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    Regulated B+ Power Supply?



    Posts : 15
    Join date : 2009-11-21

    Regulated B+ Power Supply?

    Post by vegaseddoc on Fri Mar 30, 2012 11:19 am

    I'm working on a custom chassis for my ST120. In the process, I'm going to use dual rectifiers and regulate the B+ voltage. The electrical grid at my home fluctuates wildly throughout the day (anywhere from 114 to 131 VAC) so I think regulating the power supply may help with the fluctuations. What I need to know is what would be the ideal B+ voltage supplied to the VTA board. Also, how much current is being drawn by the board through the B+ supply? I did notice that the power transformer gets a bit warm. I was curious as to the bias supply from the transformer. Is the bias supply a tap off of the main secondary or its own separate winding? If it is a simple tap off of the main secondary, I was going to add a separate bias supply transformer rated to 300mA to take some of the stress off of the main transformer. Any thoughts?


    Posts : 1441
    Join date : 2008-11-30

    Re: Regulated B+ Power Supply?

    Post by tubes4hifi on Fri Mar 30, 2012 11:48 am

    here are some answers and suggestions.
    The standard classic VTA70 board uses about 17ma at 380vdc.
    The bias winding on the ST70 is a tap on the main B+ winding, (and the bias circuit only uses 2-3 ma of current)
    but the HUGE problem with regulating a B+ power supply is not only the large amount of current needed (about 200ma) but that unless you use some extremely complex auto-tracking regulation system, fixing the B+ voltage will cause the bias to never be correct except at one particular setting, because the small variations in bias voltage will make a huge variation in bias current if the B+ is fixed.
    I discovered this 20 some years ago attempting to use the Curcio modification with B+ regulation (which by the way, is the BIGGEST problem of his modification - I've NEVER seen one that didnt' either blow up the power supply or blow up the tubes).
    If the B+ is not regulated, the bias will always track it near perfectly, since they both flucuate at the same exact percentage of flucuation in the main AC power from the line socket. But if you want to mess with some servo design that tracks and auto-corrects constantly, bring it on!! I'm sure someone somewhere has done it before, for mega-bucks.
    Whatever you do, do NOT regulated the B+ and bias supplies separately!!!!
    I suppose the easy way to do it is use a string of zeners all in series, with a tap-off for the bias on the low end (but don't forget that is a NEGATIVE voltage). You'll need to waste a minimum of 10-20ma of current in the zener string to keep it well regulated at 200ma of output.
    Personally for all the trouble I would buy one of those expensive power conditioners that are available from hi-fi places that regulate the AC power line supply by sampling the AC input and then generating and amplifying the AC to a perfect constant 120vac.
    The PS Audio P500 is a good one. I also see a newer brand for less money that looks excellent: PurePower AVPS PP1050
    IMHO tube amplifiers sound so much better and more realistic with a simple non-regulated power supply.
    Think of it as yourself, breathing normally, full deep breaths when needed (running,peak exhaustion) and resting (shallow breathing) compared to wearing a tight strait-jacket that never lets you take a full breath, but gives you just enough breath to survive. It makes that kind of difference when listening to music (unless you like low-level elevator music all the time).


    Posts : 15
    Join date : 2009-11-21

    Re: Regulated B+ Power Supply?

    Post by vegaseddoc on Fri Mar 30, 2012 7:38 pm


    Thanks for the input on your experience. A train Zener diodes was what I was thinking and I was worried about the extra current draw. I have done some regulated supplies in some preamp designs with some + and some negative results. I have been using a copper cap on my ST120 since my amp eats modern 5AR4s. I have 2 old Mullards that are on their last legs so I'm going to build a new chassis to have 2 rectifier tubes. Though the copper cap is nice and reliable, I prefer tube rectification (call me old fashioned). I'm going to purchase an external regulated supply so that all my components receive steady 120 VAC.



    Posts : 58
    Join date : 2009-01-19

    Constant voltage transformer

    Post by danf on Fri Mar 30, 2012 7:55 pm

    You could also consider a constant voltage transformer. These are sometimes available on eBay for a reasonable price. These do buzz a bit, but they work well in situations like the one you describe.


    Posts : 157
    Join date : 2012-01-31

    Re: Regulated B+ Power Supply?

    Post by hawaii.ken on Sun Apr 08, 2012 5:10 pm

    I'm thinking you should complain about the fluctuation to your power company. Sometimes it's as easy as changing out a defective transformer.


    Last edited by hawaii.ken on Tue Apr 10, 2012 4:56 pm; edited 1 time in total (Reason for editing : added info)

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