Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on September 15, 2013, 09:11:47 am

Title: Super Reverb Amp reissue bias
Post by: Colas LeGrippa on September 15, 2013, 09:11:47 am

Hi,

I have a stock super reverb amp reissue amplifier on my bench ( 2000, year of mfg ) and the bias is way too cold. I just plugged in new JJ's 6L6's and the cathodes read 15mV right b4 the 1 ohm resistors on the PCB, when bias pot is fully clockwise ( hottest ). The 430V on the plates, would call for a bias of approx 40mA. Why is the bias circuit so cold ?

Colas

Title: Re: Super Reverb Amp reissue bias
Post by: sluckey on September 15, 2013, 09:42:26 am

That is cold. The schematic I have shows -52VDC on the grids and 33mV on the cathodes.

Title: Re: Super Reverb Amp reissue bias
Post by: Colas LeGrippa on September 15, 2013, 10:11:22 am

yeah right on -52 and 33

Title: Re: Super Reverb Amp reissue bias
Post by: LooseChange on September 15, 2013, 02:56:45 pm

430v seems low too. Should be around 460v. Try another set of tubes first.

Title: Re: Super Reverb Amp reissue bias
Post by: Colas LeGrippa on September 16, 2013, 09:52:44 pm

yes sir too low indeed: GZ34 defective, with bias circuit corrected, new 6L6's showed 340V on the plates.........with new GZ34: 460V. Biased at 40 mA, new amplifier, totally.

colas

Title: Re: Super Reverb Amp reissue bias
Post by: Platefire on September 16, 2013, 10:43:05 pm

Wow! never had that before. My experiance with tube rectifiers is they are usually working at full potential or not at all. Plate

Title: Re: Super Reverb Amp reissue bias
Post by: LooseChange on September 17, 2013, 04:59:15 am

I've had my share of bad Rectifier tubes and mostly GZ34's.

Title: Re: Super Reverb Amp reissue bias
Post by: Colas LeGrippa on September 19, 2013, 05:22:16 pm

Yeah Platefire, me too, first time I see a «half defective» rectifier tube !

Colas