Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: six el six on July 09, 2012, 05:41:14 pm

Title: Voltage on AB763 tremelo tube
Post by: six el six on July 09, 2012, 05:41:14 pm

Is pin 1 mislabeled on the ab763 schematic in the tremelo circuit?

I've never heard of it being so.

I have 404v on pin 1 of the tremelo tube but it says 270v on the schemo.

What am I not getting?

Thanks,
six

Title: Re: Voltage on AB763 tremelo tube
Post by: sluckey on July 09, 2012, 06:07:08 pm

Quote

What am I not getting?

The trem oscillator is switched off. Plug in the footswitch and turn it on and the voltage will be more in line with the schematic voltage. Actually, it will probably be swinging wildly if the oscillator works.

Title: Re: Voltage on AB763 tremelo tube
Post by: HotBluePlates on July 09, 2012, 06:09:39 pm

The schematic voltage is with the trem operating.

If you turn off the trem via the footswitch (or in a stock Deluxe Reverb, by not having a footswitch), then -55v from the bias supply is applied to the trem oscillator grid. The tube is very far into cut off, and draws no plate current. With no plate current, no voltage is dropped across the oscillator's plate load. The oscillator's plate voltage then rises to equal that of Point A, or the screen voltage. Which is 410v on the schematic.

Sounds like if your trem is off, your trem oscillator is reading exactly like it should.

Title: Re: Voltage on AB763 tremelo tube
Post by: six el six on July 11, 2012, 04:15:55 pm

Thanks guys.

And HPB I appreciate the explanation on the circuit very much!

six