Modifying bias circuit for lower current

[thejaf]

You could try a voltage doubler on the bias tap, but wold probably just be easier to tap off the HT and be done with it.

[Firestorm]

The readings @ pin 5 with no tubes installed is between .500 and -44 VAC. These voltages are obtainable by rotating bias adjusters.

I assume you mean .500 and -44 VDC (rather than VAC). From the schematic, you should not be able to adjust the bias voltage much higher (less negative) than -22VDC. If you can really get to .500, the circuit is not wired right. It looks like a lot of work has been done in there over the years: with tag boards, it's too easy for someone to have wired something to the wrong post.

[9VOLT1967]

Those readings were based on resistor changes. With the original schem 4.7K in place it is adjustable from -44 tp -42 or so. I will probably be tapping the HV and totally re-work the bias circuit.

[epis]

Another thing what you can do, replace diode 1N4006 with full wave rectifier, that way you're gonna get higher negative voltage.

This will make little difference to the voltage achieved. There will be a small benrfit from a lower ripple voltage, but that will be offset by an additional diode juction drop.

You were right, my bad (I didn't think enough before posting)

[epis]

I am re-working a Sound City 50+. It seems that the transformer is hard wired to the 105v input tap.

Check out AC voltage on the heaters, if it's in 6.3V +- 10% range,
Firestorm might be right and you should completely verify wiring of bias circuit in your amp.Cheers

[9VOLT1967]

I got this schem from another forum. Looks like a workable option. Voltage doubler.

[9VOLT1967]

Well I will be utilizing the voltage doubler with separate bias controls for each tube. Pic 2 is the prototype bias circuit and pic 1 is the stripped version prior to rebuild. Should be a nice amp whem complete. I will be adding a presence control to it as well.

[Gaz]

I didn't read this whole thread, but I'd like to think you got frustrated with the bias supply and stripped the whole amp

[9VOLT1967]

Had to be sure of the bias circuit before overhaul! I had planned on re-build anyway. The amp is gonna be a single channel with hi gain circuit. It is getting new caps, sockets, and new pots. I won't deny the bias circuit was a PITA! I will post pics when complete.

[9VOLT1967]

Here is the completed amp. It sounds great and it sings nicely. My heaters are running about 6.9VAC. I left accomodations for dropping resistors. Should I drop it down of just leave it? I suppose I will leave it because it is at the +10% tolerance. The plates are running @ 520VAC. Love the tone. Started with 4 and ended up with 3 stages with plate driven stack! Here is the new pic!