[Brian]

Hi,
I just installed the Curcio driver board upgrade mod (cascode
amplifier with split load phase splitter) and I don't get any bias
voltage? The three tubes light up, the rectifyer and the power tubes
light up but I can't set the bias. Any help would be great! any idea on
where to check for what voltages.


Thanks!

[Peter Wieck]

Brian wrote:
Hi,
I just installed the Curcio driver board upgrade mod (cascode
amplifier with split load phase splitter) and I don't get any bias
voltage? The three tubes light up, the rectifyer and the power tubes
light up but I can't set the bias. Any help would be great! any idea on
where to check for what voltages.

Go over your connections again. Better yet, get someone else to go over
them. Often someone will overlook their own error over and over. Last I
looked this mod does not alter the bias circuit, so you should be
getting something.

I am sure you have already addressed the bias supply (caps & silicon
diode) and the condition of the pots. If not, go here first.

Peter Wieck
Wyncote, PA

[Brian]

Thanks Peter,

WIll do this evening. Are there any voltages I
should be looking for at any point? Thanks

Peter Wieck wrote:
Brian wrote:
Hi,
I just installed the Curcio driver board upgrade mod (cascode
amplifier with split load phase splitter) and I don't get any bias
voltage? The three tubes light up, the rectifyer and the power tubes
light up but I can't set the bias. Any help would be great! any idea on
where to check for what voltages.

Go over your connections again. Better yet, get someone else to go over
them. Often someone will overlook their own error over and over. Last I
looked this mod does not alter the bias circuit, so you should be
getting something.

I am sure you have already addressed the bias supply (caps & silicon
diode) and the condition of the pots. If not, go here first.

Peter Wieck
Wyncote, PA

[Peter Wieck]

Brian wrote:

WIll do this evening. Are there any voltages I
should be looking for at any point? Thanks

Let me look when I get home. I have to sit with the ST70 schematic &
the Mod schematic. But you should be concentrating on the Bias Supply
at this moment.

Peter Wieck
Wyncote, PA

[Sander deWaal]

"Bret Ludwig" said:


I'm astonished Joe doesn't have the builder replace the goofy value
"Biaset" resistor with a separate resistor for each cathode and provide
for two pots. IIRC the original supply comes off the wiper of the pot
so if this is to be retained, pots made when Kennedy was president may
be in for preemptive replacement, as it's the wiper that goes open.


Whenever I get a tube amp or receiver in that has adjustable bias by
potmeters, I wire them so that when the wiper comes off, the bias is
automagically set to max. negative.

Which in practice means a resistor from the negative supply, the pot
wired as a variable resistor to ground.
The entire contraption now acts as a voltage divider, and I'm using
the joint of the pot and resistor to tie the grid to via an
appropriate value resistor.

No more worries.


--
- Ever seen someone with 5.1 ears? So, what does that tell you? -

[Peter Wieck]

Bret Ludwig wrote:

I'm astonished Joe doesn't have the builder replace the goofy value
"Biaset" resistor with a separate resistor for each cathode and provide
for two pots. IIRC the original supply comes off the wiper of the pot
so if this is to be retained, pots made when Kennedy was president may
be in for preemptive replacement, as it's the wiper that goes open.

He does, it is a different mod.

Peter Wieck
Wyncote, PA