AB763 screen grid voltage question

[Planobilly]

Hi,

This is a Hoffman AB763 Deluxe Reverb with JJ6v6 tubes. The plate voltage is 438 VDC. A little high but that is what the Hammond PT is putting out.

The choke is 4H.

The issue is that the screen voltage is 439 VDC. I assume it should be about 5 volts less than the plate.

Any suggestions on what is the best way to reduce it?

Thanks,

Billy

[sluckey]

It's very common for the screen to be a bit higher than the plate with that power supply. I would not worry about it.

[PRR]

> I assume it should be about 5 volts less than the plate.

Why?

And WHAT plate voltage? When making sound, plate swings 100v to 900V.

[robrob]

I agree, your voltages are normal. Proceed.

[Planobilly]

Hi guys,

> I assume it should be about 5 volts less than the plate.

Why?

And WHAT plate voltage? When making sound, plate swings 100v to 900V.

The "why" was just about looking at Fender schematics which indicate a nominal voltage of 415 VDC plate voltage and a screen grid voltage of 410 VDC. Also, direct  measurements of a vintage amp I have access to shows similar voltages. Voltage readings were taken at quiescent condition.

I have been going back over everything in the amp due to two conditions.

1. Some hum noise when increasing the reverb control. There is zero hum on the normal channel and a small amount on the reverb channel with the  channel gain turned up past seven.

2. Some unusual buzzing sound that are audible at frequencies less than approximately 200 Hz and are harder to hear at higher frequencies. This sounds increases with larger input signal, as when the low E or A string is struck hard. Playing at low gain levels does not produce this sound. This condition occurs on both channels with and without V3 and V5 removed.

Before I go any further, I want to move the amp to another location to do some testing.

Thanks,

Billy

   

[PRR]

Fender-spec choke or something else/better?

If it has lower Ohms than what Leo was buying, the DC Volts will tend to run higher than Leo saw.

Anything 80%-102% of Plate voltage is probably all the same to the tubes. In practice the plates show a little drop in OT resistance, the screens show a little drop due to screen filter, and which comes out hi/lo is no real issue.