Mk III Rebuild

I am starting a Mk III rebuild and was hoping to post it and ask some questions, plus maybe, get some direction. I removed the transformers from one and was a little worried by the cloth installation. I put Teflon shrink tubing overs the wires.



Is there anything with that? The Teflon shrink tubing doesn't seem to shrink. Has anyone worked with it? Would you use high temp black paint for the core of the transformer?

Thanks,
Jim

PTFE begins to shrink at 650 F (343 C). The mandrel or part to be covered by the Heat shrink tubing must be able to withstand this temperature range.

Thanks Ken,

Is this the right application Teflon shrink tubing? My goal was to protect to the deteriorating cloth transformer leads. I can get it to shrink, but backed off on some of the other leads because I was worried about damaging the shrink tubing.
I am going to use Dynakit’s Teflon coated hook up wire and that got me thinking about the Teflon shrink tubing. It's also very expensive and I need more and just want to make sure its not a stupid idea.

IMHO not a "stupid" idea. How's the stiffness of the leads with the Teflon shrink?

For painting the transformers I would use Rustoleum high heat primer as the base coat.

They are a little stiffer, but not too bad. It took about 7 feet to do one transformer a lot more than I had figured. I hate to play with the wires that deep in the transformer. I was also wondering if I could put high temp silicon where the wires go into the transformer to seal it and take some strain off the wires.

Chassis stripped and Flitz applied inside and outside. The chassis has some pitting, the photo is pretty kind. Dynakit's insulating bushings were added for the transformer leads holes.

I replaced all the side pieces except for the power cord with Dynakit parts. On the RCA input I cut the insulator with a razor on each hex side once the jack was tightened. The original bolts fit in without touching the input, but the nuts wouldn't. I ended up Dremeling each down so they would fit. Preamp input is the beginnings of a Dynakit’s Bias Balance Control.

Cinch tube sockets and a new 10k Bias Pot Mil style installed.

All of the connectors are in place. Basic assembly of the bias balance control finished. Test fitted the SDS Labs quad cap board. I turned the board around so the 500k bias pot wasn't resting on one of the Panasonic caps of the SDS Board. An 8-32 threaded 1/4" standoff seems to work well with clearance for the board.
Board is a home brew:

8 Panasonic TS-ED 100uf 400V 105c
4 Solen Fast Caps .1uf 630V
8 Vishsay Metal Film 330k 1/2 watt
1 Metal Oxide 6.8k 3 watt

Harpy,
Very nice work. I rebuilt a vintage ST70 using a VTA driver board last winter totally enjoying the experience as I can tell you are. Using the two Morgan Jones books as reference it was liking taking a course about amplifiers with lab.
I love the way my ST70 sounds and have been considering another project, possibly a pair of MKIII's. I have a couple of questions for you if you don't mind.
What rectifier are you going to use, GZ34 or the Weber replacement? After that, are you going to use the twist lok capacitor along with the cap board or will you leave it there for show? Do you have a schematic of the power supply that you can share and did you plan on any mods to it? I was talking to one of the guys on the Citation forum that suggested bypassing one of the twistlok sections with an oil filled cap to make the power supply "quicker sounding".
What does your new driver board look like and do you have a schematic? My board has individual pots to bias each tube so I was wondering why you have a single bias pot mounted between the output tubes on your chassis.
I have some pictures of my build on the photo site if you care to check them out. I ran into a few issues along the way, mostly due to limited space, and I tried to photograph my solutions. For example, I wanted to use an IEC power connector on my amp without cutting the chassis, my solution was to fashion and install a "tail" scavenged from and IEC extension cord. I locked it down inside by twisting a high tensile wire around the waist of a rubber grommet and clipping it flush with the grommet inside of the amp. Worked better for me than the standard knot. I was also able to install a "safety ground" to the chassis close to the power cord's entry point.
One last thing. Is that a rectifier stack I see in front of the cap board?
Again, nice work. Keep the photo's coming. Rich

Thanks Rich,

I am going to use a matched set of Mulland GZ-34's that I am going to steal off my MIV's if this ever gets off the ground. I was going to leave the old quad cap on for show and use its grounding lug for the Bias resistor mount. Followed your advice from another post and just taking my time and enjoying the project and trying to learn what I can. I should buy the Morgan Jones and try to understand the circuit better though.
On the cap board those are 2 IN4007 diodes. For right now this is my driver board , but I see what you are saying about the pots on the other boards and future board upgrades. Hopefully the Imps still have a bit of life in them.



Would like to see those photos too.

