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    Unusual Mark II Driver

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    txteacher

    Posts : 3
    Join date : 2018-11-03

    Unusual Mark II Driver

    Post by txteacher on Mon Nov 05, 2018 3:10 pm

    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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    PeterCapo

    Posts : 561
    Join date : 2008-12-05

    Re: Unusual Mark II Driver

    Post by PeterCapo on Mon Nov 05, 2018 6:07 pm

    I vote for one-off.
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    peterh

    Posts : 1054
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Unusual Mark II Driver

    Post by peterh on Tue Nov 06, 2018 1:45 am

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

    Undocumented amps are very close to junk
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    Peter W.

    Posts : 965
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Unusual Mark II Driver

    Post by Peter W. on Tue Nov 06, 2018 9:40 am

    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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    PeterCapo

    Posts : 561
    Join date : 2008-12-05

    Re: Unusual Mark II Driver

    Post by PeterCapo on Tue Nov 06, 2018 10:15 am


    txteacher

    Posts : 3
    Join date : 2018-11-03

    Re: Unusual Mark II Driver

    Post by txteacher on Tue Nov 06, 2018 12:04 pm

    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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    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Unusual Mark II Driver

    Post by Bob Latino on Tue Nov 06, 2018 2:03 pm

    txteacher wrote:

    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.  


    10 ohm @ 10 watt IS excessive .. A 10 ohm 2 or 3 watt resistor here is probably what you would want to use ..

    Bob
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    PeterCapo

    Posts : 561
    Join date : 2008-12-05

    Re: Unusual Mark II Driver

    Post by PeterCapo on Tue Nov 06, 2018 4:33 pm

    txteacher wrote:...I have heard that the parts you can save are the transformers and chassis.
    Not necessarily only those.  There could be some other parts that can be saved/reused.

    txteacher wrote:Yes, they are odd ... ... I'll have to see if any of the other mods would make it hard to insert a new driver board.

    Would be helpful if you can post some perspective images of the top and bottom of both amps.  The images you have posted so far show an amplifier that might be customized to the point that you won't be able to just "drop in" a PC board.  Whether you get a copy of the original Dynaco PC board or some other board, it will, at least to some extent, rely upon the amplifiers otherwise being in the original Dynaco configuration.  Either that, or you'd really have to understand what's going on in your customized amps so you can successfully hook up a PC board.  At this point, without seeing additional images that might suggest otherwise, I think you're looking at a total tear-down and rebuild.

    txteacher

    Posts : 3
    Join date : 2018-11-03

    Re: Unusual Mark II Driver

    Post by txteacher on Thu Nov 08, 2018 3:24 pm

    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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    PeterCapo

    Posts : 561
    Join date : 2008-12-05

    Re: Unusual Mark II Driver

    Post by PeterCapo on Thu Nov 08, 2018 3:45 pm

    The cloth covered transformer leads actually have some kind of plasticky insulation under the cloth.  Try thoroughly warming them up with a hair dryer before gently flexing them.

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