Dynaco Mark III Fresh Dynakit Build


Project Resources

High Voltage Safety Dynaco MKIII Gallery Original Dynaco MKIII Manual KT88 Datasheet 12AU7 Datasheet GZ34 / 5AR4 Datasheet

Project Notes

This project was a rebuilding of a pair of Dynaco MKIII (MK3) amplifiers that I picked up off of eBay. I ended up getting 3 amplifiers that were all in pretty bad, well really bad shape. This project will outline what I did to get them back into circulation. The gallery has most of the pictures.

After building the Tubes4HiFi ST-70 I had the bug. I was working on the EZ260, but came across 3 BEAT Dynaco MK-III mono amplifiers. They were really rough, but looked complete. After getting them and realizing that they were in really bad shape I decided to do a face lift by tossing pretty much everything. I ended up keeping the power transformers, the power cords, cages and a few other bits.

The original Dynaco Mark-III chassis. Dirty

The original Dynaco Mark-III chassis. Dirty

The first thing to go was the burnt up PCB’s and corroded chassis. I was looking at a few upgrade driver PCB’s and decided to give the one from Triode Electronics a shot. They were pretty flexible in terms of tube configuration, so the order went out.

Next on the list was the chassis. I called the folks at DynaKits Parts for a couple of things, starting with the chassis. They make a very nice and original looking stainless steel chassis, so that was on the list. Next was some general hardware, and new chokes as the ones in the Dynaco’s looked like they were over heated as all the wax had run out of one of them.

Not onto the 3 pairs of transformers. I had removed them from the chassis and the bells were dirty and a bit rusty. Worse off was the cores has rust and were looking bad. I carefully removed all the parts and began the cleaning process. I had a new vibratory tumbler for car parts and it did a great job on rusty bolts, so I tossed them in. After a day or two of cycling them out (they all wouldn’t fit at once) I pulled the last of them out and began the primer. I used self etching gray primer and heated them up with a small room heater as it was cold. Then hit them up the next day with some glossy gray paint. After 2 coats with drying in between with the heater on, they were done. They really came out clean looking.

The transformer cores were a different problem. I pulled the bells off and put in a couple of screws to keep the lamination tight.

Ugly Dynaco Transformers

Ugly Dynaco Transformers

Most of the transformers cleaned up easily with some scotchbright pads. A couple were worse, and I had to use a wire brush to get the scale off. After some elbow grease they looked good. I primered them with black primer and then hit them with black gloss paint. After the first set dried they looked nice but odd as they were just too shiny. So I swapped the paint with black ‘Satin’ and it looked much better. The trick was to not use a lot of paint to cake on the lamination, but just enough for coverage. It almost looks like metal with the satin finish.

Transformers were re-assembled and it seems one had a different bell style, which was not a big deal since I had 3 sets. The last thing I did was to upgrade the old dirty screws that hold the transformer together with some new stainless steel ones from McMaster Carr. I also picked up a bunch of fiber washers for them as well.

Onto assembly of the Triode Electronics driver board. The board was very well laid out and a quality part. One thing I like was that it was a coated board. Some of the driver boards were just plated copper without even silk screen, that’s not a good thing. Assembly

Top view of the Triode Electronics Driver Board

Top view of the Triode Electronics Driver Board

went well but I’ll say the instructions from Triode need an update/refresh as they were scanned and a bit hard to follow until got the overall picture of how I was going to build it. This is primarily due to the many configuration options the board supports. I used Solen caps and everything else was pretty common. I chose 12AU7’s as the tube compliment, but many others could have been used.

Chassis assembly begins with mounting the easy hardware, switch, sockets, jacks etc. Then the PCB, sockets, new CE high voltage cap for power supply, and then the wiring begins.

I picked up rolls of cloth covered wiring as it looked cool in some of the older amps I saw. Not to be worried it’s cloth covered over high voltage plastic wire. I also had the upgraded bias supply to replace the selenium rectifier, and lastly a new locking bias potentiomenter.

For the majority of wiring I followed the Dynaco build manual. It was pretty easy with a bit of slowing down to make sure the wiring to the driver board was done correctly. With the new sockets and clean wiring, and all new parts it was not a bad job at all.

One last modification was to put a diode in series with each leg of the rectifier tube. This still keeps the tube in the circuit but helps prevent issues with tube flashover on start (or so I’m told).

After both amps were completed it was a matter of stuffing tubes and firing it up and setting the bias. For tubes I was happy with

Assembled Dynaco Mark-III

Assembled Dynaco Mark-III

the JJ KT88’s and so I picked up a quad of then as well as 4 JJ 12AU7’s, and finally 2 JJ GZ34’s. With all the glass in these small amps and the polished stainless chassis, these looked fantastic.

I also really lucked out as the amps came with 3 cages, 2 were in really nice shape, the third had a hold drilled in the top for easy bias adjusting so I let that one sit out. The paint was good and I had some new badges that I also picked up from Dynakit Parts. After they were burned in and the bias was stable I hooked them up for a listen and compare to the Tubes4HiFi ST-70.

How did these sound? They sounded smooth and a bit flatter then the Tubes4HiFi ST-70, which sounded crisper and the way I would describe it as a ‘faster’ amp. The Mark-III’s used a different tube set in the driver and I think that’s the primary difference. The Triode Board can use any common tubes, and I think that is what makes it a good design. The 12AU7’s have lower gain then the tubes used on Tubes4HiFi, but I think now the Tube4HiFi amp can be order with a lower gain version now. In any case the amps sounded good, and different which was interesting to me since at this point I only had one tube amp for a reference.

One thing I did find out what that I spent way too much dough on the build. By the time I added up all the money for paint, extra parts and such, I was very close to the amount of just purchasing 2 Dynakit Parts Mark-III amps. Very close. Are the original Dynaco transformers worth it? I don’t know but I would suspect no, and if you are worried about the money just buy a couple of amp kits from your favorite vendor. If you like a project (Like I do) then rebuild these amps were a lot of fun and not too crazy a project.

Have fun and build something!

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