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    A few questions re. restoring a Dynakit mk II

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    Chris Scott

    Posts : 13
    Join date : 2017-05-09

    A few questions re. restoring a Dynakit mk II

    Post by Chris Scott on Fri Sep 29, 2017 5:51 pm

    Hi everyone. I'm restoring a pair of Dynaco mk II amps, and just have a few questions. Though I have built a number of tube guitar amps, I'm not very experienced regarding electronics, so please go easy on me... Cool

    My first question is re. the diode - will a 1N4007 be an acceptable retrofit?

    Next is the 12 ohm bias resistor - as I have a set of simple octal bias probes, will these not obviate the need for installing them? ...and might I assume a 60% pd figure for a set of JJ E34L's operating in this environment to be reasonable?

    These amps came to me with the original speaker terminal strip having been replaced with a simple set of red/black speaker screw terminals. As the owner has (and will use exclusively) a pr. of 8 ohm speakers, am I wrong in thinking that I would connect the black OT lead to the black (neg) terminal, then the orange (8ohm) secondary to the red terminal (which then connects to eyelet 7 on the pc board) with the 680 ohm resistor bridged across the 2 speaker terminals?

    I hope I was clear enough here, and thank you in advance!

    Chris

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    peterh

    Posts : 859
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: A few questions re. restoring a Dynakit mk II

    Post by peterh on Fri Sep 29, 2017 7:06 pm

    Chris Scott wrote:Hi everyone. I'm restoring a pair of Dynaco mk II amps, and just have a few questions. Though I have built a number of tube guitar amps, I'm not very experienced regarding electronics, so please go easy on me... 8)

    My first question is re. the diode - will a 1N4007 be an acceptable retrofit?

    Next is the 12 ohm bias resistor - as I have a set of simple octal bias probes, will these not obviate the need for installing them? ...and might I assume a 60% pd figure for a set of JJ E34L's operating in this environment to be reasonable?  

    These amps came to me with the original speaker terminal strip having been replaced with a simple set of red/black speaker screw terminals. As the owner has (and will use exclusively) a pr. of 8 ohm speakers, am I wrong in thinking that I would connect the black OT lead to the black (neg) terminal, then the orange (8ohm) secondary to the red terminal (which then connects to eyelet 7 on the pc board) with the 680 ohm resistor bridged across the 2 speaker terminals?

    I hope I was clear enough here, and thank you in advance!

    Chris  

    The 1n4007 used as bias rectifier is ok. I't also possible to use for substitution of the
    5u4 if you like , although one usually recommends to use 2 1n4007 in series on each
    side. One may also use the 1n4007 as the dioe in the "yellow sheet mod".

    As for cathode resistors, if keeping the original design of one resistor shared
    for both cathodes in each channel, 10ohm makes the math easier, where
    one measures 1V on the measuring pont that correspond to 100mA ( for both cathodes).
    Personally i find it easier to use the biaset voltage measured on the front of the amp
    then having to remove the tubes and mount a probe in between ( if i understand your question right).
    JJ EL34 will bias just as any other EL34, and in this position 40 to 45 mA per tube is ok
    ( 0.8 - 0.9V on the measure point of a 10 ohm cathode resistor is used) The 65mA per tube
    as stated in the dynaco manual is generally felt i little high with todays tubes, thus
    40 - 45 is more conservative without loosing sound quality.

    About the speaker posts, black and orange connects to black resp. red for 8 ohm
    speakers, but the yellow one is for the 680 ohm and the feedback circuit and must be
    joined together inside the amp as the connector strip is absent.


    Chris Scott

    Posts : 13
    Join date : 2017-05-09

    Re: A few questions re. restoring a Dynakit mk II

    Post by Chris Scott on Sat Sep 30, 2017 2:45 pm

    Hey Peter thanks so much for responding!

    Re the bias setting question - using a probe is no big deal for me, as the adjustment pot is accessible from the top of the units - I just wanted to confirm that I could omit the 12 ohm resistor and take my readings from "upstairs" if you will...

    I knew I could omit the tube rectification, but as I had a few anos 5U4's and there's a place to put 'em, we'd go with glass right across the board.

    ...and I feel a bit stupid for not realizing beforehand that the 680 ohm was for the nfb circuit....the mere fact that I'm working for the first time on a tube hi-fi amp rather than a big ol' nasty distortion generator like a guitar amp has somehow confused me a bit...

