Bias voltage Doubler - Brain Flatulation


Well for some reason my logic has failed me.

I'm in the middle of moding the bias circuit on a
100 watt sound city amp. But now I can't recall
after I add another cap and another diode,
where those should all be wired to.

I know I'll have two caps in series where the
existing cap is (left). From the junction of these
two caps (center), I'll wire it to the existing neg terminal
of the cap in the chassis.

Where the one diode is, I'll have two. The other
diode will be added to the terminal on the lower left
and will be wired into the positive side of the new bias caps
(the two that are now in series).

Then there is one other connecton to be made.
The Hiwatt Biad Mod Schematic from The Tube Amp Book
shows this wired to chassis ground?
This is the junction of added diode, and the positive
side of the second cap (in series), I'm just not sure where to
solder this?

A. The plus side of the Capacitor, right next to the
existing cap.

B. The plus side of the capacitor, right next to the
existing cap and on the other side of the 220K bleed down resistor.

C. Wire them to a chassis ground point.

Actually I guess either would be correct.
One should be more correct then the other.

I've included pics under my pics here:

The are listed under my pics.
There are pics of the following:
Hiwatt mod schematic
the in the amp with the new diode (and old caps)
SC100 close up of the bias circuit

Any light on the situtation would be greatly appreciated.


VacuumVoodoo said:
You can use (a). Move ground from beetwen the caps to (+) output. But I see Villard as more elegant :)

I agree. So I'll do the Villard and see how it works. It just seems
kind of strange having a cap in the circuit first. I updated my blog
with a link to your site.

Thanks for your input.

And this:



Almost finished with the cap job too.

I'll post it with some finished pics over at the blog.
I've got a tone of other stuff that I'm finally getting
ready to post there as well so check back often.


Here is the final implemention of the voltage double bias circuit.

For a full explanation and full pic check out the blog update.
Kinda nice to have this finished too. I'll post some specs after
brining up the amp slowly and burning in the new tubes.

Using new JJ KT77s for the outputs and two NOS preamp tubes.
The 12AT7 is a NOS Mullard, while the hole shot (first gain stage)
is a NOS 12AX7 Tele or Mullard depending on which sounds best.

The other two tubes I'm leaving in there, they are fine old stock
Mullard 12AX7s that came in the amp and still test very strong.

Thanks again Alex and Eric.


Oh yeah,

Almost forgot. After finishing up and before sticking it back in
the head cab one is forced to burn it in and play through it.

Then adjustments are made to bias, tubes, or other things
are noted and back to the bench she goes.

The toughest part of all this is being FORCED to HAVE TO
though this thing. Damn, I hate when that happens.

I mean it is a tough job, but someone has to do it.



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