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    Mark III's eating rectifier tubes

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    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Mark III's eating rectifier tubes

    Post by wazowski on Sun Jan 02, 2011 8:03 am

    [b]My friend P has a pair of Mark III's he has been running for close to 3 years. For the past 2 years he has replaced 3 rectifier tubes, and just a couple of days ago, he lost another one. Curiously, the problem alternates from L amp to R amp.

    The rectifiers just quietly give out, and fail to function. The fuse also blows. The quad cap can stays cool and does not heat up when this happens.

    The amps are not original and have been rebuilt, but I do not know to what extent.

    Can anyone offer any insight? Could this be a similar issue to my other post regarding my ST-70?




    Sal

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

    Re: Mark III's eating rectifier tubes

    Post by Sal on Sun Jan 02, 2011 9:45 pm

    Unfortunately today's new manufactured 5AR4 tubes are not built like the good old USA or Mullard 5AR4 of years gone by.

    What your friend can do is a easy mod that most likely help the life of the 5AR4 tubes, he should remove the wires from pin 4 and 6 on the 5AR4 tube socket, move them to pins 5 and 7 and solder a 1N4007 (UF4007) diode from pin 5 to pin 4 and a diode from pin 7 to pin 6. The anode of the diode (stripe on diode) must face pin 4 and pin 6.

    Regards,
    Sal

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: Mark III's eating rectifier tubes

    Post by wazowski on Mon Jan 03, 2011 8:16 am

    Thank You Sal. I'll pass this on.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Mark III's eating rectifier tubes

    Post by Bob Latino on Mon Jan 03, 2011 9:16 am

    What Sal said is good advice. Two other things ..

    1. Check with your friend and see if the quad caps on his Mark III's are original Dynaco quad caps. If they are, have him replace both with the 80, 40, 30, 20 550 volt cap that Kevin at Dynakitparts sells.

    2. Mention to him never to "short cycle" his Mark III's. Short cycling is turning the amp OFF and then ON again in less than 30 seconds. If you shut OFF any Dynaco amp with a 5AR4/GZ34 tube rectifier wait one full minute before turning the amp on again. In that one minute the quad cap will bleed off virtually all the voltage it is holding and the tube rectifier (as it warms up) will be working into basically an uncharged quad cap.

    Bob

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: Mark III's eating rectifier tubes

    Post by wazowski on Mon Jan 03, 2011 6:48 pm

    Will do, and thanks again for your help!

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: Mark III's eating rectifier tubes

    Post by wazowski on Thu Feb 03, 2011 12:02 am

    Update: I did the 1N 4007 rectifier diode modification to the 5AR4 sockets on both of these mark III's and.....no more blown rectifiers! They are sounding MINT! Thank you for your help.

    Sal

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

    Re: Mark III's eating rectifier tubes

    Post by Sal on Thu Feb 03, 2011 11:53 am

    That's good news, thanks for the update.

    Sal

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Rectifiers holding up, but sound is flat

    Post by wazowski on Sun Feb 06, 2011 5:43 pm

    I guess I spoke too soon. Having never had the opportunity to listen to these Marks before so I had nothing to compare them to. Once I completed the mod and ran them for a while to test them, I was impressed with the sound. My buddy, however once he got them home and ran them for a while says that the highs and lows are "cut off" and "flat" sounding. He says the units no longer produce "full and rich" sound and mentioned he can no longer hear things he used to be able to hear on music that he is familiar with. The rectifiers, however are holding up. Can anyone offer any insight here?

    The only thing I did was install the 1N4007 diodes on the rectifier sockets between leads 7 and 6 and leads 4 and 5, with the striped ends at the 4 and 6. I then moved the red leads from the transformers to leads 5 and 7.

    Sal

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

    Re: Mark III's eating rectifier tubes

    Post by Sal on Sun Feb 06, 2011 6:02 pm

    Hi Mike,
    I highly doubt installing the 2 diodes would make a difference in the sound. Was the amp rebiased after adding you gave the amps back to your friend?

    I just so happen to have a ST-70 on my bench with the diode mods. I measured the B+ at the first section of the quad cap cap with the diodes and then with the diodes shorted out with jumper leads. The difference was with the diodes shorted out the B+ was about .5 volts higher. Absolutely no way he would hear the difference.

    Did he connect the speakers correctly and observed the polarity of the speakers?

    Sal

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: Mark III's eating rectifier tubes

    Post by wazowski on Sun Feb 06, 2011 6:10 pm

    TubeRectifier-DiodeMod" alt="" />

    This is a sketch of the modification I performed. It was provided by our good freinds at Dynakitparts.

    Sal

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

    Re: Mark III's eating rectifier tubes

    Post by Sal on Sun Feb 06, 2011 6:18 pm

    Son of a gun, I have the same yellow sheet... :-)

    Sal

    PS: Measure the B+ but if your friend can bias the amp its probably ok.

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: Mark III's eating rectifier tubes

    Post by wazowski on Sun Feb 06, 2011 6:20 pm

    Could the Eico pre amp he is using be causing the problem?

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    Re: Mark III's eating rectifier tubes

    Post by wazowski on Sun Feb 06, 2011 6:22 pm

    LOL I think that yellow sheet gets around!
    He indicated he can adjust the bias. I don't have access to them right now, but when I do, I will check the B+ as you suggest.

    Thanks again!

    wazowski

    Posts : 25
    Join date : 2010-12-28
    Location : Cleveland, OH

    It was the Eico Preamp!

    Post by wazowski on Thu Feb 10, 2011 6:07 pm

    As it turned out, the problem with the sound produced was due to a bad 12AX7 tube in the Eico preamp. The tube was not glowing as brightly as the others. When it was replaced with a tube from another amp, the Mark III's sounded better than ever.

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