Bias cirquit

[kaj]

Hi.
I`m building a "Steel String Singer" using the 002 schematic. My power trany dont have a center lead, so I am using an diodebridge and I get a B+ of 490VDC out without tubes installed.
I would like to ajust my Bias to apx. -55V before I install the tubes, but my problem is that I can`t get a negativ voltage! I`m connecting to one AC-side of the trany using the Bias cirquit as is on the 002!
Must the tubes be installed for it to work?
I`v allso tryed using bouth AC sides, still no -voltage!!
What is my problem??
I hope some smart tech can help me
Cheers Kaj

[VacuumVoodoo]

Are all 3 diodes in the bias circuit oriented correctly?

[martin manning]

A FW bridge doesn't have any negative voltage swing (to speak of), so you need to capacitively couple the bias circuit. Connect to the HT with a 47-100n cap followed immediately by a resistor to ground, 56-100k say, then the reversed diode for the bias supply.

[VacuumVoodoo]

A full wave bridge is composed of two dual phase half wave rectifiers. (confusing, isn't it?)
With secondary winding's center tap grounded you can get a positive or negative rectified voltage or both as long as any rectifier output isn't floating without ground reference.

[martin manning]

Hi Aleks, in the original post it says FWB with no CT.

[VacuumVoodoo]

Aha. OK. By the time I got to the schematic I already forgot what I read in the first post. Then reactive (capacitive) coupling is the way to go.

[kaj]

A FW bridge doesn't have any negative voltage swing (to speak of), so you need to capacitively couple the bias circuit. Connect to the HT with a 47-100n cap followed immediately by a resistor to ground, 56-100k say, then the reversed diode for the bias supply.

if I understand that correctly: From one side of the trafo HT to C15 is the supply, and the negative bias volrage will be taken from top of R31.
Correct me if I am wrong.
And yes it is a FW bridge, no center tap from the trafo.
Kaj

[martin manning]

That's it!

[kaj]

That's it!

Thank you gay`s. It`s nice to have a forum like this ! That solved my problem, I can now regulate from -48V > -56V, I had installed a 10K pot and connected that in serie with a 68K resistor. 22K-pot with 56K resister would probably have giver a wider regulating range, but now I can start testing the amp with the tubes installed. Thats exsiting':D'