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    Dynaco Mark VI vs VTA M-125

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    Bob Latino
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    Dynaco Mark VI vs VTA M-125

    Post by Bob Latino on Thu Apr 23, 2015 7:19 pm

    I had a person Email me about the Dynaco Mark VI and the VTA M-125. Essentially he wanted to know about a "comparison" of the two amps. I believe he has the opportunity to purchase a pair of Dynaco Mark VI 120 watt mono block amps. He was also considering alternatively a pair of VTA M-125 monoblock kits. The answer that I gave to him is below ... Personally, I have never heard the sound from a pair of Mark VI's so I could not give any sound comparison with the M-125's to him. The one thing that I did not put in the answer was that parts for the Mark VI are virtually non-existant. Sure, you can substitute modern parts for some parts in the amp but try to get say an output transformer would be difficult unless you can find a non-working parts amp. Answer below ...

    Bob

    "The Dynaco Mark VI was a 120 watt monoblock amp from the mid '70's. The only thing about the M-125 that was similar to the Mark VI was the fact that both amps used a parallel push pull output circuit with 4 output tubes. There are many differences between the two amps.

    1. The M-125 can use 6550, KT88, KT90, KT100 and KT120 output tubes whereas the Dynaco Mark VI driver circuit was designed for an 8417 output tube. Dynaco mentions in the Mark VI manual that there is no substitute for the 8417 tube. You can, however, change the circuit of the Mark VI to be able to use KT88, 6550 etc ..

    2. The driver circuit on the Mark VI used a single 7199 driver tube for both the voltage amplifier/inverter sections. This is similar to the driver circuit on the ST-70. The M-125 uses two 12AU7 or 12BH7 tubes. The front 12AU7 or 12BH7 is the initial voltage amplifier and the rear tube is the phase splitter/inverter tube.

    3. The Mark VI had only 75 uF of DC power storage to power the output transformer. (25 uF before the choke and 50 uF after the choke) The M-125 has 336 uF of DC power storage for just the output transformer which is about 4 1/2 times what the Mark VI had. The M-125 also has another 167 uF for just the driver board. This is not said as an indictment of the Mark VI. Back in the mid '70's the state of tube amplifier development was such that it was difficult to get a lot of DC power storage at a reasonable price. Today's electrolytic caps are smaller in relation to how much DC storage they have and are also less expensive than what was available back then.

    4. The M-125's may be operated in either ultralinear mode or triode mode at the flick of a switch. The Mark VI only operated in ultralinear mode.

    5. The Mark VI had a hard wired solid state rectifier. The M-125's may use either a GZ34 tube or a solid state rectifier (Weber WZ68 recommended). The solid state rectifier plugs right into the rectifier tube socket.

    6. AFAIK the Mark VI was intended to be operated with 4 output tubes only. The M-125's have "two output tube" option. If you don't need the full 125 watts per monoblock, instead of using four output tubes, you can also use just TWO output tubes - one left side tube and one right side tube. The power drops to 65 watts in ultralinear mode and 35 watts in triode mode and the amp must be rebiased."


    Link to photos of the Mark VI from an earlier post on our forum here ... Mark VI assembly manual link below also ..

    Mark VI thread on the Dynaco Tube Audio Forum

    Dynaco Mark VI assembly manual


    MontanaWay

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    Re: Dynaco Mark VI vs VTA M-125

    Post by MontanaWay on Fri Apr 24, 2015 12:20 am

    is the 8417 not replaceable by an EL34 without any mods?

    peterh

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    Re: Dynaco Mark VI vs VTA M-125

    Post by peterh on Fri Apr 24, 2015 1:19 am

    MontanaWay wrote:is the 8417 not replaceable by an EL34 without any mods?
    No. It's a hotted 6550 with grid closer to cathode.


    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco Mark VI vs VTA M-125

    Post by Bob Latino on Fri Apr 24, 2015 8:29 am

    There is a web page on the Triode Electronics web site that lists modifications for the Dynaco Mark VI. One of the first modifications are adjustments to the amp to allow 6550 output tubes in place of the 8417's. Link below ..

    Dynaco Mark VI modifications

    Bob

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