Ampeg B15NF bias slowly raising

[boroman]

Wow. Entertaining thread! Mine too has 190VDC on the plates in standby. And bias voltage of about -78vdc.

These go to 465VDC plate, and -49VDC bias within a few seconds of taking off standby.

I’m assuming this is normal.

For those of you with plate voltages in the 460-490 range, what kind of plate current are you seeing? Mine are only about 25ma. So by my calculations that’s only 40% of max plate dissipation for 6L6gc. Wouldn’t I need to decrease my negative bias voltage quite a bit to get it up the 65-70% target? Something doesn’t seem right. Maybe the power tubes themselves?

I have almost all my B15 from different era checking more than 500VDC with +/- 125V mains. I bias them around 50-60% good for clean bass. I think they were around 50% from the factory as far as I remember

 

[byacey]

I can just pull the center tap off the switch to test, yeah? And do you get 0VDC on your plates when in standby?

I don't have one of those amps, but there would have to be some leakage somewhere to cause those voltages to occur with the CT open. It could be capacitive leakage in the power transformer, too.

 

[byacey]

On most conventional fixed bias supplies, it's all pretty low series resistances until after the filter caps and the voltage divider. It's only the resistors that bias the grids that need to be high.

 

[Wasnex]

Disconnect the standby switch and re-test. There may be carbon track leakage inside the switch from arcing.

The amp I metioned is not a B15. Standby lifts the ground on the center tap. It has a 0.02uF across the standby switch. Perhaps the capacitor is there to charge the bias supply so the amp has a soft start when set to operate. The bias goes high in standby and then drops down when the amp is set to operate.

 

[byacey]

The amp I metioned is not a B15. Standby lifts the ground on the center tap. It has a 0.02uF across the standby switch. Perhaps the capacitor is there to charge the bias supply so the amp has a soft start when set to operate. The bias goes high in standby and then drops down when the amp is set to operate.

That cap is to reduce arcing across the switch contacts. It would also be the cause for a little bit of leakage current in standby.

 

[B-string]

The amp I metioned is not a B15. Standby lifts the ground on the center tap. It has a 0.02uF across the standby switch. Perhaps the capacitor is there to charge the bias supply so the amp has a soft start when set to operate. The bias goes high in standby and then drops down when the amp is set to operate.

When the output tubes are not conducting (in standby, no HV) the bias circuit is not fully loaded. Load the bias supply when tubes start conducting and the bias supply drifts down to normal designed voltage.

 

[bogson]

Good thoughts here…. So can others with b15n’s confirm seeing 180-200VDC on the plates and negative 70-80VDC bias when ON STANDBY?

 

[B-string]

There is no "on standby". There is "in standby" and "on" (or operate). The argument made that switching the center tap on the HV winding is hard on switches did IME prove true. There are many paths for voltage bleed that will not result in operation. Complete interruption of the high voltage can't be done safely with a direct switch accessible by a consumer, severe shock hazard at minimum and high failure rates.

 

[bogson]

Sounds good. And yes, it’s definitely hard on the switch and they do arc. I’m just curious if we’re all seeing roughly the same amount of voltage at the plates when switched to standby.

 

[byacey]

There is no "on standby". There is "in standby" and "on" (or operate). The argument made that switching the center tap on the HV winding is hard on switches did IME prove true. There are many paths for voltage bleed that will not result in operation. Complete interruption of the high voltage can't be done safely with a direct switch accessible by a consumer, severe shock hazard at minimum and high failure rates.

I don't think there is much of a shock hazard with the old.metal toggle switches. The bat lever is well grounded through the ball and bushing, which is secured to the chassis ground.

 

[B-string]

I don't think there is much of a shock hazard with the old.metal toggle switches. The bat lever is well grounded through the ball and bushing, which is secured to the chassis ground.

Take a good look at the voltage rating on those switches, it will be the first clue.

 

[byacey]

The high voltages being interrupted is certainly hard on the contacts, but I maintain the shock hazard to any fingers on the user side of the chassis is negligible.