Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Imntgrumpy on May 03, 2017, 06:24:35 pm

Title: Can't get bias below 48ma on my Princeton clone
Post by: Imntgrumpy on May 03, 2017, 06:24:35 pm

Hey gang,


My Hoffman Princeton Clone started to sound distorted and farty after 6 years of use.  Just replaced every tube and tried to adjust the bias.  Well, I can't seem to get it below 48ma.  I've already installed two 100k-3W resistors in series for my "range resistor"  (didn't have the right value available).  The original schematic calls for only 1 100k-3W.


Looking for suggestions on taming the bias.


Thanks,


Jim Koehler
Long Island, NY

Title: Re: Can't get bias below 48ma on my Princeton clone
Post by: PRR on May 03, 2017, 06:35:46 pm

Exact schematic used would help.

My blunt guess is that the bias supply is failing, perhaps a leaky cap. So where you want -50V, you only got -30V now and zero next week (infinite 6L6 current).

Title: Re: Can't get bias below 48ma on my Princeton clone
Post by: Colas LeGrippa on May 03, 2017, 08:08:38 pm

Hi buddy, I don't know your level of knowledge and maybe you've already tried what I am suggesting you to do:  remove the power tubes, fire the beast and check the bias voltage. Try to get at least -50V at pins 5 if not, replace the bias related parts: diode, cap and resistors. While you're in there, install a pot for bias adjustment and a 1 OHM resistor from the K to gnd. Check also the two 220K ( maybe 100 or 330K ) grid leaks resistors. Like PRR said, try to post a schematics.


Colas

Title: Re: Can't get bias below 48ma on my Princeton clone
Post by: Imntgrumpy on May 04, 2017, 09:12:18 am

Fixed!   :laugh:

Removed one of the 100k-3W range resistors and was able to adjust the bias to 29ma.

Title: Re: Can't get bias below 48ma on my Princeton clone
Post by: Colas LeGrippa on May 04, 2017, 09:18:05 am

Excellent, now you can play !