it is a good build and nice amp! For me looks like 6c33c heater wires are too thin..but maybe in picture it looks like it. I also feel that it is electromagnetic problem.To be sure, you can remove all of the power transformer secondary side and just load it with dummy resistor and then measure output transformer hum. good luck!
There is a hum, even if there is no output tube ...
Magnetic attack powerful transformer - output transformers.
Perhaps the output transformer is made with high induction - that's the problem.
To be measured is idling.
I keep going back and forth on this point... Yesterday, I was convinced it was not due to grounding. Today, I am unsure again...
... Since there is a new development. With only heaters on (no B+), I get a low-volume 60Hz hum. Turning on the B+ seems to change the hum into 120Hz. (It sounded higher in pitch, anyway.)
The only differences between this version of the amp and the breadboard version are:
1. transformers attached to metal top plate
2. secondaries of OPTs grounded instead of floating (removed these wires yesterday, but there was no change)
3. using a slightly modified star ground instead of a true star ground
4. output tubes are now biased through potentiometer/resistor network with meter, instead of only resistors (no meter or potentiometer) as in the breadboard version
5. heater wires not running right next to signal wires
(which makes it even more shocking that the breadboarded amp was hum-free)6. top plate grounded through thermistor
Since the 60Hz hum is present with no tubes installed and only heaters switched on, I suspect #1 or #6 as the culprit.
At any rate, it sounds quite lovely. Diana Krall is in my living room.
don't underestimate the effects of magnetic coupling!
i once had someone send me a Crate 60 amp that had a hum problem that was only resolved by relocating the output transformer and turning it 90 deg to the power transformer.
this came about after much discussion with the tech's at SLM Electronics(the manufacturer)i don't know if they ever did revise the design but it surprised me that something with that kind of problem made it out of the factory.
i once had someone send me a Crate 60 amp that had a hum problem that was only resolved by relocating the output transformer and turning it 90 deg to the power transformer.
this came about after much discussion with the tech's at SLM Electronics(the manufacturer)i don't know if they ever did revise the design but it surprised me that something with that kind of problem made it out of the factory.
1. IMO, for big tubes, like 6S33S /use You both triodes in parallel?/, this is not very good idea use 1 PS for both channels and use 1 chock /600 ma cross 5 Hn ?...wow/......humm depends from current from +B. If You use two channels with two chocks, current will be two time less cross every chock and filtering will be better for every channel.
2. Also, it's not very good idea powering prestage and output stage from 1 point of +B. Better separate these stages.....Add small chock for prestage /from Hammond or EDCOR/ or res 500 ohm after 325 V, and el. cap 200 uF to ground.Than connect plate resistor of pretube.
2. Also, it's not very good idea powering prestage and output stage from 1 point of +B. Better separate these stages.....Add small chock for prestage /from Hammond or EDCOR/ or res 500 ohm after 325 V, and el. cap 200 uF to ground.Than connect plate resistor of pretube.
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I am only using half of each 6C33C at 150mA, so the choke only has about 330mA across it. It is rated for 500mA. I will update the amp to include additional filtering for the driver stage soon...
Today, I elevated the heater potential ~70V above ground. This had no effect on the amount of hum present. I also relocated the parts which are mounted below the power transformer. Again, no change in hum. Finally, I mechanically isolated all transformers from the top plate using rubber O-rings. No change in hum, but a definite improvement in the clarity of sound.
3 Hn chock for both channel is....not seriose....very small. My chock for 1 channel is 15 Hn for 270 ma /for both heaters/. After chock I connected OT. Your idea connect together OT and prestage to +U is not good. Better separate them and connect OT after chock that must be more 10 Hn. /My hmmm is 0 in my 96 dB speakers/. Sorry, IMO, You have mistakes in Your constructions....Your project is for small tubes....6S33S are for monoblocks structures in 2 or 3 or 4 boxes, IMO.
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