Dyna MK III/VTA 125

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.

Many thanks for your quick and timely response Bob.

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 The quad cap has been replaced by caps inside the chassis.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

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!!!

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.