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Frankenstein Dynaco MK VI

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Just picked up a pair of Dynaco MK VI mono amplifiers that have been modified. That is actually putting it quite mildly as the pictures will probably speak for themselves. The price was right and the chassis was in decent shape so I though I would give it a whirl. These amps will need some major TLC and unfortunately one of the power transformers is not original. If anyone has an extra power transformer sen me a PM as I am now on the hunt.

Thought I would document the restoration process here and maybe I could get some guidance along the way. I am not planning to restore these back to original for two major reasons: 8417 and 7199. The plan as by the guidance of another member is to use a 12AU7 or 6CG7 in the front end for both the voltage amp and long tail pair phase splitter with a constant current source. I want to use KT88 output tubes and set up an individual bias scheme.

The story behind these is also quite interesting:

I picked these up off of craigslist.org and traveled pretty far to check them out. I talked for quite a while with the seller as he was pretty into tube gear. He actually had a pair of original MK VI's that he sold after getting these "upgraded" ones. He bought these from a gentleman in new jersey that he said worked for NASA. the original owner did a huge amount of experimentation on these amps. Cool part is he documented al the stuff he did and I have a bunch of the paperwork showing the various circuits he made.

Per the seller the amps were significantly better than the originals thus the reason he sold the originals. Unfortunately he moved and when he unpacked the amps found the separate power supply had been damaged. He had no clue how to fix them (understandably if you check out the pics). So they sat for 15 years till I came by.

The original owner may have made legitimate improvements but he had no concept of layout and finish. It looks like quite the hack job. I plan on gutting everything and converting it back to single amps.

Here are a few pics. I'll upload a few of the schematics I have when I get a chance to scan them. This is going to be quite interesting
 

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Here you go. http://www.triodeel.com/images/dynaxfmr.pdf
Dyna's transformers were made by TRESCO in New Jersey.

Thanks a bunch I actually just found a similar pdf. It discusses a 120 watt amplifier using the PC-706 power transformer but doesn't specifically denote the MK VI. I hear the output transformer that was actually used was A-470 and not the A-451 shown in the given circuit. I'll have to see what I can dig up

http://www.clarisonus.com/Archives/Trans/Dynaco_transformer.pdf
 
Played with the amps a little bit today. Used the "Simple Tests" from this website and the outputs so far are checking out OK

Output Transformers

Also removed the single original transformer and all secondaries are accurate. Gonna ship it of to Heyboer to make a couple copies early in the week.

Next step is to totally gut these amps and try to clean up the chassis. Slow and steady...
 
Played with the amps a little bit today. Used the "Simple Tests" from this website and the outputs so far are checking out OK

Output Transformers

Also removed the single original transformer and all secondaries are accurate. Gonna ship it of to Heyboer to make a couple copies early in the week.

Next step is to totally gut these amps and try to clean up the chassis. Slow and steady...

I strongly suggest reducing the B+. These were Horsepower Race amps, and stress the finals with too much HV. Since you are going to use two new ones, this is a very good time to take this on...:)

While you're at it, specify a conservative 125V primary. Perhaps going so far as to ask for 120,125 and 130V taps on the primary. Or just specify a volts/turn on the primary that is 80-85% of standard.
cheers,
Douglas
 
driver mods were popular

Because the driver, just like the rest of the amp, even in the Flagship one was marginal. If you want to keep the topology, use a decent triode and pentode. A 12BY7 at minimum, and something like the 12A4 for the split-load, PI.

Individual bias, will cut the grid circuit resistance( this was marginal too; it was high ). Going lower has always made an amp sound better for me, and a decent PI section will drive it w/o issue.
cheers,
Douglas
 
Here are the values I am getting with the transformer

House AC- 121.8 ACV
HV- 815 ACV
Heater- 6.68 ACV
Bias- 62.9 ACV

There is also a winding that combines with one end of the heater to supply the lamps. This is 10.29 ACV

My plan was to use 12AU7 or 6CG7 as the voltage amp and phase splitter. My idea (with a lot of help from others) was to build a long tail pair phase splitter with a constant current source (still researching on how exactly I want to build the CCS).

As said before, I amp planning to use KT88 output tubes and definitely will be including individual bias ;) .

Right now I am just trying to make sure I have the power supply/transformer modeled correctly. I have very little experience with this so its a learning process.

Again with some help, I have modeled a B+ of just around 505 DCV for B+ using the original transformer. This should be well handled by the KT88 based on my review of the data sheet. What is meant by "finals"... the output tubes? Any recommendations on what I should spec the transformer?

Thanks
 
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