Re: B+ rectifier question

Hi Peter,
I can leave the preamp on for hours on end and the PT only gets slightly
warm. You can hold your hand over it and it's not uncomfortable at all.
I'll do what you suggest and give you an update. I used an volt meter and
measured voltage from the chassis to ground. It's pretty low (around 8
volts) and just a little difference when changing the position of the plug
in the outlet. The preamp had 6 Telefunken 12AX7's and I replaced them with
Sovtek 12AX7LPS. Do you think the Sovtek's require more filament current
than the Telefunken? I guess now it a matter of getting the voltages lower
so I can safely use it without a variac.

Thank to all,
Edward Morris

"Peter Wieck" <pfjw@xxxxxxx> wrote in message
news:8e503221-8a35-464d-840e-c101ec00c309@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Ed:

Please note the interpolations:

On Dec 15, 2:26 pm, "Edward Morris" <emor...@xxxxxxxxxx> wrote:
Hey guys,
I appreciate all the input. I replaced all the bumble bee capacitors.
I replaced all the electrolytic capacitors (cans if possible). I couldn't
find the can for the filament circuit so I had to go with 3 separate
capacitors for that circuit. I went with keeping the capacitor voltages
the
same as the originals or at least close. I replaced the resistors in the
power supply as well as the filament bridge which was originally 4
separate
diodes mounted on a strip and I replaced the original 2 diodes for the B+.
I have to run the preamp on a variac at 110v because the filament voltage
tested at each tube is right at 12 volts.

Actually, the filaments for a 12AX7 are 12.6V nominal. Running at 12V
will help tube life somewhat, and not materially affect peformance.

My goal is to get the resistance
added necessary to be able to plug the preamp into the wall outlet about
123V but I also don't want to add too much more heat to the innards. The
filament circuit pulls a lot of current. I've got a 6 amp bridge for the
filament circuit and it runs warm. The resistors (2-7.5 ohm 5 watt) run so
hot you can even touch them. But they were that way before the new parts.

Too hot! Nothing should get that hot inside that amp. I know that in
many cases OEM equipment does have dropping resistors that do run
seriously hot - my preference would be to replace them with a higher
wattage resistors - but in this case, pay _special_ attention to
fusing. Heat and equipment are enemies. Heat and phenolic circuit
boards are enemies.

So, if you have put in silicon diodes and are running at 110V to get
12v at the filaments with 7.5 ohm resistors, I would *suggest* you go
with 20 ohms (additional 12.5 ohms) at 5-watts and see what you get at
124V. Again, you are working with single-element (originally) diodes,
so a massive amount of additional resistance over the OEM 7.5ohms will
not be needed.

The preamp at this point sounds great. I've used it running from the
variac
for a number of months now with no problems and the PT runs just a little
warm.

What is "just a little"? Egg-frying hot after a few hours/minutes/
ever?
Or warm-to-the-touch-but-no-more forever? T

The latter is what you want.

I have a number of resistors to be able to experiment. It's just
getting the thing unhooked and out of my cabinet. I know I sound lazy. I
guess I just don't like the idea of getting it out again. It was a lot of
work at time to (rebuild it). You guys know what I mean? I have the
schematic so I know the recommended voltages. I know just about enough
about electronics to be dangerous. If you go towww.nosvalves.com/C22.htm
you can see the 2 grey B+ diodes at the left of the pictures. I still have
them but they don't have stacks per se like I've seen some other selenium
rectifiers have. Do you recommend for the B+ using resistance before the
diodes or after?

Always _after_ the diodes.

Also the 2 HT wires Red and Red/Yellow; do I need to
install a resistor between the capacitor and the Red/Yellow wire; the
voltage is running a little high, or will reducing the B+ voltage take
care
of that?

It should. But when you finally walk away from the amp, the voltage on
the caps should be at/slightly below the listed voltage(s), and B+
should be at or slightly below the listed voltage(s). But you would
like to work to as few points as possble, not have new resistors
spread all over the place. Further, be sure you document what you have
done so you or your successors do not have to re-invent the wheel each
time you service the amp in the future.

Peter Wieck
Melrose Park, PA

.

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