Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: lkrasner on March 12, 2015, 01:40:04 pm

Title: BlackFace Pro Reverb Not Enough Bias
Post by: lkrasner on March 12, 2015, 01:40:04 pm

I promise to shut up about this amp soon... if you help me fix it.

I simply can't get enough bias voltage. even with the pot all the way up, I'm only getting a voltage drop across the OT primaries of about 3V which works out to about 35mA. My plate voltage is 455, so this is less than 60% dissipation for a 6L6GC. does this indicate a problem, or should I just change the range resistor to suit my needs. If that is ok, what value should I use?

Title: Re: BlackFace Pro Reverb Not Enough Bias
Post by: sluckey on March 12, 2015, 01:56:43 pm

Quote from: lkrasner on March 12, 2015, 01:40:04 pm

I simply can't get enough bias voltage. even with the pot all the way up, I'm only getting a voltage drop across the OT primaries of about 3V which works out to about 35mA. My plate voltage is 455, so this is less than 60% dissipation for a 6L6GC. does this indicate a problem, or should I just change the range resistor to suit my needs. If that is ok, what value should I use?

Actually, it sounds like you have TOO much bias voltage which is causeing the amp to run cooler than you want.

Replace the 470Ω/1W on the bias board with a 1KΩ/1W. This will decrease the bias voltage which will allow your tubes to run hotter.

Title: Re: BlackFace Pro Reverb Not Enough Bias
Post by: lkrasner on March 12, 2015, 01:58:37 pm

Oh yeah, i guess I wasn't thinking about the right direction. could this have been caused by replacing the electrolytic capacitor in the bias circuit? I'm just trying to figure out why it would have gone out of spec.

Title: Re: BlackFace Pro Reverb Not Enough Bias
Post by: sluckey on March 12, 2015, 02:06:44 pm

Quote

could this have been caused by replacing the electrolytic capacitor in the bias circuit?

Possibly. If the old cap was drying out then the bias voltage would have been somewhat less. Putting in a fresh cap would have increased the bias voltage in this case.

Quote

I'm just trying to figure out why it would have gone out of spec.

What spec is out? The only reference you have to go by is the -51V on the wiper of the bias pot.

Title: Re: BlackFace Pro Reverb Not Enough Bias
Post by: lkrasner on March 13, 2015, 11:17:11 am

Quote from: sluckey on March 12, 2015, 02:06:44 pm

What spec is out? The only reference you have to go by is the -51V on the wiper of the bias pot.

Well clearly the amp was originally intended to have a proper range of bias, so something has changed. I'm not terribly concerned, I just wnated to make sure this wasn't indicating a larger problem.

The currents on each tube seem fairly mismatched too, even though the tubes are a new matched set of JJs. This could just be error in my measurements of the OT primaries (throwing off the ohms law calculations), so I will check again, but I wanted to point this out in case it gave any more clues.


Thanks for all the help everyone!

Title: Re: BlackFace Pro Reverb Not Enough Bias
Post by: sluckey on March 13, 2015, 11:34:27 am

Quote

Well clearly the amp was originally intended to have a proper range of bias, so something has changed.

It was originally intended to be adjusted to -51v. Can you adjust yours to that value?