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    Dyna MK III/VTA 125

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    tubed1

    Posts : 17
    Join date : 2012-01-03

    Dyna MK III/VTA 125

    Post by tubed1 on Mon Nov 02, 2015 5:47 pm

    Greetings Dynaco Tube Gods, Modifiers and Electrical Engineers,

    I realize I may be stepping on M125 kit toes here.  Has anyone ever wondered about the M 125 board in a MK III chasis?  Just wondering how far the stock MK III power and output transformers can be pushed.  Could always upgrade with Bob's new transformers if needed.  There certainly does seem to be enough real estate on the standard Dynaco MK III chasis for four 6550 possibly KT 90 tubes. KT 120 and 150 vacuum tubes may pose a problem with the glass egg envelopes.  I have not measured, but eyeballing the chasis looks like a fit!  The VTA M 125 chasis definitely has more real estate.  I'm trying to get on board with these amps and do appreciate your feedback. Pun intended.


    Last edited by tubed1 on Mon Nov 02, 2015 5:56 pm; edited 1 time in total (Reason for editing : typo)

    Bob Latino
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    Location : Massachusetts

    Re: Dyna MK III/VTA 125

    Post by Bob Latino on Mon Nov 02, 2015 6:54 pm

    Sometimes things look possible but when you get right down to it, they turn out not to be feasible ...

    1. The Mark III chassis is only 9 inches X 9 inches. (81 square inches) There is no way you can stuff four output tubes and two Mark III transformers on that chassis. If you did somehow squeeze 4 output tubes on a chassis of that size, the four tubes would be maybe an inch apart. Way too much heat concentrated into too small an area.

    2. The output transformer of a Mark III has a core size that is too small to give 125 watts out at 20 Hz to maybe 50 Hz. It would probably work at higher frequencies at 125 watts out but the output transformer would get very hot.

    3. The power transformer on a standard Mark III was rated at 200 milliamps. Just biasing 4 output tubes @ 60 milliamps each (240 milliamps total) exceeds the rating on the Mark III's power transformer. That doesn't even include the milliamps used by two driver tubes and a tube rectifier. (if you are using a tube rectifier in the amp ...)

    The VTA M-125 is made on a 10 inch by 12 inch chassis (120 square inches or about 50% larger than a Mark III). The output tubes on the VTA M-125's are 3 1/8 inches apart side to side and 2 5/8 inches apart front to back. As a comparison, the output tubes on each channel of an ST-70 are 2 3/8th inches apart. The power transformer on each M-125 is rated at 450 milliamps - more than twice the rating of a Mark III power transformer.

    Bob

    tubed1

    Posts : 17
    Join date : 2012-01-03

    Re: Dyna MK III/VTA 125

    Post by tubed1 on Thu Nov 05, 2015 8:01 pm

    Many thanks for your quick and timely response Bob.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dyna MK III/VTA 125

    Post by Bob Latino on Sun Nov 08, 2015 7:35 pm

    It is interesting that I ran across a pair of amps where someone tried to convert his Dynaco Mark III's to a "4 output tube mode" for extra power. This amp has a custom driver circuit with (what appears to be) a bias pot for each of the four 6550 output tubes. The rectifier is gone and the amp is using solid state rectification under the hood. The quad cap has been replaced by caps inside the chassis. With the stock Mark III power and output transformer this amp will probably make no more than 70 watts. The power transformer cannot supply enough current to four output tubes to make 120 - 125 watts and the output transformer is sure to saturate at anything above 70 watts at lower frequencies in the 20Hz to 50Hz range. Look how close the glass of the four output tube are to each other. Maybe 1/2 to 3/4 of an inch apart.

    Bob







    Bob

    Guest
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    Re: Dyna MK III/VTA 125

    Post by Guest on Mon Nov 09, 2015 9:44 pm

    I'm surprised there is no hum and other unwanted noise with that rats nest of wiring under the hood. Or maybe there is?
    The proximity of the power tube to each other should not be an issue, seen much closer units happily working for years.
    It is amazing though what people will try and do to get more oomph out of their tune amps. Sometimes, someone knows just enough about tube circuitry to be dangerous!!!....and sometimes they are just clueless!!!

    tubed1

    Posts : 17
    Join date : 2012-01-03

    Re: Dyna MK III/VTA 125

    Post by tubed1 on Mon Nov 16, 2015 12:27 am

    There is just something obscene about what is going on with this MK III chassis and I like it!!!  Nothing like the look of old Dyna Iron.  This build certainly looks like it was at least influenced by Andy Fuchs with those resistors sticking straight up and the individual bias pots.  Amplifier pornography at it's best.

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