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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


    Optimal voltage for our amps

    Mr C
    Mr C

    Posts : 94
    Join date : 2019-07-11
    Age : 65
    Location : Visalia CA.

    Optimal voltage for our amps Empty Optimal voltage for our amps

    Post by Mr C on Wed Sep 30, 2020 1:55 pm

    I'm wondering if there is an optimal voltage for our amps. I've read they should be okay up to 120. The use of a variac is recommended. Also a bucking transformer can be used. My variac is usually set to 116-118. I was playing with a bucking transformer and I get about 110 off one tap and 120 off the other tap when my mains are at 124+. Is 110 too low? So I'm wondering what is the sweet spot? At what voltage are these amps running exactly in spec as designed?
    PeterCapo
    PeterCapo

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    Join date : 2008-12-05

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    Post by PeterCapo on Wed Sep 30, 2020 6:12 pm

    Mr C wrote:I'm wondering if there is an optimal voltage for our amps. I've read they should be okay up to 120. The use of a variac is recommended. Also a bucking transformer can be used. My variac is usually set to 116-118. I was playing with a bucking transformer and I get about 110 off one tap and 120 off the other tap when my mains are at 124+. Is 110 too low? So I'm wondering what is the sweet spot? At what voltage are these amps running exactly in spec as designed?

    It depends which amplifier you're talking about and how its power transformer primary is wound.  Specs are usually quoted at a certain value of AC mains across the power transformer primary.  Once that's established, the amplifier should meet its specs optimally at that value of AC mains.  There is some functional tolerance on how wide the value of AC mains can vary, though I don't think I can remember seeing any data on how the amplifier's performance changes with different values of AC mains.

    Generally, I'd think if the AC mains runs lower, the filaments and the B+ will be lower, unless the amplifier has some kind of built-in regulation.  If the AC mains is running lower, the amplifier won't make as much power, but I can't tell you how much the difference would be.  A lower filament voltage may also affect the amplifier's performance, and I am mentioning this because I remember reading posts here and there where some folks have reported noticing a sonic difference depending on how high or low the filament voltage is running.

    With somewhat more specificity, and speaking of the original Dynaco circuits, the original manuals contain information about allowable range of AC mains.  For instance, specifications were established for many original Stereo 70s with 117VAC on the PT primary (later Stereo 70s may have had a 120VAC PT primary).  Offhand I recall that the acceptable upper limit of AC mains for a Stereo 70 with a 117VAC power transformer primary is 125VAC.  With later 120VAC PT primary, the maximum would be 128VAC.  Similar information should be available in the original manuals for the Mark II, Mark III, Mark IV and Stereo 35.  It's up to the amplifier's designer to specify the optimal value of AC mains along with its upper and lower limits.  

    I am not familiar with bucking transformers, but I suspect that they share in common with variacs that they do not regulate the AC mains.  In other words, if you set a variac to 117VAC, it won't stay at 117VAC output if the mains from your wall varies.  Furthermore, the output from a variac shouldn't be set by the scale or meter on the variac itself.  The variac's output needs to be adjusted with the use of a separate meter, such as a handheld multimeter.


    Last edited by PeterCapo on Wed Sep 30, 2020 7:39 pm; edited 1 time in total (Reason for editing : Insert quotation.)
    Mr C
    Mr C

    Posts : 94
    Join date : 2019-07-11
    Age : 65
    Location : Visalia CA.

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    Post by Mr C on Wed Sep 30, 2020 6:29 pm

    I have a VTA ST-120, So that's what I'm asking about. I usually have to make small adjustments on the variac when I turn on the system. Bucking transformers give a percentage voltage reduction. Like 7% - 12% depending on transformer. As for VTA amps I was wondering what the sweet spot voltage is.
    PeterCapo
    PeterCapo

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    Join date : 2008-12-05

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    Post by PeterCapo on Wed Sep 30, 2020 6:50 pm

    I have a Latino Stereo 70, however I have not yet had a chance to delve into it.  But, IIRC, its power transformer primary is wound for 120VAC.  My assumption would be that it's the same for the ST-120, in which case I think this would be the answer to your question.  But Bob would have to confirm whether or not his PT primaries are wound for 120VAC, in which case they'd presumably both function optimally with 120VAC across their respective primaries, exclusive of a thermistor or anything else in series with the PT primary.  

    I am not clear on their respective upper and lower range of tolerance.  I'm sure they function with something less than 120VAC down to some lower limit, that, again, Bob would have to quote.  The upper limit should probably be of greater concern as damage could occur if the voltage across the PT primary goes too high for the design.
    PeterCapo
    PeterCapo

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    Join date : 2008-12-05

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    Post by PeterCapo on Thu Oct 01, 2020 6:51 pm

    So I just looked at the schematic that came with my Latino Stereo 70.  The power transformer primary appears to be marked as 117VAC.  But, here's another way to tell.  The filament for the 5AR4 rectifier is 5.0VAC and the other filaments are 6.3VAC.  If you adjust your variac so that you get 5.0VAC on the 5AR4 filament and 6.3VAC on the other filaments, then you should also be at the "optimal" voltage on the PT primary that you could then measure to find out if it is 117VAC, or 120VAC or something else.
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    vinylb

    Posts : 7
    Join date : 2020-09-08

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    Post by vinylb on Fri Oct 02, 2020 5:11 pm

    I have seen mention on this forum that some folks use UPS or “power unit/conditioner” with AVR. I tested the output voltage on my Monster power unit and was alarmed to find it at 125v. Subsequently my APC ups with AVR also had output of 125v. I called 2 manufacturers and they claim it’s within 10% of 120v so performing as expected. My choice is to get power company to turn down the juice at the transformer near my house or install my bench variac in my main system and set it to 119v, then connected my APC ups to it and the lcd screen shows output of 119v. Of course I did the latter for now and listening to this fine amp with no worries of shortening life of tubes or the entire system. I once had an ARC blow tubes and resistors and an expensive visit to ARC repair tech made me very wary.

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