Dynaco MKii/iii triode kit project

Bradbrews

Bradbrews

Super Member
I recently picked up a couple of mono blocks.
One is an essentially finished MKiii that has had a triode USA driver board installed, and the caps replaced with new ones in series to raise the voltages. Other new parts are in the unit as well. It seems ready to go, but I've yet to test it and measure Bias voltage.
The other one is what appears to be an original MKii, with very minor changes.
Included is another triode board, an SDS labs capacitor board, and a set of tubes.
The P.O. was going to mod the MKii to be like the MKiii but lost interest in finishing the project. I was told that both amp were working when they were pulled from service.
IMG_4395.JPGIMG_4396.JPGIMG_4397.JPG
The MKii has the large circuit board, so I'm going to need to craft a spacer to be able to mount the triode board in it. The only thing that seemed not original in the amp was that it has an 11.2 ohm resistor instead of a 12 ohm. It still has the Selenium rectifier.
Is there any reason not to just proceed on with modding the MKii as the P.O. planned?
I will need a few more parts for the bias circuit and maybe add a choke instead of the 50 ohm resistor.
Should I try out the MKii with the tubes it has before I start into it? It still has the original old cap can.

Is there any reason not to just hook the finished one up and try it out? I have a DBT, but would need to borrow a variac. I'm a tube amp newbie, so tips appreciated.
 
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Brad- You have a nice pair of amps. there. I would continue to upgrade the Mk.II to Mk.III status. They were essentially the same amp. except the Mk.III added the choke and the 6550 output tubes. Definitely replace the selenium rect, and the bias capacitors, if they haven't been done yet. Stable bias current is crucial.
The "biaset" resistors should be the same in both amps for ease of reading. As an alternative, you could use 10 Ohms and just divide the reading by 10 to get the bias current. I like 1%, 1/2w. resistors here, ends the inaccuracy jitters.
 
If the Mk II has the A-430 output transformer then it has only 8 & 16 ohm outputs, I believe. Could be an issue depending on speakers.
Nice projects.
John
 
I rebuilt a pair of MkII's with the Triode board for my first project. Well worth it and a great education. And yeah, one chassis was large board. I just filled in with a piece of aluminum. And definitely get the choke.
 
Made some progress on the MKii. I've removed the old board, the old can cap, and the selenium rectifier to make way for the new boards. Once everything was out I did a little cleanup of the chassis. Not a full blown, remove everything clean up, but just a little buff with some fine steel wool. I fabricated a new spacer panel to allow the Triode board to fit and installed it.
Next is to wait for the rest of the parts. I'm going to convert all the bias circuit parts to the MKiii design so both amps are the same. I'll add the choke - Thanks for the confirmation bean_counter. A new gold RCA jack to match the other amp also needs to be added.;)
The two amps will never look like indentical twins, as the existing MKiii has a 431-70 output transformer which is in a square can, vs the 430 output in the MKii. That also means that the only common output terminals will be the 8 ohm and the 16 ohm, as nerdorama mentions.
However, with covers on that won't matter as much. Speaking of which I only have one cage, and would like another one...
Here are some pictures:
IMG_4405.JPG IMG_4407.JPG IMG_4409.JPG
 
The parts for rebuilding the MKII showed up, and I had a chance to install them. I added a choke, and completely replaced the bias circuit and turned the amp into a MKIII so that I don't have to be concerned with remembering different voltages between the two units.
The SDS cap board makes for a tidy installation.
The wires from the old transformers were very hard and brittle. They wanted to stay in the position they've been in the last 60 years! I ended up cracking the insulation on the center tap wire even while being as careful as possible. It had to move the most, as it was routed to the old can cap, and now needs to circle back to the SDS board. I put heat shrink tubing around the crack - which was just in the rubber, the cloth still holds everything in place. Anyway, as I used a heat gun to shrink the tubing, I noticed that once the wire got good and hot, it was much more pliable. Had I used the heat gun on the wire in the first place, I suspect I could have rerouted it without issue.
IMG_4439.JPG Now one more check of all the connections and its time to set bias and see if all is well!
I noticed that the first amp has a small cap in parallel with the 47 uF cap on the bias POT. Are there any high frequencies in the circuit there that really make this necessary, or is this just in the name of "It can't hurt". None of the mods I've seen mention this. You can see it in the photo in the first post.
 
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Bradbrews said:
The parts for rebuilding the MKII showed up, and I had a chance to install them. I added a choke, and completely replaced the bias circuit and turned the amp into a MKIII so that I don't have to be concerned with remembering different voltages between the two units.
.
Click to expand...
I noticed in your photo that you have grounds in at least 2 places. This may work out but ideally you would want to have all the power supply grounds and the circuit board grounds and the bias supply grounds on one point. Some builders use a seperate ground for the input and output but those grounds could also be put with the single point. From a technical point having a star ground is considered best. Otherwise looking good. regards, Primo
 
Primo, You're correct, that was probably not ideal. I rationalized it after noticing that the original aluminum can cap, the 11.2 ohm bias resistor, and the bias capacitor hooked to the center leg of the bias Pot all are grounded to different places on the chassis. This seems to be true in even the new Dynakit instructions. I should probably move the black ground wire that is the SDS cap bank over to the transformer ground - one bolt over.
I did go an add a small 700 pico F cap in parallel with the bias cap on the POT just because.
After that I set bias on both amps and after successfully testing them on a junk speaker for ~15 minutes, decided to hook them up for real.
IMG_4445.JPG After about 45 minutes of operation, I noticed that the power transformer on the amp on the right, the one that started life as a MKII, was hotter than the transformer on the other amp. Getting out the infrared thermometer revealed that it was continually rising in temp as well. After about an hour operation, the temp hit 150F, and I power everything down. The other amps power transformer stabilized at 110F.
I suspect a problem, so back to the bench to look for issues that could drive higher currents that intended. I suppose worst case is a short in the winding.

