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    Don't run just one channel on a Dynaco ST-35 power amp ...

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    Bob Latino
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    Location : Massachusetts

    Don't run just one channel on a Dynaco ST-35 power amp ...

    Post by Bob Latino on Tue Jan 26, 2010 7:55 pm

    Today I was changing out the 4 coupling caps on my personal ST-35 amp to some Russian PIO coupling caps. I did one channel and thought "OK - maybe I will just check out that channel to see if it works OK". So what I did was to put in the driver tube and the two EL84 output tubes for just that (it was the left) channel and check to see if the channel with the changed out caps played OK. I left out the tubes for the right channel. No sense in putting them in because I know that channel plays OK. I turn the amp on and the left channel is playing OK at first but after a minute or so the sound is distorted and I quickly notice that both EL84 output tubes have cherry red plates affraid > a sure sign that those two tubes are drawing WAY too much current. What the hell is wrong? And then it dawns on me ... In the ST-35 with the stock bias system all the current for ALL FOUR OUTPUT TUBES is drawn up through a single SHARED 95 ohm bias resistor. The current drawn up through that resistor (maybe 140 to 160 milliamps) is supposed to be shared by FOUR output tubes. Each output tube is supposed to draw about 35 to 40 milliamps apiece. But now, with only TWO output tubes in there, each of the TWO output tubes is getting 70 to 80 milliamps apiece which is DOUBLE what they are supposed to draw. No wonder those two tubes lit up cherry red on the plates. What did Forrest Gump say? > "Stupid is as stupid does .." I was lucky because the tubes did not burn up and were OK after the altercation.

    As a side note - this "running a single channel problem" won't happen with a stock ST-70 which has a separate bias resistor for each channel or a VTA boarded ST-70 which has a separate bias control for each output tube.

    Bob

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Re: Don't run just one channel on a Dynaco ST-35 power amp ...

    Post by scott6058 on Wed Jan 27, 2010 6:36 pm

    Just goes to show your learning curve is still healthy.
    Just kidding.
    I'm a newbie to my st35 and have a question.
    What could cause my one input tube to glow in the leads below the bottom plate at initial start up. It only happens to this one tube regardless where I place it. If I shut it down and fire it up a minute later it behaves fine.
    Thanks for all your efforts Scott

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

    Re: Don't run just one channel on a Dynaco ST-35 power amp ...

    Post by Bob Latino on Wed Jan 27, 2010 7:01 pm

    scott6058 wrote:Just goes to show your learning curve is still healthy.
    Just kidding.
    I'm a newbie to my st35 and have a question.
    What could cause my one input tube to glow in the leads below the bottom plate at initial start up. It only happens to this one tube regardless where I place it. If I shut it down and fire it up a minute later it behaves fine.
    Thanks for all your efforts Scott

    Hi Scott,

    From your description it appears that what you describe is probably normal for the filament in that tube. If the amp plays OK I wouldn't worry about it. Is that 12DW7/7247 tube possibly a Mullard, Amperex or some other tube that could possibly have been made by Mullard but "rebadged" to some other manufacturer's name? The reason I ask is that on one of the other posts up here I mentioned about the "Mullard Flash" that occurs upon turn on with nearly all Mullard small signal tubes (12AX7, 12AT7, 12AU7, 12DW7 etc.) When these tubes are dead cold they will flash brightly for a second as soon as you turn the amp on and then settle down to a normal glowing filament. This is perfectly normal with these Mullard tubes and does no harm to the tube. If you turn the amp off and then on again while the tube is still warm they won't "flash". There is a link below that explains this in more detail ...

    Mullard Flash

    Bob


    Last edited by Bob Latino on Wed Jan 27, 2010 8:43 pm; edited 1 time in total

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Re: Don't run just one channel on a Dynaco ST-35 power amp ...

    Post by scott6058 on Wed Jan 27, 2010 8:32 pm

    Hi Bob
    The tube is a Bogen branded Mullard 7247.
    Thanks for the help and the interesting link.
    Scott

    Sal

    Posts : 223
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: Don't run just one channel on a Dynaco ST-35 power amp ...

    Post by Sal on Thu Jan 28, 2010 7:45 am

    Hi Bob,
    Just wire in the ST35-BCU bias upgrade kit from Dynakitparts.com and that should solve your mono testing techniques... :-)

    The ST35-BCU works great on a SCA-35 also.

    Regards,
    Sal

    PS: Might be hard to bias the stock ST-70 with only 2 tubes installed.

    Bob Latino
    Admin

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

    Re: Don't run just one channel on a Dynaco ST-35 power amp ...

    Post by Bob Latino on Thu Jan 28, 2010 7:59 am

    Sal wrote:Hi Bob,
    Just wire in the ST35-BCU bias upgrade kit from Dynakitparts.com and that should solve your mono testing techniques... :-)

    The ST35-BCU works great on a SCA-35 also.

    Regards,
    Sal

    PS: Might be hard to bias the stock ST-70 with only 2 tubes installed.

    Hi Sal,

    I have biased just one channel on a number of ST-70's with no problems. Because you have only one channel's tubes in there, the voltage drop is lower and the B+ high voltage is a little higher. There seems to be enough latitude built in to the ST-70's stock bias system to run just one channel. It works OK with the VTA driver board's bias sytem also.
    Bob

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