Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tiny_Daddy on April 07, 2012, 04:46:36 pm

Title: Ampeg J12R re-issue runs zero bias?
Post by: Tiny_Daddy on April 07, 2012, 04:46:36 pm

This one came into the shop with cooked tubes. Replaced tubes and expected cathode bias, nut NO! There is no bias. Don't have a schematic so I traced the cathode: connects to the grounded filament circuit. Grid measures 240K to ground, verified that too. Anyone else seen this?

Title: Re: Ampeg J12R re-issue runs zero bias?
Post by: PRR on April 07, 2012, 10:31:56 pm

Aint right.

Pull tubes, power-up, quickly look for negative voltage at grids.

If the cathodes are hard-grounded, yet grids are at zero, suspect a short in the bias supply.

Title: Re: Ampeg J12R re-issue runs zero bias?
Post by: PRR on April 07, 2012, 10:37:16 pm

> cathode: connects to the grounded filament circuit.

On the original...

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_J12R_.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_J12R_.pdf)

...cathode connects to heater circuit which is NOT grounded.

There should be a 10 Watt resistor; maybe easy to find.

Heater-wiring short to ground or shorted cathode-cap on that 10W resistor?

Title: Re: Ampeg J12R re-issue runs zero bias?
Post by: Tiny_Daddy on April 08, 2012, 08:05:01 am

Nope, no 10 watt resistor. I have traced the grids and cathodes to a star ground. No negative voltage on grid circuit, just zero volts. Hard to believe. I plan to install cathode bias.

Title: Re: Ampeg J12R re-issue runs zero bias?
Post by: LooseChange on April 08, 2012, 06:51:47 pm

Are you implying it works with zero bias voltage?
Sure looks like cathode bias on the schematic.

Title: Re: Ampeg J12R re-issue runs zero bias?
Post by: Tiny_Daddy on April 08, 2012, 07:19:30 pm

Looks like I was wrong, cathode not going to ground. More on this later.... Wish I had a schematic.

Title: Re: Ampeg J12R re-issue resolved!
Post by: Tiny_Daddy on April 14, 2012, 08:13:33 pm

Problem was the tubes were cathode biased with a 120 ohms 5 watt resistor and would try to draw 60mA each! Since I have to install and guarantee the tubes, I find 220 ohms gives more reasonable bias and the amp still has plenty of power. This re-issue does not match the old schematic and has an op-amp and a pair of 12AX7.