I also noticed that the cap called C3, on the circuit board closest to the KT88's got hot on both amps. They both stabilized at about 190-200F. This seems hot to me for a capacitor, but this is my first time with tube amps.
 
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I’m going to revive my old thread as I’ve been listening to these and have decided to work on them some more.
When I got these I knew almost nothing about tube audio, and only basics about solid state. I’m a little further along now, and looking at the amps I decided to upgrade them.
- add dual bias
- revise the grounds, I’ve got all of them going to the chassis in at least 4 different spots.
- revert to a 2 prong plug as originally done.
- possibly add a switch to go between 12.6v heaters and 6.3 v on the driver tube.
- replace the two piece ceramic sockets as they have been a problematic pain in the…
Here’s where we stand now.IMG_9637.jpegIMG_9636.jpeg
 
primosounds said:
I noticed in your photo that you have grounds in at least 2 places. This may work out but ideally you would want to have all the power supply grounds and the circuit board grounds and the bias supply grounds on one point. Some builders use a seperate ground for the input and output but those grounds could also be put with the single point. From a technical point having a star ground is considered best. Otherwise looking good. regards, Primo
Click to expand...
This would be a good time to revise your signal ground wiring. The CT of the HV winding should be extended and run to the ground terminal of the cap board nearest to the input end. Then you won't need the other black ground wire running to the AC power ground. That should make the hum noise in the background quieter even though the feedback may be covering it up. The signal ground then will be entirely in the cap board and on the audio board with no connection to chassis. This has always worked to be a quieter solution for me. Sometimes I've added a ground isolator (bridge rectifier, cap and resistor) circuit between signal ground and chassis.
 
This is the foil of the SDS cap board.
It looks like I would need to cut the traces between the bolt and the rest of the ground plane to do that.
Otherwise this connection undoes the benefit right?
Do you move the input jack ground to this plane also?
 

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I used nylon threaded standoffs on my cap boards so there is no connection through that bolt. But you will get benefit of having the CT wire connect directly to a ground point on the pc board as it will keep the highest ripple current out of the chassis and return them to the nearest point in the supply to the first cap after the rectifier.
 
Bradbrews said:
I recently picked up a couple of mono blocks.
One is an essentially finished MKiii that has had a triode USA driver board installed, and the caps replaced with new ones in series to raise the voltages. Other new parts are in the unit as well. It seems ready to go, but I've yet to test it and measure Bias voltage.
The other one is what appears to be an original MKii, with very minor changes.
Included is another triode board, an SDS labs capacitor board, and a set of tubes.
The P.O. was going to mod the MKii to be like the MKiii but lost interest in finishing the project. I was told that both amp were working when they were pulled from service.
View attachment 1164380View attachment 1164385View attachment 1164389
The MKii has the large circuit board, so I'm going to need to craft a spacer to be able to mount the triode board in it. The only thing that seemed not original in the amp was that it has an 11.2 ohm resistor instead of a 12 ohm. It still has the Selenium rectifier.
Is there any reason not to just proceed on with modding the MKii as the P.O. planned?
I will need a few more parts for the bias circuit and maybe add a choke instead of the 50 ohm resistor.
Should I try out the MKii with the tubes it has before I start into it? It still has the original old cap can.

Is there any reason not to just hook the finished one up and try it out? I have a DBT, but would need to borrow a variac. I'm a tube amp newbie, so tips appreciated.
Click to expand...
wrong title, this is no longer a Dynaco MkIII, it's a derivate that differs
 
I totally agree. Do you think I should have started another thread instead of continuing on this old one from 6 years ago?
It’s on the same mono blocks…
 
For revising the grounds in this unit, I can move three of them to the ground plane of the SDS cap board quite easily, the red lines proposed on the sketch.
However, the bias supply grounds and the cathode resistor ground are a little harder, especially the sense resistor, as it's tied to the tube socket ears. I could run a parallel wire to the cap board ground plane, but that's a parallel circuit for the ground. Those are the red circles.
When I add dual bias, I'll have two sense resistors, so another ground.
What's the best path here?
Ground ideas.jpg
 
I think if you move the HV CT wire to the cap board ground nearest the end with the first cap stack and then the black wire that now it tied down at the power entry ground can move to the other ground connection on the board. At this point I wouldn't worry as much about the sense resistors but you need to leave the cap board grounded to the chassis, as you have now, so the cathode currents have a return path to audio ground on the board.
 
You can retain the grounded cord and just isolate the signal grounds from the earth ground to deal with any ground loop issues. That would make it safer than the original design without the problems that can be created with 3 prong cords. Just a thought.

Extra easy if you're re-working the grounds to a single chassis tie point. Instead of going to chassis, land it on the bridge rectifier / loop breaker. The output tube cathodes have to be grounded to the same point as the power transformer center tap if you do this, not to chassis.

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I had a chance to rework the grounds.
The loop breaker is interesting, but I just went back to a 2 prong plug for now.
The signal grounds now all go to the ground plane of the SDS board. It’s then grounded to the chassis at the same point as the bias supply. Turns out my board is Not grounded to the bolt so it needs a wire.
I’m going to give it a listen now before moving on to dual bias.IMG_9647.jpeg
It’s maple syrup season so progress is slow…
 

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