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Old 20th March 2010, 11:34 PM   #401
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If you want to, you could probably also get rid of the second stage of filter caps. The regulator is really good at getting rid of 100-120Hz ripple. They certainly won't hurt anything, though.

Here is a picture of my heatsink, salvaged from an industrial motor inverter. It's overkill.
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Old 21st March 2010, 01:17 AM   #402
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Another question:

What do R44 and R45 do?

I also had another thought. You could use a 5U4 to drop a little more voltage to get regulator dissipation down.

Anyway, none of these suggestions are meant to push you to do this a certain way. I'm just trying to open your eyes to some options.
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Old 21st March 2010, 03:13 AM   #403
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Quote:
Should R17 go to ground?
R17 in theory should go to ground. My latest version of this design has R17 going to the wiper of a pot. The ends of the pot go to + and - 2.2 volts generated by using LED's as zener diodes. Why, to allow nulling out those little offsets that add up in a direct coupled design. It's adjustment drastically lowers the distortion at high output voltages, but may not be needed here.

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Should R56 be there?
No.

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What do R44 and R45 do?
They are anti-blow-up resistors. If the pot wiper goes open the gate of the mosfet floats. A nano amp of leakage in a coupling cap will put +75 volts on the grids of the output tubes, making then very unhappy!

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I'm a bit surprised to see the plates powered by the Miata regulator.
Early on in this multi year project Chris scored a pair of Hammond 300BX power transformers. The unregulated B+ from these would be a bit much for a 25 WPC 6L6GC amp, even by my standards. Chris opted for active regulation over a choke input filter, and this is a wise choice for an AB1 or AB2 amp. A class AB amp likes a low power supply impedance. I will probably build two amps using this design. One will be a monster using a BFC across a typical CLC supply. The other will be a lower powered amp, but I won't know the power supply requirements or its details until I figure out what the amp is.

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Why not use the full 400V to supply the first stage?.....As for the full 400V for first stage, I suspect that tubelab first drew it as you describe, I will go back and check...
No, I drew it like it is shown here. My first few itterations of this design had low frequency instability issues, so R24 and C16 were added. They are still present on my latest circuit boards, although in theory they should not be needed. Never got around to testing without the parts.

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The mosfet in the regulator is configured as a follower, so as current increases, the 11 Ohm resistor will start dropping voltage when more current is drawn through the regulator. I think that the purpose is to make the regulator short-circuit tolerant.
The 11 ohm resistor is for current limiting and maybe short circuit protection. That is the explanation given in Michael Maida's original paper. I can tell you that even with the resistor in place the mosfet will violently explode if the output is shorted after the circuit is already on. It may offer some protection if the short is present at turn on.

I have been gone all day. I traveled to a hamfest where I got 93 new test subjects including some more 6HJ5's from ESRC, his supply of these tubes is nearly gone. I also got a big box full of assorted used tubes (mostly sweep tubes) for $5. I just got to hook this breadboard up. There is some glow testing to be done.
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Old 21st March 2010, 03:23 PM   #404
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Quote:
Originally Posted by tubelab.com View Post
The 11 ohm resistor is for current limiting and maybe short circuit protection. That is the explanation given in Michael Maida's original paper. I can tell you that even with the resistor in place the mosfet will violently explode if the output is shorted after the circuit is already on. It may offer some protection if the short is present at turn on.
I shorted the regulator in my preamp twice. It's based on the original paper. Both times it instantly blew the transistor, the LM317, the series resistor, and the zener.

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...some more 6HJ5's from ESRC, his supply of these tubes is nearly gone.
Dang, and I just ordered some more from him. I guess we'll see if he can actually fill it.
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Old 21st March 2010, 08:16 PM   #405
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Dang, and I just ordered some more from him. I guess we'll see if he can actually fill it.
I got 20 more 6HJ5's plus about 70 more dollar tubes from Stan at a hamfest on Saturday. He told me that someone had just ordered some, plus my 20, lowered his supply. He had about 200 of them, and said that no one ever buys them, in January.
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Old 21st March 2010, 09:24 PM   #406
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I guess we'll see what happens. I ordered them via his website, which doesn't really provide any feedback. Parts just show up one day.
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Old 21st March 2010, 11:10 PM   #407
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Thanks everyone for the continued help. I have tidied up the schematic:

R17 to ground instead of the negative supply.
R56 removed.

Questions:

Should I reduce the value of R32, or is the consensus that the 40.2K is OK? I have the 40.2K part already in a TO220 type ready to mount on the heatsink. With 5U4GB (if used) the current through that zenner will be down to under a milliamp...

I think I recall tubelab saying that he had trouble with the IRF820 Mosfets blowing. I had added protection zenners to the MOSFET followers D9 and D10. Are these incorporated correctly?

Good Point about the 5U4G rectifier dropping more volts. Chassis space is tight and the ones I have are the big-bottle style... Also a consideration, is a direct-heated cathode a problem with faster B+ application? Bias supplies are solid state rectified, but have VR tube rectification. Not sure how long the VR tube takes to come on. In any case, I have quite large heat sinks ready for the regulator and pass MOSFET.

With 400V transformer CRC 110u/100R/110u and 180mA, PSU II is showing about 485v with 5AR4 and 433v with 5U4GB. The Hammond transformers do run over-voltage however, so expecting that the voltage to be dropped by the regulator will probably be more like 100v for 5AR4 and 50v for 5U4GB
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Old 24th March 2010, 03:45 AM   #408
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I like the idea of the resistors to protect against an open wiper.

chrish, it's your call on the 40.2K. I think it will work, but Jones is right that noise on the gate of that mosfet will be lower with more current.

As for the zener, I have been putting back to back zeners to the other side of the stopper (since I have the stopper hooked directly to MOSFET pin and zeners are on a circuit board). I don't know what is better, zener tied to gate pin or the other side of the stopper.

VR tubes come on quick since they don't have a heater/filament. They are cold cathode tubes.

I think you are wise to assume that your output voltage will be a bit higher with the Hammond transformer.
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Old 24th March 2010, 03:57 AM   #409
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I hope nobody minds the off-topic but we have been discussing George's universal driver here. I just wanted to show some early results with 6SN7 at 780V supply and 10M90S cascode plate loads. It is 200V/div with the 100X probe here. Is this enough for G2 sweep tubes or do they need more?
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Old 24th March 2010, 04:22 AM   #410
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Thanks for taking the time to answer my questions, I really appreciate it!

I have been planning chassis layout and assembling the power supply, no soldering yet, just organising the parts on some perf board. I figure I have plenty of Amps available from the transformer, so opted to follow Jones' advice, used a 12K in that spot.

Not quite sure what you are describing re the zener protection of the MOSFET, but I think it is as per the attached image...

I did some measurement of the Hammond transformer for some accurate PSUII modelling. Unloaded voltage was 436VAC, the secondaries were 48R and 50R, primaries 6.5R. PSUII calculated a voltage of 499 volts when loaded with about 180mA (99 volts to drop in the regulator, close to estimation). I will try with a 5AR4, then if the voltage turns out to be too high and things are getting a little warm, I will try a 5U4G.

When I am sure I have the right setup for protection zeners, I will re-post the schematic.

Hopefully I will have the power supply ready to test tomorrow. I am assuming that since I have about 8mA passing through the voltage set divider in the reg (data sheet says 5mA min), I should be able to test it with no other load to confirm it is working OK?

Regards,

Chris
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