    Thanks again mate!
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    peterh

    Posts : 859
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: A few questions re. restoring a Dynakit mk II

    Post by peterh on Sat Sep 30, 2017 3:22 pm

    Chris Scott wrote:Hey Peter thanks so much for responding!

    Re the bias setting question - using a probe is no big deal for me, as the adjustment pot is accessible from the top of the units - I just wanted to confirm that I could omit the 12 ohm resistor and take my readings from "upstairs" if you will...

    I knew I could omit the tube rectification, but as I had a few anos 5U4's and there's a place to put 'em, we'd go with glass right across the board.

    ...and I feel a bit stupid for not realizing beforehand that the 680 ohm was for the nfb circuit....the mere fact that I'm working for the first time on a tube hi-fi amp rather than a big ol' nasty distortion generator like a guitar amp has somehow confused me a bit...

    Thanks again mate!

    No, i would not omit the 12 ( or 10 ) ohm resistor. It's more accurate since it's always there.
    Omit the probe instead.

    Chris Scott

    Posts : 13
    Join date : 2017-05-09

    Re: A few questions re. restoring a Dynakit mk II

    Post by Chris Scott on Tue Oct 03, 2017 5:17 pm

    Ok cool, I will leave the 12 ohm in...just to be clear, the bias probes are not meant to be installed permanently - not sure if you knew this - and I'm curious - can one use the probes with the 12 ohm resistor installed? - they're basically just extensions with a 1 ohm resistor from the cathode to ground, so I'm wondering if the 12 ohm resistor would interfere with the reading taken with them?...

    ...my ignorance is indeed quite obvious here, so thanks in advance for your patience. Cool
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    PeterCapo

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    Join date : 2008-12-05

    Re: A few questions re. restoring a Dynakit mk II

    Post by PeterCapo on Tue Oct 03, 2017 9:53 pm

    I am not exactly sure what these bias probes are, to which you refer.  I *think* it sounds like you plug your bias probes into the octal sockets for the power tubes, then plug the power tubes into the bias probes, and then set the bias, is this correct?  I think I have seen these kinds of things while looking around online.

    I’d want to know what (as in voltage or current) and where (as in which corresponding tube pins) your bias probes are supposed to measure, so that you’d know what value to adjust for.  What is the "60% pd figure" to which you referred, please?  I think you are referring to power dissipation, is this correct?  Dynaco's original specification for biasing the Mark II's pair of EL34/6CA7 power tubes is 130mA (130mA combined total for both power tubes, or 65mA for each power tube). You could calculate the power dissipation from there.

    It drives EL34 hard, and some feel today's current production EL34 can't take it.  If this is a concern for you, an alternative is the Electro Harmonix 6CA7 (big plates). You can also run 6550/KT88 with an adjustment to the bias as described in the manual.  If you lower the bias current to go easier on the power tubes, the distortion rises, but, again, there is debate on how much this matters.

    Dynaco used their Biaset scheme (where you retain the 12Ω resistor), which was very simple to use.  Instructions were included in the original Dynaco manual.

    My suggestion, if you have not already done so, would be to obtain a copy of the original manual and, if possible, the corresponding pictorial wiring diagrams for your particular Mark II.  This should help with all phases of your restoration.  The tricky thing is that I know of at least three or four different original variations of the Mark II.  Adding to the confusion is that I have seen Mark II manuals accompanied by the wrong pictorial wiring diagrams.

    Any chance you can post a series of photos of your Mark II top and inside?  Might be helpful in figuring out what you have.

    Try this for starters, but we should still try to identify your particular Mark II: http://www.tubes4hifi.com/Dynaco%20MK2%20manual.pdf


    Last edited by PeterCapo on Wed Oct 04, 2017 1:36 am; edited 2 times in total (Reason for editing : Add comment.)
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    peterh

    Posts : 859
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: A few questions re. restoring a Dynakit mk II

    Post by peterh on Wed Oct 04, 2017 12:50 am

    Chris Scott wrote:Ok cool, I will leave the 12 ohm in...just to be clear, the bias probes are not meant to be installed permanently - not sure if you knew this - and I'm curious - can one use the probes with the 12 ohm resistor installed? - they're basically just extensions with a 1 ohm resistor from the cathode to ground, so I'm wondering if the 12 ohm resistor would interfere with the reading taken with them?...

    ...my ignorance is indeed quite obvious here, so thanks in advance for your patience. 8)

    The best you can do is to forget the bias probe. It does not add anything , rather it
    adds potential problems by repeated removal/insertion of the tubes.
    The measurement device that the probe adds to some ( badly designed) amps are already
    present in your amp, it is the 12 ohm resistor. All you need to do is measure
    the voltage across this resistor to figure out the current drawn.

