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Modified Dynaco Mkiii

Jeff Litt.

Jeff Litt.

New Member
Hello Everyone,
To start out, I have visited this forum over the last 3 years for research and information. I finally made an account to post some information about my equipment. This site has been very helpful in research and decision making on my equipment / tube selection.

I have a pair of Dynaco MKiii amps that have Constant-Current Differential-Amplifier (CCDA) by ADI conversion. They have also been converted to run on EL34's. I have posted a few pictures to go along with the post. There is more documentation on the ADI conversion, it was just to much to post.

These amps belong to my father and about 4 years ago he gave them to me. He was tired of messing and constantly adjusting them. He switched to a Lynn integrated amp. As the story goes, he purchased the original Dynakit back in the 60's and built the factory amp. He wanted to upgrade the amps and did the CCDA change, plus converted it to EL34's. I guess KT88 where getting hard to find good quality in the 70's. The person that did the upgrades also took some liberties in upgrading the design how they sought best. It's made it a little challenging for me as the amps do not match to original Dynakit or ADI conversion prints.

The 2nd year of having them I lost sound out of one of the amps. Due to their age I decided it was time to recap them. I successfully recapped the first one and found the cause of sound loss (broken resistor on the 400V path). The pictures here are of the other amp, not re-capped yet. I will be going through it here over the next weeks to recap.

Thanks,
Jeff L.

IMG_4170.JPG IMG_4171.JPG IMG_4172.JPG View attachment 2388298 View attachment 2388299 IMG_4175.JPG IMG_4178.JPG IMG_4179.JPG IMG_4180.JPG
 
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The best you can do is to revert it to dynaco configuration and get a pair of 6550 tubes.
The world is full of strange "imprivements" most of them undocumented and full of misconceptions.
As it looks the chassies is undamaged, chances are high that the end result can
be a pair of good working and stable Dynaco MkIII
 
Great project!... Let me know if you find repros of the original circuit board... or you choose something else. I'll be watching for updates on progress.... Never heard of your ADI mod. Any reviews on it?


II
 
I confess to being intrigued!
The design looks solid and well-built.
Since they are intact, why not check the tubes and listen to them for a few weeks before reverting one as a parity check.
 
If you ever decide to put it back original, it will cost some for new boards, etc., and the time to do it. Assembly instructions and new boards are still available too.
The original instructions are very good and easy to follow. But if it had been working and sounding just fine as is until that single part failed, it would cost less to just recap them as is and enjoy. If they were mine I would ditch the JJ rectifier tubes. They don't have a good track record for lasting very long. I had one fail myself. Maybe use a Mullard, Tungsol or TAD brand. The JJ's do put on quite the fireworks show though when they let go. :D
 
Will you copy the other amp's values to make it a match? You could make your own schematic of the resulting circuit and post it here for opinions on the mods performed.
 
I disagree with "put it back to original". These output transformers are oversized for EL34 but the extra 20W of the 6550 is just not worth it. Returning the cathodes through the output transformer secondary is also an excellent mod (think Audio Research). I have done both of these things myself.

There is nothing in that CCDA circuit that I would not have done myself back in the day, had I the resources ($$$). I suspect these amps sound a heck of a lot better than the original and I think you are darn lucky to have them!
 
VetPsychWars said:
I disagree with "put it back to original". These output transformers are oversized for EL34 but the extra 20W of the 6550 is just not worth it. Returning the cathodes through the output transformer secondary is also an excellent mod (think Audio Research). I have done both of these things myself.

There is nothing in that CCDA circuit that I would not have done myself back in the day, had I the resources ($$$). I suspect these amps sound a heck of a lot better than the original and I think you are darn lucky to have them!
Click to expand...

I agree. I had a pair of renewed-original Mk IIIs and they were pretty good but certainly not exceptional. The description of 'self-balancing' and 'minimal feedback' suggest that this may be an improvement worthy of evaluation. Dynaco had some great designs for their time, but many have produced valid improvements on those designs. From the description and the observation that the changes do not greatly complicate the layout, I would side with those who say stick with the mod.
 
Whats the impedance on those transformers? Thats really the question that needs to be answered to figure out if they make sense for an EL34. The EL34 will handle considerably more voltage than the Mark III runs at so no worries there.

driver is interesting. Using the speaker secondaries for cathode feedback has shown up in a few Dyna mods. The two differential amps wired like they are is a bit odd though. I've never seen that config before. Usually you get one diff amp used as the phase inverter only.
 
I've checked the internet for more info and found nothing. This mod looks to advance Mk3 specs with a simple circuit... I would like more comments from our experts to see how good it is and updates included. This is so intriguing.
 
crispycircuit said:
I've checked the internet for more info and found nothing. This mod looks to advance Mk3 specs with a simple circuit... I would like more comments from our experts to see how good it is and updates included. This is so intriguing.
Click to expand...
There seems to be zero independent evaluations of this mod. Thus, rebuild to original is my advice.
 
The MKII used the same output xfmr with EL34s. The primary imp. is 4300 Ohms, the MKII might have been missing the 4 Ohm output tap.

That mod is from TU-BE or not TU-BE mod manual by Eisenson. The problem I had with this manual is it doesn't explain why a unit is modified and/or what the mods will accomplish.

Craig
 
The grid resistors are high per the 6550 datasheets. Fixed bias specs a max of 50k ohms. Not sure if that also applies to UL but I assume so. Otherwise I like the circuit. I've used LTP's with ccs tails a number of times. I would repair and try it out.
John
 
The first downside to the mod is the intrusion of solid state devices on the signal path. I realize many audiophiles disagree with this philosophy, but maintaining "transistor-free" design concept has served me well for a very long time. The second issue, which is shared by the original design, is lack of AC balance adjustments. It has been my experience that these adjustments, which are common in "legacy" LTP driver designs, can be instrumental in reducing overall amplifier distortion.

Jack
 
The LTP with a SS ccs absolutely forces AC balance in the LTP stage. The global feedback should work on the output stage issues. SS does some things well, tubes other things. My opinion of course. I also understand and enjoy the "purist" approach. My favorite amp is differential in both PP stages with ccs's and all transformer coupled.
John
 
That is remarkably like the first set of mods I did to my Mk.iii's. I ran a 6SL7 input stage to a 6SN7 driver. It worked quite well. I did come up with better, but that is another story. I did upgrade to a DN2540 cascode CCS before I built something else from another set of Iron( A441's).
cheers,
Douglas
 
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