• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

First OTL breadboarded

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Brit01

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Joined 2008
The supplies really do need to be as close as possible, otherwise you will get an offset. I built up a small Mosfet circlotron a while back, I had issues with offset until I added regulators to my power supplies so that they matched.
Click to expand...

I was thinking of regulating them but wasn't sure if it was viable for floating supplies.

I would need some careful planning for these as the power supplies draw some large amounts of current for several tubes in parallel.
Lets say min. 200mA per triode. 4 or 5 in parallel!!

Looking at a transformer that can supply 1 amp min, so some headroom, at least 1.5 amps, preferably 2 amps per secondary winding at 100-140 volts.
A regulator circuit that can handle 1.5 amps?? Any ideas?

I was quoted 120 USD for 1 tranny with 2 secondaries of 110 volts at 1.5+ (200 watts to be on the safe side) amps each. So a core that can supply over 400 watts.

I would go for a step down (220-110)(200 watts) as these are only about 37 USD each and easily available here but then I would need 4 big step downs on a chassis for 2 channels!!!!
(not including the B+ and heater transformers:eek::eek:)

I will get some more quotes.
 
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Brit01

Member
Joined 2008
Originally Posted by Brit01 View Post
Could a cascode DN2540 be used as a sink to ground on the cathode?
Only if you want superb balance and overall performance with a simple, inexpensive circuit.
Click to expand...

I could possibly implement a trimmer between the cathodes, with the wiper connected to the CCS to balance the 2 cathodes equally.

At present I have the CCS connected directly in between the 2 cathodes. Probably not the best most balanced design:(


Will try this.
 
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Brit01

Member
Joined 2008
The 9-10 volt difference in grid bias required to balance the output to zero DC offset seems to be caused by the FETs.

I swapped the source follower FETs and it followed the FET. Didn't expect that.

So I needed for example -49 and -58 volts on each rail to balance the output.
These only changed rails when i changed the FETs.

I will try another FET.
 
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