Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on May 24, 2023, 03:14:59 pm
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Using a PS like the one on this old Philips PA
There is a way to obtain a negative voltage (say -150v in this arrangement) adding parts to this PS ?
Many Thanks
Franco
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need more information
will the -150 be referenced to the amps ground?
are you using the amp AND want -150?
or
do you just need -150 for something other than the amp?
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Ciao Shooter
The amp is not yet existing, more, the plan of the amp is not yet existing
The circuit you can see attached is a proposal and the use will be between a preamp and the Power Tube of an SE Power Section, in the middle between the two
it was only an idea that was abandoned because of the necessity of the negative voltage
But as I'm curious I opened this thread as to know if it will be feasible to obtain such a negative voltage starting from that kind of PS
The Philips PA used 150v and 300v, the idea used the same PS but starting to from a bit higher AC voltages to arrive to have 200v and 400v
Franco
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Bogan PA amps derive their negative bias voltage from the B+ winding, they don't have a bias tap on the secondary. They use a .22 uf capacitor to tap off the B+ winding, and then the circuit for the negative bias voltage. There's no reason you couldn't do that to get negative 150 volts.
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Bogen did it, as Amber says. I've always been afraid of doing it wrong. (Well, actually, Bogen used the 2-diode doubler, not the center-tap bridge.)
A heater transformer, backward, is simple and harder to do wrong.
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Thanks AmberB & PRR
The preferred solution will be to take the negative voltage via a cap from the HT winding
but the unusual way of the PS that feed G2 using the CT put on me doubts about the feasibility
to add a small Transformer will give a more sure result but as it is a new project and at the moment there isn't a PT suitable, may be it will be ordered custom so with a dedicated winding
Franco
Franco