Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tdvt on March 23, 2023, 10:50:19 am

Title: Hammond AO-29 Field Coil PT question
Post by: tdvt on March 23, 2023, 10:50:19 am

Hoping to use up a salvage AO-29 PT (field-coil version) in an amp project for a friend, what would essentially be a single-channel Deluxe Reverb w/no trem. I would like to use a tube rectifier & land at around 420V B+.


From reading about these PT's in other threads, I had mentally filed away that a 700Ω power resistor in series on the center tap, taking the place of the field-coil, would bring the voltage down into usable range.


Having just rebuilt the PS section of an ultralinear Pro Reverb that uses a two power-resistor voltage divider to drop voltage for B+ node "B",  I am wondering if that is a more viable method to deal with the high PT voltage instead of the CT resistor.?

Or even a combination of the two methods?

Title: Re: Hammond AO-29 Field Coil PT question
Post by: sluckey on March 23, 2023, 12:45:01 pm

The single resistor in the CT looks good to me. In, addition, it will provide a free source of negative voltage for your bias circuit.

Title: Re: Hammond AO-29 Field Coil PT question
Post by: tdvt on March 23, 2023, 01:20:56 pm

Quote from: sluckey on March 23, 2023, 12:45:01 pm

The single resistor in the CT looks good to me. In, addition, it will provide a free source of negative voltage for your bias circuit.

Thanks for checking that out & good point about the bias supply.