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Best Dynaco MKIII driver circuit for hobbiest build?

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I have a pair of MkIIIs and another for experimenting on. I was wondering if anyone had a circuit (schematic) for the input/driver that's way better than stock?

I was thinking of rolling my own, using a 12AX7 with a 1.6 ma constant current diode to make a true diff input, then cap coupling to a driver stage with a 12BH7 or 6SN7.

It'd be a lot easier for me if someone just said "use this - sounds great".

I know I can buy replacement driver boards for these amps, to use someone else's design, at $250 a pop...

I'd prefer to not spend that kind of money. I'm an EE who can solder. I've owned and modified Acoustat direct drive amps, and I'm still alive typing this in.

If you'd like to help me out, but dont want to post something publicly for me to see, write me at joejas@ihpc.net

Joined 2001
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jjasniew said:
I have a pair of MkIIIs and another for experimenting on. I was wondering if anyone had a circuit (schematic) for the input/driver that's way better than stock?

The front end i'm planning on using in my ST70 (and a pr of monobloc SCA35s) is Allen Wright's differential cascode circuit -- it should work wonders for Mk IIIs too.

An externally hosted image should be here but it was not working when we last tested it.

All the schematics and picturescan be found way down the page on the Vacuum State schematics page (PP-1C Power Amp).

Thanks, Joe

I still havent got my Audio Express CD for last year's article, nor the paper copy I've asked for to take a look at that circuit.

Your implementation brings the number of circuits to choose from to three! The response to this question has been great!

I wonder why no one has suggested some "single ended driver with interstage transformer" type circuit? Why would the MKIII iron be unsuitable for this type of arrangement?
The se driver driving a interstage transformer I think would be a good solution - it all depends upon the quality of the transformer. I'm new to this forum so I don't know what may be available in the archives however I do know that there are many discussions on the audio asylum on the use of IS transformers as phase splitters in pp power amplifiers.

Differential input to ST70

Hope I'm not hijacking this thread but I have an ST70 I'd like to run balanced in. I noticed the offerings on Joe's site are different for the st70 and the MKIII. Should I try to replicate the premium MKIII driver or go with the standard ST70 model?
Cucio input board installed!

Well, it only took me a year and a half...two year old, you know; he likes Daddy's attention ;')

The amplifiers work flawlessly and the sound is excellent; easily transparent to the artifacts of my input source, IMHO.

I have a couple questions, doubt it's going to get answered in a year and a half old thread, but here goes.

I used WIMA coupling caps for the output coupling and cascode bypass applications. I happened to have them on-hand and they fit in the board.

1) I used 0.22 instead of the 0.1uf the design called for, figuring all it needs to do is hold the voltage firm on the grid. While perhaps rational thinking, is it OK? Would a better quality cap at this point in the circuit effect the sound? (IMHO, I dont think there's any audio signal current going through these, so I'd guess "no")

2) What capacitor type or brand (a list is OK ;') would sound better than the WIMA 0.47 400V jobs I now have coupling the driver tube to the output tubes?

Again, I'm really happy with the sound as it is. However, I am open to that it could still be better and I have some $ to spend on these kind of components. Just got a set of KT88s for each amp in the mail, so I have an excuse to go into the amplifiers again too. (I mean, once you take the cage off, might as well...)

Best Regards,
Best Mk-3 mod

I have built and used Joe Curcio's differential cascode input stage (I didn't try the companion output tube HV supply regulator however). In a few words it was overall the best of the bunch. I also used Kevin Kennedy's (KTA) ST-70 input circuit (but modified slightly for the 6550 output tubes) in another pair of MK-3s. Also, a very nice sound...... slightly more of the traditional "tube" sound than the above mentioned Joe Curcio design. Either is a good bet, the KTA being a less complicated affair.

I run a pair of cascaded diff amps in my Mk.iii's. 6SN7>>6BX7. Put a CCS under the 6SN7, a tail resistor to the bias supply( which you will reinforce) for the 6BX7 and hang its anode loads off the corresponding screen of the power tube( pin 4). No Loop NFB will be needed anymore and you will have a spare B+ filter stage to parallel for the B+ of the power tubes.
This works absolutely brilliantly, and 2x 12B4 can be sub'ed for the 6BX7 if you want. I can show you pix of mine and detail circuit diagram if you are serious.
A friend bought the real KTA Mk-3 mod kit. Being somewhat of a novice (at the time) he asked me for some assistance with the project. While I had it in my possession I had a look at the circuit... it was similar to what you suggest (in concept) but using a 12AX7 on the differential input stage and if I remember right 12BH7's on the driver stages. He had the cathodes of the input pair tied to a RC network that was connected to the negative supply which also supplied the output tube bias. Bet it is a nice amp mod. I never heard this particular setup, being that my friend changed his mind and returned the boards to KTA.

Too bad Kevin closed up. He had some nice designs.

And what about Joe's circuit in some Heath W5M's

Oh yes,
I have some stock Heath's that have been rehab'd with good quality parts, new capacitors all the way around. I have been toying with the idea of introducing Joe Curcio's driver circuit to the Heaths and then giving them a listen on my ESLs (oddly, I liked the KTA Mk-3's better......the Heaths sounded "darker")

Joe C, if you are listening, care to comment?


p.s. 12AT7's in the Curcio circuit? Anyone?
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