Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blind Lemon on August 01, 2015, 11:49:00 am

Title: Ampeg V2/4 bias circuit......Missing something
Post by: Blind Lemon on August 01, 2015, 11:49:00 am

Does this circuit only show a true close bias voltage when its loaded? First time in one off these and I'm a little "corn-fused". When I come off SB the bias goes to nothing, both tube start to red plate (known good tubes).

Here are the unloaded Voltages. All parts are under board or I'd replace diode and .047 with thinking, guess I'm being a bit lazy. :dontknow:

Title: Re: Ampeg V2/4 bias circuit......Missing something
Post by: sluckey on August 01, 2015, 12:04:41 pm

Pull the output tubes and lay them aside until this is resolved. When the STBY switch is closed, the bias voltage on the tube socket grid pins should be about -68vdc. When the STBY switch is open that bias voltage will likely change to something crazy because the STBY switch breaks the normal ground path for the bias circuit. However, the bias circuit will find a sneak path thru the B+ supply.

Title: Re: Ampeg V2/4 bias circuit......Missing something
Post by: Blind Lemon on August 01, 2015, 12:18:58 pm

Hey Steve, miss the agency? Don't...... its not the one we started in.

Ok, so with the tubes out BV should go to ~-65VDC? Its -1.1 VDC.

Title: Re: Ampeg V2/4 bias circuit......Missing something
Post by: Blind Lemon on August 01, 2015, 01:26:47 pm

Ok, so I was lazy and it was a snake in plane view and it bit me..............so I changed it.  :l2: