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Rare Dynaco Mark VI Frankenstien

F

fmajewsk

AK Subscriber
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So I picked these up a while ago! They are the Japanese Dynaco Mark VI with potted transformers. An NY tech called 'Tube God' in Hudson, NY, modified this by adding external power regulators with 1800uF caps that regulate the B+, a 6ns7/J-Fet input, and a regulated bias supply. These modifications significantly enhance the amplifier's stability. It also runs 8417 output tubes (fun ones that can thermally run away). I managed to go through both units, bias them, and get them working! Sounds nice, but I am a little hesitant to use them all the time because of the output tubes.
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It sounds like he may have taken care of the stability issues? There may vary well be other modifications done to them as well. I probably wouldn't run them too long without replacing the electrolytic capacitors in the amps and regulator boxes. Unless this wa done recently. If those a.ps were mine I would use them with the cages removed for better air circulation and to be able to see those power tubes in case one started red plating.
I almost bought a pair of these at a very reasonable price years ago until I read up on them.
Those TO-3's sitting on top of those boxes almost look plastic and translucent in that picture.
 
Much of the "instability" of the MK VI was due the 8417 tubes themselves. Many (but not all) amplifiers using this tube typically had major burn out issues around the output stage. The MK VI ran them at a lower B+ level in UL than many did, and wasn't so bad about it.

Dave
 
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Customer walked through my stores door in about 1998 with a pair of Dynaco MK VI un-built kit's in the box.

So we built them as Triode connected 8417's using the UL windings.

What killer amps! They did 70 watts RMS and sounded like an ST-35!

Ahhh, The good old days.

Pictures were scanned off my old polaroid's so the Left Right got reversed.

Cool amps! :)

The small variac in the back is still my most usable door stop...

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Lavane said:
It sounds like he may have addressed the stability issues? There may well be other modifications as well. I probably wouldn't run them too long without replacing the electrolytic capacitors in the amps and regulator boxes unless this were done recently. If those amps were mine, I would use them with the cages removed for better airflow and to be able to see those power tubes in case one started red plating.
I almost bought a pair of these at a very reasonable price years ago until I read up on them.
Those TO-3's sitting on top of those boxes almost look plastic and translucent in that picture.
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Oh yeah, I put new filter caps, fixed the reg bias circuit setup, tested all the cathode resistors; also, it is set up for individual bias per tube. Found 2 open bias pots, etc., and I'm hoping to post the updated pics.
 
There was a pair of these at the auction at Kutztown. I saw in the box next to them the cages and at least some 8417 tubes, no idea if it was a full set. I admit there was a bit of morbid curiosity but I really don't need another amp, especially a pair that are this big, or that use such an expensive tube set. The idea of getting a pair and re-configuring to run KT88 or whatever is interesting too but I really didn't want to spend the money or add yet another thing to my to-do list.
 
gadget73 said:
There was a pair of these at the auction at Kutztown. I saw in the box next to them the cages and at least some 8417 tubes, no idea if it was a full set. I admit there was a bit of morbid curiosity but I really don't need another amp, especially a pair that are this big, or that use such an expensive tube set. The idea of getting a pair and re-configuring to run KT88 or whatever is interesting too but I really didn't want to spend the money or add yet another thing to my to-do list.
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Hopefully the modifications done to the OP's amps made them mu h more stable. I would love to try some 6550's in my PPP amps, but thw cost of eight 6550's has kind of killed my curiosity. I already had a quad of KT-88's when I bought the amp kits, so I only needed four more. Big power tube amps get expensive when using a quad of power tubes per channel.
 
gadget73 said:
they aren't cheap, but price around for 8 NOS 8417 and it seems like a bargain. I don't want to buy either.
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At leased I can still use those mono blocks with only two power tubes per amp if I loose a tube. Then they're basically just Mklll's @ 60 watts instead of 125 watts. Having to buy just four 6550's would be cheaper than eight if I wanted to try some.
 
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