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In Reply to: RE: Experience with RC coupled power stage? posted by twystd on January 22, 2015 at 15:55:52
The 3K resistor is in AC parallel with the OPT, for a net load impedance of ~1500 ohms. For good results you would like the resistor to be 10 times the load impedance; that's 30K and at 75mA will drop 2250 volts. That's why you've never heard this before!
Follow Ups:
I must admit I'm puzzled.
I can see why the 3.2K resistor is in parallel with the output transformer for a net of 1500 approx. What puzzles me is what is wrong with a load of 1500 for an Rp of 500 which is what I thought would have been the typical three times ratio.
Obviously I am about to learn something which will be useful since I have used this topology in the past.
ray
, good question!
I was assuming the circuit and voltages were already chosen. To operate into a smaller load impedance, the tube would need to run more current and lower voltages. Such a re-design would exacerbate the voltage problem.
Your post made me look closer, and I see that making the change I suggested would leave more voltage available to drop across the DC load resistor, so it would have a higher resistance, and eat up less of the power. So it's more practical than I thought.
twystd - if you see this and want to follow up, post the circuit and your flexibility about it ...
Paul, a friend of mine suggested a choke that could get the job done, sounds good, and is affordable. I could probably sell the EXO-003s for about the same money that the higher capacity chokes cost, so looks like I'll go for a more conventional design.
I know what you mean by adjusting the op points and tweaking the power supply. I modeled changes to the supply in PSUD2, and played with load lines and op points, but think the more conventional approach is best. Thanks, it's been interesting. I'll post the amp once I have it finished. Probably looking at a power supply chassis, and a separate signal chassis.
twystd
Good chokes are hard to beat...the 801 SET am putting together is an 8 Henry with DCR of 35... center tapped to power transformer and rectifier filament... the center tap configuration was thought up by Baldwin engineers and yields a VERY quiet amp.
The Mind has No Firewall~ U.S. Army War College.
Edits: 01/25/15
That's what I was afraid of, guess I'll have to pop for some bigger chokes.
twystd
Of course, you could make it parallel single ended so it's tuned for the 1500 ohms. Double the power but half of it dissipates in the resistor so the same power as originally envisioned. It's just a single monoblock then though. :^)
I'm going to be throwing away power anyway, and tubes and sockets are cheap, and 5.5 watts is enough power. With RC coupling I avoid loss of highs because of choke capacitance, and I've read that RC coupling gives a better damping factor, so really should have increased audio bandwidth. Did a quick model of the power supply and even drawing 195ma gives enough voltage. BTW I have a very accurate model of the power supply in PSUD2. I might just do it. Thanks again Paul.
twystd
See post above. You might get more like 2/3 the power with a single tube if you'll adjust them for more current, less voltage.
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