Unusual Mark II Driver

I'm posting this here to find out if anyone has seen something like this before.  I just got two of these units, a heavily modified Mark 2 and Mark 3, because I was curious about them.  Identically modified, all custom.  The driver circuit is custom on perf board, and uses a 7199 tube.  Output tubes were old GE 6550 tubes, but they were destroyed in shipping.  The soldering indicates that it was probably not a first time amateur.  The wire is teflon insulated. The Mark 2 plays well with new 6550's, but the Mark 3 has a bit of a hum, so it's on the bench now.  The big lesson so far is that shipping costs are a killer, and if the shipper leaves the tubes in the sockets, it's a bad day when they turn to glass dust.  Only one went to dust, but I assumed the others were damaged beyond use from bouncing around.

Top view - some broken tube glass can be seen:



Bottom View:




If anyone has seen anything like this, I'd be curious to know if there is a schematic somewhere, or if it's a one-off, before I part them out and start over.

The switch changes the tone and level, and seems to switch more caps and resistors into the cathode of the 7199 pentode section.

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I'd order new boards from dynakitparts and don't try to decode the homebuilt ones.

Undocumented amps are very close to junk

That circuit is weirdly close to the OEM MKIII - the only question is to discern the "why" of it.

a) The OEM board was damaged at some point.
b) Adapting to the 7199?
c) Wanted a tone switch?
d) Other?

In any case, I agree, it would be good to look into new boards and bring the entirety back to the original design.

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Thanks for the replies and the links.  Yes, they are odd - I have heard that the parts you can save are the transformers and chassis.  I'll have to see if any of the other mods would make it hard to insert a new driver board.

One last question - any idea why the builder would use a 10 ohm, 10 watt resistor for the bias set resistor?  Seems excessive.  The bias can be set and it holds, adjusting the value to account for the 10 ohms.  There is also a 530 uf cap on the bias supply, and there does not seem to be a reason to do that.






Thank you all for the advice and help.

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I have discovered that tube amps are like onions - they have layers. Fix one thing, other items show up.

So now it's determining how deep to go.

Re-capped the Mark III - played great until the solid state rectifier replacement board went out. Fuse blew on the 10A variac powering both amps at the time.

Thought it was a shorted B+ transformer, but no, measuring against the working Mark II, the transformer is OK. Measures very close to the same.

But, in the process of comparing measurements, I discovered that the Mark II was running on only one leg of the B+. Didn't think that was possible. B+ measured OK, so I never gave it a thought.

The builder had used the much more traditional cellophane tape, rather than heat shrink to insulate a splice - and the splice broke under the mess. Half-wave I guess, but there was no hum, nothing odd.

I have a whole bunch of new diodes, they are 800V. I can't think of any reason not to just use them before adding more cash to the money pit. The current diode board has 4x 1n4007: to per side in series.

Oh, any hints for handling the very old cloth covered wire, other than be really gentle with it? Almost ready to dispose of them both as parts units, or gut and start over.

The lessons continue...

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Txteacher You did not state what the original goal of the amps would be. I assume that you are trying to make them the same although they were different models (early MK II and a MK III). The early MK IIs have the wider circuit boards that are very prone to lifting of the traces. That probably forced the conversion to what you see now. I have not found a supply of the early wider boards for sale. I jumpered out the bad traces on my MK II but your board is gone so that is not and option. At best you can buy the narrower MK III type board and then make a mounting plate to cover the remaining hole.

Although a lot of work I would not give up on them yet. The running of only one leg on the B+ would still give a pretty high voltage but it would sag badly under load. It sounds like you have the fix figured but the effort is significant. Also if the tubes were not turned to dust I think they may be fine and not to be given up on either. The early MK IIs do not have a 4 ohm tap. Assuming that is not a problem both amps could be rebuilt to match each other.

Good luck with them WLT

Thanks for the info.  They do match exactly - both have the A-431 transformers and the 4, 8, and 16 ohm taps.  My goal was to use them in my office system.  I fixed the apparent issues with both of them, and they now operate.  It was not too much work to replace capacitors, fix the broken B+ connection, replace a few diodes that had issues, get new tubes, etc.  A few Mouser orders later, they worked.  I've had them playing for a few days now.  

However, I think I may pass them on at this point because a complete teardown exceeds my available time.  Might be a fun project, and I can see the potential, but it's like building a boat - spend time building or spend time using.  It's easy to underestimate the effort getting something back to its original state.  Save the transformers, junk the rest, buy new chassis, put the original power supply back.  I envision a big box of parts in the corner for up to a year.  I did look for the square Mark II boards too - they don't seem to be made.

You were right about the tubes too.  Three of the original 6550 output tubes, and both 7199 tubes are intact, but I assumed them to be damaged from bouncing around, but they all worked too - one of those hit the switch, hide behind the desk with the variac power switch, wait for the explosion moments.

So the lesson for me was important:  a rebuild is a big deal, vintage vacuum tube gear requires a little babysitting.  Solid state seems to fit what I do, driving my home office speakers (1986 LaScalas with an early 90's ULD-18 sub) that seldom use more than a watt.  That combo, with the sub turned way down, is actually really nice.

a total rebuild would only take 4-6 hours per amp, just junk the existing board and caps, put in a new driver board (either stock or the VTA board),
new quad caps, and you're good to go for another 50 years of pleasure.
But if that's too much work for you, I'm sure there are plenty of readers here who'd take them off your hands for $300 or so.
Shipping is the killer.