Ya know, I thought I read somewhere those Pyramid caps were notorious for going bad or drifting. I have box of them from the 70's that I'm afraid to use. I used K40Y-9's on my board and I can listen to that amp all day long. I'm also using vintage Mullard EL34's, a vintage Mullard GZ34, a Telefunken input tube and two RCA cleartop phase splitters. One of the reasons I wanted the VTA board was so could roll tubes, there's more 12au7 types made than I can begin to remember. Having individual bias adjusters for each EL34 tube meant they don't need to be exactly matched to work well together. I thought that was a huge advantage.
The two Morgan Jones Books are excellent. "Building Valve Amplifiers" will explain the best way to lay out things, for example, where the best place for the Star Ground is, where the best place for the safety ground is, how to lay out the heater wires, soldering, etc.. It's a real "how to" book. I've been doing this hobby for more than 40 years and was humbled when I read the book. "Valve Amplifiers" is more theory and was helpful to me. For example, when and why to use a metal resister or a carbon resister in a particular position. Like I said, Class with lab.
I attached some more pix. Rich
[img][/img]

Hey Rich,

Thanks for the pictures. I also found "Building Valve Amplifiers" used on Amazon, and with my Prime Membership it should be here Weds. Would also like to get "Valve Amplifiers" because where to use carbon vs. metal resistors is a question I have.
Those Imps and the tube socket look pretty grim. Sonicap will make custom values for the price of the next cap size and tighter tolerances too for an added cost. I know you wouldn't hear difference between .22uf and the stock .25uf, but it's not that much extra per cap. I have Sonicap custom values in my speaker cross-over and some regular Sonicaps in my cross-over. I still mulling over all the different options before I start wiring, so I may take an operational pause for reading and research. I am still waiting for the flexible Teflon shrink tubing to arrive to do the other transformer. Hopefully, I am not ruining them with the high heat needed to shrink the tubing. I was careful not to get the wire too hot and didn't go near where they went into the transformer (used regular shrink tube near the where the wires went into the transformer).

Take your time...that's a good idea, believe me. There's a couple of articles on the net about resister noise you should read.
http://www.aikenamps.com/ResistorNoise.htm
http://www.diyaudio.com/forums/tubes-valves/119683-where-low-noise-resistors-matter.html
http://www.eetimes.com/design/automotive-design/4009614/Strategies-for-minimizing-resistor-generated-noise

Very interesting articles heyraz, thanks! in the forum thread there's a discussion about fuses. one note about fuses is that they are generally non-linear: as they heat and reach their fuse point, their current transfer is non-linear. they are not a good choice to be used in an application which requires a linear response. I believe I read about this in an AES paper I downloaded. something to consider. fuses really should only go on the mains if at all possible.

It would help to be in the right manual too. The 929003-2 manual has one of resistor values as 4.7k while manual 11593 shows a 1k value. Mine matches the 11593 on the bias mount. Thanks for posting those they were very interesting.

I ran into my first question. Bob had warned someone in another post about the cloth leads color fading over time. This picture shows the other amps wiring to the speaker output. According to the manual pin 1 on the bottom is Black, then brown, orange, and yellow. The 16 ohm tap (4 yellow?) seems to have a thinner gauge wire. There is also an unused wire with the shrink tube covering the end that looks brown. Both amps were wired the same. I just wanted to confirm if this looks wired right or if someone has monkeyed with it. Also, what is that unused wire?



Thanks,
Jim

harpy wrote: ...Also, what is that unused wire?

Thanks,
Jim

There was an optional version of the MKIII with provision for a 70.7V (constant voltage) output, which is used in commercial distributed sound systems. That may explain the extra transformer lead.

Best Regards,
Charlie

Interesting Thanks. The transformers are back on with Dynakit’s isolation mounts, but larger head bolts. I decided not to paint the cores or covers. Cleaned covers with Hoppes 9 Solvent after trying something milder then wiped some WD 40 to protect areas that had been damaged.
Wiring up the transformers was a pain. The Teflon shrink tubing made the leads harder to work with and the old wires were brittle if you played with them too much. We shall see I guess.

You might want to measure the resistances between the different wires on the transformer's secondary side to verify the taps. Also--Is that a rectifier stack I see bolted in the photo? Is it original? I thought they were unreliable.

They ARE unreliable. However, selenium rectifiers' failure mode is high resistance or open, and even when they work, silicon diodes have a lower forward resistance, so bridging a silicon diode across a selenium rectifier, using the original selenium device as a "terminal strip," works just fine. In the photo I see what MIGHT be such a silicon diode, just so mounted on the selenium rectifier.

Yes, that's how both were wired, bridged with diodes that were not original. The selenium rectifier uses a different resistor value than the diode on the two post board setup (1000 ohm verse 4700 ohm). I decided to remove selenium rectifier and go with the two post board set up mainly for my ease of understanding. Thanks for bringing those points up.
Installed the 4700 ohm resistor, diode, .02 Uf, and Vishay 100uf caps.

I had no idea, thanks Bob. When I looked at the manual with the plus and the minus I thought it was just a terminal and the diode was supposed to be there. I switched to back to manual 929003-2 that has the diode setup. I am having fun doing the steps.
I had to correct my speaker output wiring. I think the wire connected to the 16 ohm tap was the white commercial wire. These amps may have been used commercially, but did not have the 5 output terminal strip. They both had slots cut in each cage too. Ohmed out the output transformers per Bob's guide and it checked out O.K. Metered the SDS Labs Board got good Uf readings at the coupling caps. Installed quad cap board. Wasn't sure the best way to route the Cathode wire. Decided to run it high so at least is not resting on any other wires. All wiring are mounted from underneath the SDS board.

Dynakit's Bias Balance Control Kit wired up.