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    Reducing st35 output.

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    dmag

    Posts : 30
    Join date : 2010-10-05
    Location : North Shore,Ma.

    Reducing st35 output.

    Post by dmag on Fri Jul 13, 2012 7:05 am

    Lately Ive come to really enjoy EL84 tube amps One is my st35.I'm beginning a rebuild of two amps.I would like to use Genelex Gold Lions for the output tubes.
    I have been advised that this amp overdrives them and they would last a very short time.
    Any suggestions how to reduce the total power tube dissipation to 11 watts?
    Idle current reduction?
    Also how would this relate to a sca35,
    as that will be a project once I get my hands on a project quality donor.
    Thanks in advance.

    Sal

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

    Re: Reducing st35 output.

    Post by Sal on Fri Jul 13, 2012 9:19 am

    I would highly recommend Dynakitparts.com ST-35 BCU bias control upgrade kit as you can individually set the bias of each EL84 tube. You can then set the bias to your desired level to increase tube life. Go to http://dynakitparts.com/store/product.aspx?id=293 for more information.

    You can also install this upgrade in a SCA-35.

    Regards,
    Sal

    Captain Coconut

    Posts : 209
    Join date : 2009-10-13
    Age : 64
    Location : Great White North

    Re: Reducing st35 output.

    Post by Captain Coconut on Fri Jul 13, 2012 9:22 am

    Or, you could implement D. Gillespie's Enhanced Fixed Bias which knocks down quiescent current to 27mA from 35mA in the output tubes.

    Bob Latino
    Admin

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

    Re: Reducing st35 output.

    Post by Bob Latino on Fri Jul 13, 2012 10:56 am

    Both Sal and Captain Coconut have given good advice. A quick and inexpensive thing you could also do is to replace the 95 ohm common cathode resistor found on the quad cap with about a 120 ohm 5 watt resistor. This would (depending on your line voltage) also bring the idle current from 35 mA per output tube to about 27 to 28 mA per output tube.

    Bob

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Reducing st35 output.

    Post by Jim McShane on Fri Jul 13, 2012 8:23 pm

    Bob Latino wrote:Both Sal and Captain Coconut have given good advice. A quick and inexpensive thing you could also do is to replace the 95 ohm common cathode resistor found on the quad cap with about a 120 ohm 5 watt resistor. This would (depending on your line voltage) also bring the idle current from 35 mA per output tube to about 27 to 28 mA per output tube.

    Bob

    Bob, that's true - but the lower current reduces the load on the power supply and the voltage increases. So you have a bit of a balancing act to do.

    I agree, the two prior posters gave very good advice. The Genalex will not take more than 12 watts dissipation but I recommend a max of 11 in case of a high AC line or similar that increases the dissipation.

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