Modifying bias circuit for lower current

[9VOLT1967]

I am re-working a Sound City 50+. It seems that the transformer is hard wired to the 105v input tap. I am ok with b+ at 482, but my bias current is way too high. Would it be easiest for me to tap the HV tap with a 220K resistor and re-work the bias circuit or is there an easier way to achieve my goal?

[Dr-Joned]

From the looks of it, the transformer has the seperate winding for the bias. I would check the filtering cap for the bias and also check the AC straight off the bias winding. Something has to be causing it to be high, so I would start with those and hope that the winding doesn't have a problem.

[9VOLT1967]

Replacing the bias resistor would prevent this high bias current? I believe the caps to be ok. 2-47uf 100 v in parallel. The tap directly off tranfo should be approx. 50 -60 VAC? My bias current is 80ma + and I have verified the tubes as well. It will not adjust to my desired setting. The amp has being used on a variac to prevent tube failure. Is there another way to fine tune bias circuit?

[dcribbs1412]

Replacing the bias resistor would prevent this high bias voltage? I believe the caps to be ok. 2-47uf 100 v in parallel. The tap directly off tranfo should be approx. 50 -60 VAC? My bias voltage is 80ma + and I have verified the tubes as well. It will not adjust to my desired setting. The amp has being used on a variac to prevent tube failure. Is there another way to fine tune bias circuit?

Looks to me like the schem shows a 1N4006 to a 100uf cap to 2 10k bias pots...is this the same amp?

here's a thread discussing bias adjustment...tictacs' idea may work for you
https://tubeamparchive.com/viewtopic.php?t=16839

[eddie25]

My bias voltage is 80ma + and I have verified the tubes as well.

Are you saying your plate current is too high, or did you mean to say 80 V? When I think of the term bias voltage, I think of the voltage at pin 5 in this case.

[thejaf]

If you need to lower your bias VOLTAGE, then just lower the value of R43 (4k7 from the schematic). Simplest way wuld be to stick a resistor in parallel. How much do you need to lower it? You could figure out the math I suppose, but it may be quicker to just start with something like a 10K and see where that gets you (ballpark).

Sounds like you're saying the bias voltage is too high becauase the PT primary is hard wired for 105 VAC, instead of 120VAC, thus pushing the whole amp higher.

[9VOLT1967]

Played with value of R43 went from 470R to 499K. The bias will not go below 74 to 80ma. The bias tap off the transformer is 33VAC. The readings @ pin 5 with no tubes installed is between .500 and -44 VDC. These voltages are obtainable by rotating bias adjusters. The schem shows a voltage of -36, but at that reading obviously will not bias within spec. Any insight would be helpful. Thanks for the help.

[diagrammatiks]

you might have a leaky coupling cap.

actually how are you reading the bias voltage?

is there a separate 1ohm resistor from pin 8 to ground on each power tube socket?

your schematic shows a 68ohm resistor from pin 8 of one tube to ground and a 22ohm resistor from the other.

Are you adjusting for that?

[9VOLT1967]

The bias caps?

[9VOLT1967]

I either check it with a weber bias rite meter or a fluke meter. I do not have the 1 ohm resistors on this amp. There are individual adjusters on each tube. It shouldn't matter as long as I bias each tube separately

[Firestorm]

Played with value of R43 went from 470R to 499K. The bias will not go below 74 to 80ma. The bias tap off the transformer is 33VAC. The readings @ pin 5 with no tubes installed is between .500 and -44 VAC. These voltages are obtainable by rotating bias adjusters. The schem shows a voltage of -36, but at that reading obviously will not bias within spec. Any insight would be helpful. Thanks for the help.

470R??? Schematic shows 4K7; other SC models have 1K, even 10K. Check that. Just 470R would be bad.

[epis]

Hi,
it looks to me that you are confused with bias voltage and bias current.
Bias voltage on the scheme marked as -36V is DC voltage, you can't measure it with tubes disconnected and AC voltage range on your meter.
Bias current, what you did measure with Weber probe ( in mA DC) is actually idling current with zero signal at input .
Bias current depends directly on bias voltage, higher bias voltage means lower bias current.
So, if you got 80mA DC of bias current (what is more than twice higher) you must increase bias voltage.If you can't increase it by increasing value of R43,you're gonna have to take it from high voltage windings.
Another thing what you can do, replace diode 1N4006 with full wave rectifier, that way you're gonna get higher negative voltage.

[9VOLT1967]

Appreciate the info. You are correct! I need to increase bias voltage to decrease bias current. Yes my 2 options are: Take it from HV tap or rectify the bias tap with full wave rectifier. Where could I find an example of these applications? Ex: schematics. Thank you for the help!

[tubeswell]

See the Brown or Blackface Princeton schematics for a bias supply from the HT winding.

[martin manning]

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.