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    dynaco mark IV

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    mirco64

    Posts : 3
    Join date : 2011-06-20
    Location : ITALY

    dynaco mark IV

    Post by mirco64 on Thu Jan 26, 2012 3:02 am

    I have Dynaco Mark 4, the power transformer is vibrating, it can be? Hello and thanks

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: dynaco mark IV

    Post by Bob Latino on Thu Jan 26, 2012 8:27 am

    Hi,

    First - try tightening the four nuts that hold the transformer to the chassis. Sometimes a simple tightening of the nuts can reduce a mechanical vibration in a power transformer. You can also try removing the nuts, then lifting the transformer and placing a "00" neoprene flat (plumbing) washer between the transformer and the chassis (on the TOP of the chassis) on each of the four contact points and then tighten the nut again. All VTA amp kits come with these "isolation washers" to reduce any mechanical vibration that may be present in the power transformer.

    Bob

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: dynaco mark IV

    Post by Sal on Thu Jan 26, 2012 10:20 am

    Is this a restored Mark IV? The only reason why I mention this is the possibility of the transformer pulling to much current with a bad quad capacitor, I have seen this in a ST-70 where the power transformer was vibrating until I replaced the quad capacitor. Putting neoprene washers under the transformer as Bob says is a good idea as sometimes these transformers just vibrate, but make sure you don't have an "overload" problem too.

    You can remove the 5AR4 tube so there is no B+ (high voltage) going to the quad capacitor to see if the vibration goes away as a quick test.

    Sal

    heyraz

    Posts : 58
    Join date : 2011-10-26

    Re: dynaco mark IV

    Post by heyraz on Thu Jan 26, 2012 12:13 pm

    Sal,
    I recently completed a vintage ST70 rebuild using a new 80-40-30-20 quad cap (HV version) from Dynakit Parts and a vintage Mullard 5AR4. I also wired two UF4007 in series between the secondaries of PA-060 and pins 4 and 6 of 5AR4 to protect 5AR4 from transformer flashback.
    With that configuration my amp's PA-060 also vibrated and became quite hot. In addition, I thought the bass was very full and deep, but a little flabby. Because of the heat and vibration I swapped in another 5AR4 (Vintage HH Scott made in USA) which reduced the heat and vibration but also thinned the bass up a bit. Same result using a new Sovtek 5AR4.
    Pulling the 5AR4 does eliminate the vibration but how can I tell if I have a problem with the new cap? Before installation all 4 sections tested fine. And I applied power slowly with a variac for the initial power on. Is the first section of the capacitor (40uf) that PA-060 sees too big? A DIY friend suggested I add another 22uf to the 80uf section to improve bass but I'm concerned that 5AR4 is becoming overtaxed.
    Any thoughts?
    Rich

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: dynaco mark IV

    Post by Sal on Thu Jan 26, 2012 3:18 pm

    Rich,
    If you have a DC milliamp meter, remove all the tubes including the 7199 tubes but leave the 5AR4 tube in place. Disconnect the wire on pin 8 of the 5AR4 tube and connect your milliamp meter from pin 8 to the wire you disconnected to measure the current. Your meter must be connected in series. Be very very careful as there will be over 500 volts on pin 8 of the 5AR4 tube. You should measure a few milliamps at most after the capacitor charges up. It might be a good idea to use your variac so the voltage does not exceed 525 volts going to the quad cap.

    Sal

    PS: You are right about a 5AR4 tube causing a vibration in the transformer, I forgot I had another ST-70 with a bad Sovtek 5AR4 tube causing the transformer to vibrate.

    heyraz

    Posts : 58
    Join date : 2011-10-26

    Re: dynaco mark IV

    Post by heyraz on Thu Jan 26, 2012 5:44 pm

    Sal,
    How many mA is too much? Is there a range it should be within?
    Also-inserting the mAmeter at pin 8 measures current draw between the 5AR4 and the capacitor. How about the current drawn from PA-060 from 5AR4? Any range it should be in? What current can PA-060 safely supply without overheating?

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: dynaco mark IV

    Post by tubes4hifi on Thu Jan 26, 2012 9:14 pm

    with no tubes in the amp, the current draw should be less than 1ma, that would all be capacitor leakage

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