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Wbittle
 
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Hi Chad,
You will need to install around a 5 ohm resistor right off the new
silicon bridge rectifier. This is to drop the voltage to 48 or lower
where it is attached to the preamp tube filament string. The bias
circuit in a 299C not only delivers negative voltage to the grids of the
four 7591A output tubes but 48 volts (original spec was 44 volts) to the
four 12AX7 preamp tubes who have their filaments in series. This allows
the tubes to have DC voltage on their filaments so as to reduce hum.
It is critical that you do NOT exceed 48 volts here. Since the amp
has both DC bias and balance adjustments, you will have enough
adjustment on the bias pots to take up a few volts either way.
The 299C will generally be biased correctly with around -21 volts at
the control grid of each 7591A. To check the actual bias correctly on a
299C, you will need to install a 10 ohm cathode resistor between pin 5
and ground of each 7591A and measure voltage drop across this resistor.
Spec is 35 ma per tube, so a voltage of .35 volt is what you want. You
can bias to .3 volt (or 30ma) and the amp will run much cooler with
almost no sonic effect.
Because this amp runs so hot and is so compact, you MUST replace all
the power supply capacitors as well as all the coupling capacitors.
Bill B.

Tube amp design, building, and service at:
http://home.alltel.net/wbittle1

Chad Wahls wrote:

I have a Scott 299C that I am getting ready to recap and do a general
restore as it is getting noisy and amazingly has all the original caps.

This is in a listening system so I'm gathering all my eggs before proceeding
because I don't want to thake the system down for long.

I am going to replace the selinium rectifier with silicon, do I need to put
a dropping resistor in to compensate for the efficiency of the silicon or
should I be fine with a direct replacement? BTW this one has bias and
balance for each pair of 7591's much like the early 299D, seems I have a
weird one!

I know I could experiment but surely someone has done this many of times and
has a value needed if any.

Any help would be greatly appreciated.

Chad