp without anode voltage is just a refined heater.
Do not forget that this is a tube device. Some of the tube cathodes
are heated with 6.3 Volts alternating current of 60 Hz. If there is a
cathode-heater-leak (due to tube age) you get AC into the signal path.
Note that the preamp stages are operated with 400 Volts off the
stabilizer circuit around V9 - V10. So have a look at C20 / C2 - they
should neutralize AC ripple there.
I'm scratching my head about connectors 3 and 4 on TS2 named
regulator filament. Somehow TS2 in the ps-schematic doesn't seem
to be TS2 in the amp schematic.
Regards,
H.
It's a separate 6.3VAC supply for just the 6DR7 tube which is in the
same circuit at the 0A2 voltage regulator, attached to 13 & 14 on the
TS in the amp. I've isolated the problem to the power amp circuit -
preamp section tests fine - and most likely something around the
voltage divider/6DR7/0A2 portion of the circuit. When the biasing pot
(25K unit in that circuit) is adjusted, the pitch of the buzz varies,
which leads me to believe that the problem is somewhere in that
circuit.
<snip>

Altering the bias changes the standing current in the output tubes. If this
changes the *level* of the buzz, then that indicates a power supply problem,
most probably - but not necessarily, given the on-going work that's been
done around the rectifier circuitry - to do with the filter caps. If
altering the bias pot actually does affect *pitch* - ie frequency - of the
buzz, then you've got a problem, as this would indicate that something is
oscillating at low frequency ...

Arfa

It changes the frequency not the amplitude. Changed out *all* caps in
the power supply, replaced the sarkes tarzian units with 5R4 tubes,
double checked the replacement silicon diodes - they're fine, all
power outputs are spec at the amp under load. It's gotta be something
in or around the 6DR7/0A2 part of the circuit. Used a 2.0uf Auricap
for C20 but C21 is an electrolytic. Maybe replacing C20 with a
2.0@450 would do the trick... Checked all resistors in and around
the power amp circuit and all test good and within tolerance.
Low frequency oscillation, once called motorboating,
might be caused by feedback along the supply path.
Try a fat capacitor added to the supply to see whether
that occures.
Also check other de-coupling caps, shunt them with a bigger one
to test.
.

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