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In Reply to: RE: " In the bias supply of my push-pull amps" posted by Salectric on July 23, 2018 at 07:36:37
I'm curious if you meant one hundred, rather than one thousand micro-farads.
I'm asking because I squirreled away this tidbit of info from Joe Curico regarding bias supply cap value (link below) some years ago. Although he doesn't say what "too large" is when he mentions bias supply caps, he clearly cautions against using "large" caps.
If you did mean 1000 µF, I take it this hasn't been your experience?
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
Follow Ups:
Not a typo. I hadn't seen that comment by Joe Curcio before, nor indeed anything cautioning against using too high a cap value in a bias supply, but I agree that this value is higher than normal.
All I can say is that my particular amp came with these Nichicon 1000uf caps already installed and the amps worked fine. In my quest to improve the sound quality, I wanted to replace these "cheap" caps with something better, only to find that all other caps I tried did not sound as good. The other caps I tried were more conventional values---50 to 200uf---so now that I think about it I can't say for sure whether the better sound may have had something to do with the value as well as the type of capacitor. However, I have tried the same 1000uf FW caps in other power supply positions and was pleased with the sound.
I posted regarding the "how much is too much?" question over six years ago. Needless to say, my MK-III project was left in mothballs longer than I expected.
I only got one answer to my question, which is linked at the bottom. If you search DIY Tube for "Regulated bias" you'll get 200+ answers, most of which say not to regulate bias without regulating B+, but nothing I read was clear regarding how much capacitance would cause a bias circuit to act as if it were regulated.
Back to the cap type. I used a couple of Nichicon KW on the B+ "in" position on my Millett LR phono board. I just took a look at the KW vs FW data sheets. If flowery prose means anything, the FW says "Standard, For Audio Equipment", while the KW gushes, "Realization of a harmonious balance of sound quality, made possible by the development of new electrolyte" . Sheesh, someone gets paid to write that stuff.
FW data
KW data
I was going to comment that I'd never heard of the FW before today, but that's not true. I just checked, and have two 22,000 uF on my shelf, waiting to go into [ahem] your preamp (Last PAS). I've grown tired of these two 16 oz beer can 120,000 uF caps dangling off the back after so many years.
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
Let us hear how the big FW caps do on your heater supply (in place of the really big caps!).
I actually ordered a bunch of FW caps to try on my phono heater supply. I have had some old Panasonic HFQ caps there for over a decade. At the time the HFQ caps were the best sounding of the several heater caps I tried, but that was before I found the Nichicon FW.
By the way my bias supplies have a full wave bridge rectifier. Not sure if that makes any difference given the low current.
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It will be interesting to see if I can live with a 2/3 reduction in heater supply capacitance. As you noted in your article, I've always heard improvements when I've temporary added more , as shown in this picture from many years ago. There's about 68,000 uF normally on the heaters. The day of the picture I attached an extra 54,000 uF, along with some ASC motor runs, which all made an improvement.As you can see, I went with the transformer externally mounted, a la Van Alstine, when I built it, so there is room inside for two more 22,000 FW caps.
I've always wanted to try regulating the filaments just to see if I liked it, but the voltage doubler's 25V stopped me. I have a 12.5 regulated supply here, but was reluctant to cut traces just as an experiment. Nothing ventured, I suppose...
Regarding your full wave bridge, there are those who swear by full wave or full wave bridge bias over on DIY Tube, and those who say a half-wave is more than adequate. Typical objective vs subjective banter, but I have no experience to add to the discussion. I have decided to use a Cree diode though, since they're cheap enough, and some swear by them, even in half-wave configuration.
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
Edits: 07/23/18
Your photo brings back memories for sure. As I recall, the most I ever had was four 72,000 uf caps with each pair in parallel. That was back in the day of electronic surplus stores. I paid $5 each for those caps.
Just like you described, each time I increased the capacitance the sound was better. I wish I had spent more time though trying different supplies—for example, a full wave bridge instead of the doubler or a battery like Trevor Lees recommended. One thing I learned considerably later was the importance of using separate transformers for the heater supply and the B+. That is one of the easiest and most cost effective improvements to any piece of tube equipment.
I thought it might. I had to dig a little to find it, but thought you could appreciate it.
I got my two 120,000 uF caps used, at a hamfest, I believe. I check them periodically with my ESR meter, and they still check good, even after all these years.
As you noted at the time, the PA-211 PAS transformer couldn't supply the current, so the doubler had to be used. It's interesting that we're having this conversation now, because I've been discussing my options with a friend of mine over the past several weeks.
I have an extra, unstuffed regulator board sitting here, since I bought two from Old Colony at the time, "just in case". I've always had the idea of trying it dual mono, which you also mentioned as a possibility. Of course that would require a separate chassis, attached via umbilical(s). If I were going to get that complicated I'd do exactly what you've said, and use a separate filament transformer.
I've had other people tell me "Leave it alone and just build something else". Maybe it's because it was my first successful build, but there's always been this urge to see how far I could take it.
I've said this to you once before, but thanks for the way you wrote up the project, like kit instructions. It gave me (and other new builders) the confidence to learn more, and its worked flawlessly for going on thirty years now. There's been a lot of musical pleasure squeezed out of that little box!
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
You're certainly welcome. I am glad that it has brought you a lot of musical pleasure over the years. A lot of people got their start in DIY with those PAS mods including a number of well known names in the industry.
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