Re: Selenium rectifier question

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. I had replaced all the signal caps with Auricaps, following
the black lead in/red lead out scheme recommended by the manufacturer,
but C20 (I think) is a power supply decoupling cap and not just a
signal coupling cap. The manufacturer recommends that in this
application, the red should be the input and the black to ground. It
was reversed, I put it in the right direction, but it didn't stop
motorboating in the speaker. Auricaps aren't supposed to have
polarity, and originally the old caps that had been replaced by
another person were nonpolarized poly's and everything seemed to work
fine. Guess that's what you get when you try to "upgrade"...

This is what's happened so far - the replacement of the seleniums (how
this all got started in the first place) really wasn't a problem; the
filament voltages which is all they affect are fine. When the PS/AMP
were originall fired after replacing all signal caps with Auricaps and
the large electrolytics with new units either separately or ganaged
per a multi-section unit being replaced, the fuse in the PS blew after
starting the amp, waiting for the relay/cap charge delay, and the b+
circuit engaging per the relay. Found that C21 was conducting,
replaced it, and everything worked fine with no signal. Applied a
1khz signal to 4-5-6 on the feedback input terminal strip of the amp
(goes directly to the power amp), something crackled briefly and the
amp started "motorboating". When started now, the output is clear
other than a bit of presence once the heaters heat up, but when the b+
kicks in the noise starts. I'm suspecting it might be the 80/450 caps
and possibly the 100K resistors in the power supply that might have
been damaged by some sort of issue with the b+ during the fuse blowing
fiasco. I'm about to measure the frequency, but I think it's 120 hz
and not 60 which as stated might indicate the power supply.
.

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