    Chris Scott

    Posts : 13
    Join date : 2017-05-09

    Re: A few questions re. restoring a Dynakit mk II

    Post by Chris Scott on Thu Oct 05, 2017 4:39 pm

    Thanks Peter, and I agree that repeated cycling of pins in and out of the sockets doesn't help anything.

    Best,
    Chris

    Chris Scott

    Posts : 13
    Join date : 2017-05-09

    Re: A few questions re. restoring a Dynakit mk II

    Post by Chris Scott on Thu Oct 05, 2017 4:50 pm

    PeterCapo wrote:I am not exactly sure what these bias probes are, to which you refer.  I *think* it sounds like you plug your bias probes into the octal sockets for the power tubes, then plug the power tubes into the bias probes, and then set the bias, is this correct?  I think I have seen these kinds of things while looking around online.

    I’d want to know what (as in voltage or current) and where (as in which corresponding tube pins) your bias probes are supposed to measure, so that you’d know what value to adjust for.  What is the "60% pd figure" to which you referred, please?  I think you are referring to power dissipation, is this correct?  Dynaco's original specification for biasing the Mark II's pair of EL34/6CA7 power tubes is 130mA (130mA combined total for both power tubes, or 65mA for each power tube).  You could calculate the power dissipation from there.

    It drives EL34 hard, and some feel today's current production EL34 can't take it.  If this is a concern for you, an alternative is the Electro Harmonix 6CA7 (big plates). You can also run 6550/KT88 with an adjustment to the bias as described in the manual.  If you lower the bias current to go easier on the power tubes, the distortion rises, but, again, there is debate on how much this matters.

    Dynaco used their Biaset scheme (where you retain the 12Ω resistor), which was very simple to use.  Instructions were included in the original Dynaco manual.



    My suggestion, if you have not already done so, would be to obtain a copy of the original manual and, if possible, the corresponding pictorial wiring diagrams for your particular Mark II.  This should help with all phases of your restoration.  The tricky thing is that I know of at least three or four different original variations of the Mark II.  Adding to the confusion is that I have seen Mark II manuals accompanied by the wrong pictorial wiring diagrams.

    Any chance you can post a series of photos of your Mark II top and inside?  Might be helpful in figuring out what you have.

    Try this for starters, but we should still try to identify your particular Mark II: http://www.tubes4hifi.com/Dynaco%20MK2%20manual.pdf

    Thanks Peter for your input, and yes they're just simple sockets designed to help set the bias on tube guitar amps, and the 60% is indeed a plate dissipation figure considered "safe" for today's new stock power tubes. I normally solder in a 1 ohm precision resistor tying the cathodes to ground, making it easy enough to see how much current each tube is drawing, but I got a bit confused with the 12 ohm value called for in the Dynaco kit build manual...I guess my ignorance level is still pretty high. I will try and post som pics here for you in the next few days - we're getting some weather rolling through at the moment (I'm in Costa Rica) so forgive me if it takes a bit longer...

    Best,
    Chris
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    peterh

    Posts : 859
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: A few questions re. restoring a Dynakit mk II

    Post by peterh on Thu Oct 05, 2017 5:22 pm

    The 12 ohm resistor has a history behind :

    Bias is supposed to be adjusted by the user, and the user may not have an calibrated good voltmeter.
    Here dynaco came with a brilliant idea : using a standard drybattery cell as referens, any meter
    could be used to adjust the bias, only make it as large as teh voltage of a battery cell ( 1,56V )

    Today measuring voltages with precision is a nobrainer with any cheap DMDM. But not 1958!


    Chris Scott

    Posts : 13
    Join date : 2017-05-09

    Re: A few questions re. restoring a Dynakit mk II

    Post by Chris Scott on Fri Oct 06, 2017 12:09 pm

    peterh wrote:The 12 ohm resistor has a history behind :

    Bias is supposed to be adjusted by the user, and the user may not have an calibrated good voltmeter.
    Here dynaco came with a brilliant idea : using a standard drybattery cell as referens, any meter
    could be used to adjust the bias, only make it as large as teh voltage of a battery cell ( 1,56V )

    Today measuring voltages with precision is a nobrainer with any cheap DMDM. But not 1958!


    Ah ha! - now it makes sense...thanks again for everything Peter!

    Best,
    Chris

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