Dynakit MK-IIs and MK-III – What Do I Have Here?!?!?

Hello all

A good friend of mine inherited four vintage mono-block amps from his late father:
A Knight Linear Basic Deluxe
One Dynakit MK-II
One Dynakit MK-II (that I think had been moded to a MK-III)
One Dynakit MK-III

https://flic.kr/p/Qx8o6T
https://flic.kr/p/Qx8o4i
https://flic.kr/p/RLUB9c
https://flic.kr/p/Rxp9LL
https://flic.kr/p/RLUAZz
https://flic.kr/p/RLUAPp
https://flic.kr/p/Qx8ncZ

My friend does not have time to deal w/them, and he knew that I’m into vintage gear, and have the basic skill set to deal w/them, so he handed them over to me – let’s call it a semi-permanent loan  

o The Knight and the MK-II are running and sounding fine after a clean-up, but I’ll go ahead and recap them for good measure.  
o The one MK-II (that I think has moded to a MK-III – due to the fact that is has KT-88s and a GZ34/5AR4) does not power up.  
o The MK-III powers up, but has no audio.  

I’ve done a lot of web research in the last month since I’ve got ahold of these amps, and know the basics of what I need to do the attempt to refurbish them, but what has got me puzzled is the presence of what seem to be “extra dual bias” adjustments, and ¼” sockets on the Dynacos – see the attached pictures.   Following the wiring diagrams and schematics, the wiring on these amps seem to-spec up until the point that they get to the bias adjustments, then after that, go back to spec.
o The “moded” (to a III) MK-II has the normal bias adjustment, and then the extra two potentiometers.  
o The MK-III does not have the normal bias adjustment, but the extra two potentiometers.  
o Yes, I know that the GZ34/5AR4 is in the wrong slot on the MK-III, but that’s how it was found.  And when we moved it to where it was supposed to be and powered the amp up, it arced  

So the question is – what are these mods???

o For the MK-II that’s up and running, I’m pretty sure I’ll just recap it, but how and where to measure the bias?
o For the moded (to a III) MK-II, and MK-III, I’m thinking the simplest refurb path would be to just start from scratch, ditch the extra potentiometers, and build to original specs.

Your thoughts and advice are much appreciated.

Thanks in advance.

it appears that all of the Dynaco amps have been modified (at least the bias adjustment).
You can download the original MK2 and MK3 manuals here . . . http://tubes4hifi.com/downloads.htm

Very little getter-flash on those 6550s....

Do you have the means to test tubes?

Peter W. wrote:Very little getter-flash on those 6550s....

Do you have the means to test tubes?

Unfortunately not

If I were you I would strip those guys down and rebuild them with modern circuits, parts, wiring and a fresh set of tubes. It's a great way to learn more about these amplifiers and to ensure more consistent performance. Since you already have the transformers the parts cost shouldn't be too damaging. If you really want to be creative then consider using a different box. There are so many options to make these look and sound their best.

Unfortunately not [/quote]

Where are you? Anywhere near the Philadelphia, PA region? I also know of 'available' testers in several other regions.

I'd say: forget the 6550's  Get fresh matched one from a good dealer and you won't look back.
Btw, you don't need individual bias with matched tubes, thus the original circuit will do just fine.

BTW, these are not 6550, it's KT88. Both works identically in a MkIII, 6550 tends to
be somewhat cheaper, but get what you can find.
KT120 or KT150 is not an option here.

Peter W. wrote:Unfortunately not

Where are you? Anywhere near the Philadelphia, PA region? I also know of 'available' testers in several other regions. [/quote]
I'm in the San Francisco Bay area --> Sunnyvale.

Thanks

corndog71 wrote:If I were you I would strip those guys down and rebuild them with modern circuits, parts, wiring and a fresh set of tubes.  It's a great way to learn more about these amplifiers and to ensure more consistent performance.  Since you already have the transformers the parts cost shouldn't be too damaging.  If you really want to be creative then consider using a different box.  There are so many options to make these look and sound their best.

Thanks corndog71.  I agree.  I'm 95% sure that a rebuild is the route I'm gonna take.