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    Dynaco Power Transformer Substitution



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    Join date : 2012-07-15

    Dynaco Power Transformer Substitution

    Post by Bobzilla on Sun Jan 13, 2013 4:53 pm

    What effect (if any) would their be to replacing a Dynaco MKIII transformer (400V-0-400V @ 200ma.) with a Hammond 373HX ( 364V-0-364V @ 403ma.) ? The physical dimensions are inconsequential to me. The other electrical specs are equal to or better and the higher current of the B+ is desirable, it's only the plate supply voltage being lower that is my concern. Am I over thinking this?
    Incidentally, I have a pair of the 373HX's.
    Thanks for the help!!
    Bob Latino

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco Power Transformer Substitution

    Post by Bob Latino on Sun Jan 13, 2013 8:07 pm

    According to the specs at the link below, the Hammond 373HX has 350-0-350 VAC on the secondaries. This is well below the 430-0-430 secondaries of the ORIGINAL Mark III. This 430-0-430 VAC on the original Mark III's was based on the prevailing 115 VAC mains of the 1950's and 1960's to give a B+ in the 475 - 500 VDC range with a GZ34 tube rectifier. Modern Mark III type amps have 410-0-410 VAC secondaries. With 120 volt mains, the Dynakitparts PA-782 replacement power transformer will give the proper 475 - 500 VDC B+ that a Mark III should run on.

    Hammond 373HX specs

    IMHO the Hammond 373HX has secondary voltages which are too low. A rebuild with this Hammond 373HX transformer will work but overall power of your rebuild will be lower than normal. My recommendation is to get the Dynakitparts replacement PA-782 power transformer for your Mark III. The Dynakitparts PA-782 is also considerably less costly than the Hammond 373HX.



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    Re: Dynaco Power Transformer Substitution

    Post by GP49 on Mon Jan 14, 2013 11:22 am

    SOME of the B+ deficiency could be gained back by substituting silicon diodes, with their lower series resistance, for the 5AR4 tube rectifier. Another slight gain could be had by the power transformer not having to provide the filament current for the 5AR4.

    MAYBE by doing this you could get away with it, given the apparently better regulation of the B+ voltage at high current, where the Hammond MIGHT sag less than the Dynaco. If your home's line voltage is consistently on the high side, that might help, too.

    But does the Hammond have a tap for the bias voltage? There is a circuit in Dynaco's transformer catalog which shows a way to derive the bias voltage from one of the main transformer taps, using capacitive coupling of the AC prior to rectification. I tried it when I attempted to replace the burned-out power transformer on a Mark II and it worked, but it could only provide barely enough current to properly bias the Mark II's EL34s.

    If you already have the Hammond, there's nothing stopping you from trying it out. But the way the math works out: if you can gain 10% on the Hammond's B+ voltage by applying all the factors described, and if the "modern" Mark III replacement transformer gives you a 60 watt amplifier, you will still have only a 50 watt amplifier.


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    Join date : 2012-02-18

    Re: Dynaco Power Transformer Substitution

    Post by rmyauck on Sat Jan 26, 2013 12:14 pm

    The Hammond may allow you to roll in less tough tubes like 6550 etc. May even help KT-88 last longer even if you bias lower as a bonus. Also if your line voltage is too high like in many areas it may be good thing there also. I like the 400mA rating for lots of headroom and I bet if you run SS diodes and bigger PS caps as that's possible you will be happier with the bass.

    Some tubes may sound better with a lower bias setting besides lasting longer also, so it doesn't hurt to experiment there also.

    I think there is a suitable bias tap on the 300 series Hammonds.

    Just wondering how it worked out.


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