Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: GODI on July 01, 2011, 11:55:42 pm

Title: Bias question
Post by: GODI on July 01, 2011, 11:55:42 pm

Hello, I'm building a Princeton Reverb style amp, I saw and read this page :
http://www.el34world.com/charts/Biascircuits.htm (http://www.el34world.com/charts/Biascircuits.htm)
The last images at the bottom show a ''Better bias system'' witch I like, however I don't know if the A/C voltage input is from a +/- 50 Volts bias tap from the power transformer or from pin 4 of the tube rectifier ? 
In my case, I have a Deluxe Reverb transformer that have a bias tap wire. Now if this circuit is desing for the +/- 50 Volts, what would be the value of the resistor just before the diode at the input ?  I will use the typical Princeton 6V6 output tubes.

This is the page that I will use to expose this build : (not much thing yet, but it is comming ...!)
http://jeangodbout.com/princeton/reverb-amp.html (http://jeangodbout.com/princeton/reverb-amp.html)

Thank you, Jean

Title: Re: Bias question
Post by: EL34 on July 02, 2011, 06:27:37 am

Posted in wrong board - moved

Title: Re: Bias question
Post by: Fresh_Start on July 02, 2011, 08:49:04 am

Jean - I used the attached bias circuit in a Super Reverb build.  Unfortunately, I didn't record the raw voltage off the bias tap.  Also, that amp needed bias voltage of about -55 where the PR wants something around -40.  You will have to find the right value for the resistor between the PT and the diode by experimentation.  That's why Doug calls it the "range resistor".  It sets the range of bias adjustment.  Note, you also can modify the value of the 27K resistor going to ground or the 1K to adjust the range.  It's just a voltage divider.

For optimal filtering with two 10uf caps, you really need at least 15K between the two caps.  In my case, 15K would not have left me enough adjustment range.  That's why the bias pot itself (plus the 1K) serves as the isolation resistor between the two caps. 

Depending on the raw voltage of your bias tap, you might be able to use the bias circuit from the attached PR schematic.  Obviously, you would greatly reduce the value of the range resistor from 82K to something like 470 ohms (for starters).  The 18K isolation resistor could just as well be 15K and 47uf probably is overkill for the two caps.  10uf probably is fine.

Hope that helps more than confuses.

Chip

Title: Re: Bias question
Post by: GODI on July 02, 2011, 10:49:35 am

Thank you Chip, YES that's help and I'm not confuse at all, I will implement this.

Thank you, Jean

Title: Re: Bias question
Post by: tubeswell on July 03, 2011, 01:43:27 am

FWIW the schematic from my PR build from last year

Title: Re: Bias question
Post by: GODI on July 03, 2011, 02:18:29 am

Thank you tubeswell, this will be very usefull for me, especialy with the standby section. I also like the way you draw the separate ground with different colors, well done.
Thank you very much, Jean