SoundCity bias

[drewl]

having trouble with the bias range too high on this L50
What do you think would be the easiest way to lower it?
Raise or lower R43 (4.7k)?
I wouldn't bother asking but I'm at work and can't tinker with it right now!
thanks.


http://www.schematicheaven.com/barga...city50plus.pdf

[Satamax]

What do you mean, you can't go low enough? Like stuick at -32/36 and want to go lower? By changing the resistor, you won't be able to go higher than pot maxed out, so you'll need to build a voltage doubler, or take your bias from HT winding like ampegs.

Bye.

Max.

[drewl]

The tubes are drawing too much current so I need to lower it which means I need more negative bias voltage.
I was thinking of redoing the bias supply like a Marshall. If the pots open up, there will be no bias voltage which would fry the tubes, the pots are intermittant so I believe this is what cooked one of the output tubes.

[Tom Phillips]

Because you have a bias adjust pot for each tube you can’t just simply change to the Marshall style pot wiring because the two pots will become highly interactive. One trick is to put a 100k resistor from the high side of the bias pot to the center lug. This may seem strange but if the wiper goes open circuit the bias voltage to the tube will actually move more negative and protect the tube.

Tom

[drewl]

Bias Doubling?

Are you refering to the mod in the Pittman book?
if so, the description doesn't match the little schematic!

[Tom Phillips]

Confused about question

You talkin to Max or Tom?

[drewl]

Anybody!
I rebuilt the bias supply using the voltage doubler circuit and all's well!
They were right about the stock supply not being able to deliver enough bias voltage.
thanks all!
this baby makes me want to get another SC!
It's much more volume friendly than my 120 was!

[Mars Amp Repair]

Bias Doubler

Say,
Can you show where you found the bias doubler ckt? I have the same issue on a HiWatt DR-103 & cannot find a decent doubler ckt. I know it's not complicated, just have no reference. Thanx, glen.

Anybody!
I rebuilt the bias supply using the voltage doubler circuit and all's well!
They were right about the stock supply not being able to deliver enough bias voltage.
thanks all!
this baby makes me want to get another SC!
It's much more volume friendly than my 120 was!

[drewl]

You'll need to reverse the polarity of the diodes because it's a negative voltage that's use for bias!


http://www.tpub.com/neets/book7/27m.htm

[Rob Mercure]

To "Mars Amp Repair"

Drewl did you good with the links. To add a simple thought: If the bias winding is separate from the B+/HV winding and isn't already grounded you can easily add a "full wave" doubler which has a 120Hz ripple which is easier to filter out. If the winding is grounded - a tap on the B+ winding - you're stuck with one of the half-wave doubler/triplers/etc. As most "historic" bias supplies were 1/2 wave anyway not being able to use a full wave doubler isn't that critical (although I still like the option) and 1/2 wave gives you the option of "tripling" "quadrupling" etc. the supply to desired voltages. I've seen a few designs where low C- (bias supply) voltage was required that tripled the heater supply for C- (one side of the heaters grounded).

Rob