Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jbefumo on March 30, 2016, 08:12:01 am

Title: Tapping Bias Supply From Tube-Rectifier Power Supply?
Post by: jbefumo on March 30, 2016, 08:12:01 am

I'm in the process of building my second design utilizing a tube rectifier; the first with fixed bias. The attached illustration shows two scenarios.
The first is how I've always tapped my bias supply when using a transformer with no separate bias tap (and no center tap).  This always worked fine.
Last night I tried the second scenario (tube rectifier), and couldn't for the life of me get it to work.  I verified that I'm getting (~220V) A/C to the .047 capacitor, but as soon as I connect the ground to the ground of the amp, that drops to something like 8 volts or so.
Ultimately, I just stuck an tiny transformer on the chassis and used it to supply my negative bias voltage, but I'm mystified as to why it didn't work as expected.
Any enlightenment will be appreciated.
In the future I'll make sure I buy transformers with a bias tap, but for this build I was limited to the parts I have on hand.

Thanks.

Joe

Title: Re: Tapping Bias Supply From Tube-Rectifier Power Supply?
Post by: sluckey on March 30, 2016, 08:29:29 am

Totally remove R3. Replace C3 with a 100K to 220K resistor. Call the resistor a bias range resistor and change the size to provide the range of negative dc voltage you desire.