Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tbone55 on May 11, 2019, 07:23:01 pm
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Hi All, I have a circuit that I would appreciate some help with for an amp I'm building. I need help to verify that my understanding of how this portion of the circuit works is correct. The circuit acts as a variable DC supply to provide power output from around 5W's to 12W's. There is a power level switch/pot that turns the amp on. At idle, the circuit supplies about 100vdc to the input stage and at max the full voltage of around 450vdc is applied.
Here's how I see this circuit works.
The circuit is identified on the schematic as VRM. When the power level switch (VR5 S1) is at idle, the voltage on the + side of the 1st filter cap (C12) is around 100vdc. The voltage at the + side of the 2nd filter cap (C13) is the full rectified voltage, 450vdc. The voltage here doesn't change because there is a jumper wire that runs from the junction of the two diode rectifiers to the + of this cap. As the power level switch is turned further away from the idle position, the voltage at the plus side of C12 increases until at max you reach the full rectified dc voltage (450v). The + side of C12 feeds the OT so as you vary the level of the power level switch/pot the dc voltage feeding the OT varies which in turn varies the dc voltage feeding the power tube cathode at pin 3. The only time the dc voltage at + of C12 is at full rectified voltage is when the power level switch/pot is at it's maximum. That's when the amp is putting out it's max power.
I've attached both a layout and schematic of the amp for reference. In either case, hoping you can confirm or deny. :help:
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Mostly correct(*). Not sure what you are not sure about.
The drawing was cramped-up onto a too small paper. (I suspect the VRM was wedged into a non-VRM drawing.) When you find things like that, un-scramble them left to right, top to bottom, until it looks like some conventional forms.
The basic idea is a Voltage Divider, adjustable. You can't hang a 10 Watt load on a 1/4 Watt pot, so they buffer the load with a Cathode Follower (done with a MOSFET so we say "source follower"). Draw all that left-right, current flowing down through devices, and it is clearer.
Plain 440V DC supply, steady. A hundred Ohms to limit surge, now near 435V. Voltage divider and "cathode" follower, to allow some desired fraction of 435V to be delivered to the amplifier.
I have omitted some parts which are wise in practice but not needed to show how-it-works. You have bleeder resistors for your safety but hardly affect operation. Real MOSFETs *MUST* have Gate protection but in normal operation they do not do anything.
(*)> varies the dc voltage feeding the power tube cathode at pin 3.
Typo there. And it would be sufficient to say "the B+ to the amplifier".