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Old 10th August 2006, 03:40 PM   #11
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Quote:
Originally posted by corbato

Ouch...I would move that stand-by switch before the PSU caps.

should be some crazy arcing, eh? it was just an idea. :P
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Old 10th August 2006, 03:46 PM   #12
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Quote:
Originally posted by SY
A few things.

First, a general note- you might want to use one of Hafler's later designs as your starting point.

Specifics: the LF rolloffs from the RC coupling are still only spaced by a factor of 3. You really want them much further apart. Five times should be a minimum, ten times is best.

Also, you've got almost 700u as the input to the pi filter in the power supply. That will cause unneccessarily large ripple currents, and very likely cause the 4007s to pop. Something like 60u would be more appropriate.

Well, I can put in a bigger cap than two .22uF, maybe I should put one .47uF cap and then later a .047? Nice, wide spacing? In any case, what do you mean about ripple currents? PSUDII didn't say that the 4007's would pop, but if need be, I can always leave out the extra 470u, or move it to parallel the second 200uF rather than the first, if that's better. I just had it lying around and figured it could help. I don't have a whole lot of room, ideally I'd have less capacitance and another choke, so this is kind of a quick'n'dirty approach . I kind of made a mistake in the PT I got as well, if it was 360-0-360 I'd have tried very hard to fit another choke (or even two) under the hood for well-filtered awesomeness. Ah well, such is life.

As for Hafler's later designs, I might be stuck with this one unless the later ones can use the 364V B+, the 6L6's and the 6600 ohm 60W OPT's? I don't mind changing the non-power tubes, but it means more hassle and ordering different parts.
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Old 10th August 2006, 03:57 PM   #13
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Your likely saturating your power xfmr making noise even if the diodes (and possibly why) dont pop, stick those caps on the other side of the choke. <100uF is more inline for input.

Quote:
In any case, what do you mean about ripple currents?
I think SY is refering to the very high peak currents when the diodes are conducting.
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Old 10th August 2006, 08:23 PM   #14
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Hmm. I know the power transformer's good for a half amp for the HV, and I'm not drawing the maximum of 5A for the filaments (probably only 3.6) so I think it'll be all right. I'll try sticking the extra 470u capacitor after the choke to start off, but would really rather have reduced hum, even if there should be some ripple currents. The worst that can happen is it's a bit noisy, or maybe I blow a couple 1N4007s, right? After either of which, I'll definitely change some things.
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Old 10th August 2006, 09:12 PM   #15
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You don't want a big cap before the choke, and you really don't need a particularly big one after. Hafler showed a 10uF input choke in the original; that would be OK, but anything up to 50-60uF or so will be fine. The cap following the choke doesn't need to be huge- you hit the laws of diminishing returns pretty quickly, and the amp circuit has pretty good power supply rejection. Maybe 100-200uF at most.

BTW, Hafler's adaption of the Williamson, including a discussion of staggering the low frequency rolloffs, appears in the same volume of Audio Anthology (vol 2) as the circuit you're posting. It's an article well worth reading.
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Old 10th August 2006, 10:02 PM   #16
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I ran PSUDII with a 47uF cap to start and a 150uF after the choke, and got a hum of about 1/8V, which is quite acceptable, but a voltage of about 261V as opposed to 264. I'm hesitant about deviating any further from what I think the schematic calls for, which is 385V, but on the other hand, it's not going to make that much of a difference, right?
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Old 10th August 2006, 10:28 PM   #17
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261 versus 264 is not significant. 264 versus 385 is very significant. If the sim is accurate, you need to use a different power transformer. But I have doubts regarding the sim- it's difficult to believe that the voltage could be that low if you really have a beefy 275-0-275 transformer. If you give me primary and secondary DCR, I can try the sim myself.
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Old 10th August 2006, 10:44 PM   #18
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Quote:
Originally posted by sorenj07
I ran PSUDII with a 47uF cap to start and a 150uF after the choke, and got a hum of about 1/8V, which is quite acceptable, but a voltage of about 261V as opposed to 264. I'm hesitant about deviating any further from what I think the schematic calls for, which is 385V, but on the other hand, it's not going to make that much of a difference, right?
I assume you mean 364V? I would not sweat this, since your line voltage will vary substantially regardless. SY has a very good point about the caps ... The recent tube amps I've had in my system have not had any caps bigger than 40uF and they have each been dead silent (they are choke-input, however). You can have a fair amount of ripple on the output stage B+ and never hear it in a push-pull amp. The important part is having low ripple and good isolation for the first stages of the amp, which is much easier to achieve. In fact you might get more satisfactory results using smaller value film capacitors than monster electrolytics. Something like a 40uF first cap and 80uF second cap would be more than sufficient for silent operation, and would give you the option to add tube rectification at a later date. if you feel like spending a bit of $$$ down the road, you can pick up some ebay motor-run film/oil caps in these values for $10-12/each...which would be a tremendous upgrade from electrolytic. I swapped out a 40/40/20/20 multisection for a 50/50uF LCR electrolytic and a pair of 12/12uF film caps... gigantic improvement in clarity / neutrality.
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Old 11th August 2006, 12:39 AM   #19
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yeah, i did mean 364/361V. The nice thing is, I don't have to go out and buy a holesaw to chassis-mount the beast LCR cap I had, which was 200x200. It's about 1.75" thick and 4" long 0_o. Unfortunately, now I have no reason to use the thing. And it doesn't compare to the 3 550uF 385V can caps I had left over from another project I'm rebuilding . I picked up two 47uF 450V's and have a couple pulls, namely 100uF and 150uF at 400V, so I tihink I'm good to go for both this power amp PSU and the preamp. Now, to wait.
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Old 11th August 2006, 01:00 AM   #20
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Also, for my next project, I've discovered the GLORY of "motor start" or "motor run" capacitors - they're oil, from what I can tell, and VERY CHEAP - a simple ebay search of "motor capacitor" will show you what I mean. If only I'd found these things before buying that stupid 200x200uF 500V can, I'd be a lot better off
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