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    Dynaco mkII voltages

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    Chris Scott

    Posts : 20
    Join date : 2017-05-09

    Dynaco mkII voltages

    Post by Chris Scott on Fri Oct 27, 2017 1:59 pm

    Hi all, I just wanted to post the voltages off one of these amps - I do not have any way to hear them at this time as I've no system, so until the client brings in his system I just wanted to run the info past y'all..

    Turns out the owner finally fessed up to the OT on one of these units having been rewound, which explains the difference in secondary wire color coding from the other unit as there was no evidence of any splicing when I popped the covers off...I'm going to sort it out and replace it if need be.

    So here's the voltages off the (original OT) amp, which we can call amp A, with the other (non-orig. OT) unit being amp B;

    Amp A

    Wall voltage 120vac

    5U4GB---------522vdc pins 2&8, 428vac pins 4&6
    JJ E34L's------- pins 1&8......0v, as I've installed a 1 ohm 1w precision resistor here from cathode to ground.*
                         pin 2...... 3.31 vac
                         pin 3.......521v
                         pin 4.......522v
                         pin 5....... -53v (this figure represents approx. 60% plate dissipation)
                         pin 6....... -53v
                         pin 7....... 3.31 vac

    The plate voltage on amp B is considerably lower, running at around 460v which I thought might be more consistent with an original so I'm curious if these voltages are too high.

    *I know I was advised to leave the original setup here, but since I've installed this setup in all the guitar amps I've built, I opted for this as I'm so used to it...

    Thanks in advance for any input, and again pardon my ignorance here.
    Chris
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    MarcVBelgium

    Posts : 94
    Join date : 2013-12-06
    Age : 64
    Location : Antwerp, BELGIUM

    Re: Dynaco mkII voltages

    Post by MarcVBelgium on Sun Oct 29, 2017 6:09 am

    I've had a similar problem on my Mk IV's (thread is on this forum). They originated 1959 and within its lifetime one of the power supply transformers had been rewound, giving way too high voltages. Since the B+ voltages are generated by the power supply, I suggest its not the output transformers (if that is what you mean by "OT") that are faulty, but one of the power transformers.

    Chris Scott

    Posts : 20
    Join date : 2017-05-09

    Re: Dynaco mkII voltages

    Post by Chris Scott on Wed Nov 01, 2017 12:23 pm

    Hey thanks for responding Marc. Yeah, the PT looks a bit suspicious as well, and will certainly explain the higher B+. At this point I'm just trying to get the OT in "Amp B" to function in order to see if it even works, irrespective of the higher B+ voltage.

    How high is (was?) the voltages on yours?
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    PeterCapo

    Posts : 455
    Join date : 2008-12-05

    Re: Dynaco mkII voltages

    Post by PeterCapo on Wed Nov 01, 2017 10:59 pm

    Chris Scott wrote:
    Wall voltage 120vac

    5U4GB---------522vdc pins 2&8, 428vac pins 4&6

    428 VAC on 4&6 is fine. Why you are/were getting 522 VDC on 2&8 I am not sure. Your AC mains being at 120 VAC instead of 117 VAC might account for some of it. Are you sure you had a 5U4 and not a 5AR4?

    Chris Scott

    Posts : 20
    Join date : 2017-05-09

    Re: Dynaco mkII voltages

    Post by Chris Scott on Thu Nov 02, 2017 11:41 pm

    PeterCapo wrote:
    Chris Scott wrote:
    Wall voltage 120vac

    5U4GB---------522vdc pins 2&8, 428vac pins 4&6

    428 VAC on 4&6 is fine.  Why you are/were getting 522 VDC on 2&8 I am not sure.  Your AC mains being at 120 VAC instead of 117 VAC might account for some of it.  Are you sure you had a 5U4 and not a 5AR4?

    Yeah, this didn't make any sense...it's most def. not a GZ34, and I tried another just to check...all was revealed though when I bumped the bias voltage down from around -52v to -37v.
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    PeterCapo

    Posts : 455
    Join date : 2008-12-05

    Re: Dynaco mkII voltages

    Post by PeterCapo on Fri Nov 03, 2017 12:21 am

    Chris Scott wrote:Yeah, this didn't make any sense...it's most def. not a GZ34, and I tried another just to check...all was revealed though when I bumped the bias voltage down from around -52v to -37v.

    I guess I'm scratching my head a bit on this one.  I wouldn't have thought the B+ would rise to the point where it might exceed the ratings of parts in the Mark II unless the bias were set correctly?  I can't help but wonder if something else might be afoot, but I cannot really say. Did you have either of the power tubes removed when you got the high B+ reading?

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