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    Dynaco Mark III and IV Startup Procedure

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    Gregory Braun

    Posts : 5
    Join date : 2009-02-18
    Location : Brookfield, WI U.S.A.

    Dynaco Mark III and IV Startup Procedure

    Post by Gregory Braun on Wed Feb 18, 2009 3:54 am

    I found a pair of Dynaco Mark III 60 watt amps
    along with a pair of Mark IV 40 watt amps.

    They haven't been used in ten years. They
    worked when I packed them up.

    Visual inspection doesn't reveal anything obvious.

    Is there anything special I should do before
    powering them up for the first time?

    I'd like to put them back in service driving
    a set of Dahlquist DQ-10 loudspeakers.

    I ran across Dynakitparts.com recently, which
    would make repairs possible if need be, but
    I think I'd also like to improve them if possible.

    Are there aftermarket upgrade boards for these
    Mark III and IV models?

    Most seem to focus more on the Stereo 70.

    Isn't a Mark IV simply "half" of a Stereo 70
    that produces slightly more power because the
    power supply is not shared with a second channel?

    -gb-

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco Mark III and IV Startup Procedure

    Post by Bob Latino on Wed Feb 18, 2009 9:42 am

    Gregory Braun wrote:I found a pair of Dynaco Mark III 60 watt amps
    along with a pair of Mark IV 40 watt amps.

    They haven't been used in ten years. They
    worked when I packed them up.

    Visual inspection doesn't reveal anything obvious.

    Is there anything special I should do before
    powering them up for the first time?

    I'd like to put them back in service driving
    a set of Dahlquist DQ-10 loudspeakers.

    I ran across Dynakitparts.com recently, which
    would make repairs possible if need be, but
    I think I'd also like to improve them if possible.

    Are there aftermarket upgrade boards for these
    Mark III and IV models?

    Most seem to focus more on the Stereo 70.

    Isn't a Mark IV simply "half" of a Stereo 70
    that produces slightly more power because the
    power supply is not shared with a second channel?

    -gb-

    Hi Greg,

    If your amps have not been used in 10 years they should be started up slowly with a Variac and bought up to full voltage over 1/2 hour or so. If you hit them with the 120 volts immediately you *could* risk smoking the quad cap. Variacs start at about a 5 amp model at maybe $50. A 5 amp model would work fine in your situation.

    Roy Mottram at tubes4hifi has upgrade boards for the Mark III and Mark IV at the link below.

    http://www.tubes4hifi.com/MK3-4.htm

    Yes - The Mark IV is half of an ST-70 with each monoblock having its own power transformer and its own quad cap. One difference though is that Dynaco ran the tubes on the Mark IV real "hot" by today's standards - hotter than on the ST-70. With a 13.5 ohm bias resistor for a pair of EL34 tubes, when you biased your amp at their recommended 1.56 volt DC setting that worked out to 62.5 milliamps per EL34 tube. The conventional wisdom today is to bias an EL34 at about 40 milliamps per tube OR 80 milliamps per each PAIR of EL34 output tubes. That would be a bias voltage setting of 1.08 volts on a Mark IV. Dynaco got away with the higher bias setting only because the Mullard EL34's they shipped with the Mark IV were extremely durable tubes. If you bias present production EL34 tubes at 1.56 volts on a Mark IV your EL34 tubes won't last long.

    Bob

    danf

    Posts : 58
    Join date : 2009-01-19

    Older amp safety

    Post by danf on Thu Feb 19, 2009 8:49 pm

    I always replace the original two wire line cord with a grounded cord before operating any tube amplifier. These tube amps all have metal chassis and grounding the chassis properly is a vital safety issue in my view. I have yet to encounter any hum issue from grounding the chassis. You usually have to enlarge the hole for the line cord to install a proper strain relief for a grounded cord. An IEC socket is also a possibility if you don't mind cutting a fairly large hole in the chassis. I usually use an attached cord for classic amps and an IEC socket for new builds.

    Gregory Braun

    Posts : 5
    Join date : 2009-02-18
    Location : Brookfield, WI U.S.A.

    Re: Dynaco Mark III and IV Startup Procedure

    Post by Gregory Braun on Fri Feb 20, 2009 4:08 am

    Thanks Dan,

    A very good idea. Will do.

    -gb-

    Gregory Braun

    Posts : 5
    Join date : 2009-02-18
    Location : Brookfield, WI U.S.A.

    Re: Dynaco Mark III and IV Startup Procedure

    Post by Gregory Braun on Fri Feb 20, 2009 4:13 am

    Thanks Bob,

    Are you saying I can simply adjust the
    bias to 1.08 volts and be good to go?

    Or must I change the value of the two
    13.5 ohm bias resistors?

    -gb-

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco Mark III and IV Startup Procedure

    Post by Bob Latino on Fri Feb 20, 2009 6:25 am

    Gregory Braun wrote:Thanks Bob,

    Are you saying I can simply adjust the
    bias to 1.08 volts and be good to go?

    Or must I change the value of the two
    13.5 ohm bias resistors?

    -gb-

    Greg,

    Those bias resistors don't give out very often so they are probably OK. What you probably should do is measure the resistance of those two 13.5 ohm resistors to ground. You can do it from the bias measuring point on the front of the amp. If they are OK I would just bias both amps at 1.08 volts. If one or both measure "OFF" you can get two new 13.5 ohm bias resistors from Dynakitparts.

    Bob

    Gregory Braun

    Posts : 5
    Join date : 2009-02-18
    Location : Brookfield, WI U.S.A.

    Re: Dynaco Mark III and IV Startup Procedure

    Post by Gregory Braun on Sat Feb 21, 2009 1:55 pm

    Thanks again Bob,

    I powered up each of the four amps
    individually last night on my bench,
    and they all work! I was amazed.

    Aside from a missing fuse holder cap
    on one of the Mark IV units they've
    all survived their long sleep.

    I dug out a couple of Sound Valves
    preamps to use with them and they
    both work too!

    Using a Sound Valves VT101i and the
    pair of Dynaco Mark III amplifiers
    I drove the Magnaplanars in my main
    listening system for several hours.

    I'd forgotten just how good VT gear
    can sound. The Dynaco amps do not
    drive the Maggies to the same SPLs
    that my Rotel 200 wpc solid state
    amp, but they're loud enough for me.

    All four of my Dyna amps are stock.
    I'm wondering if the aftermarket
    upgrade driver boards will make a
    noticable difference in sound.

    I think I'm going to upgrade the
    Mark III units first. I've found an
    upgraded stainless steel chassis at
    Triode Electronics that will give
    me IEC power capability along with
    punched openings for proper banana
    connectors.

    I have some questions about my PCB
    upgrade options, but I think I'll begin
    a new thread.

    -gb-

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