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    Power supply resistor question

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    colin86325

    Posts : 40
    Join date : 2014-04-01

    Power supply resistor question

    Post by colin86325 on Sat Jan 03, 2015 6:09 pm

    I am trying to figure out what wattage the resistors are in my power supply.  The power supply uses 2 can-style capacitors (100-100uF, and 80-40-30-20uF).  I attached a diagram of the circuit but I'm not very good at this stuff, so please forgive the crude drawings.  i also attached a picture of the resistors.  What wattage is safe in this application?  I was thinking 2 watts, but wanted to ask.  This amp has SS rectification.





    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Power supply resistor question

    Post by Bob Latino on Sat Jan 03, 2015 6:22 pm

    Hi Colin,

    I believe that those original Dynaco 22K and 6.8K resistors were two watt. If you want to replace them, two watt resistors would be fine ..

    Bob

    colin86325

    Posts : 40
    Join date : 2014-04-01

    Re: Power supply resistor question

    Post by colin86325 on Sat Jan 03, 2015 10:46 pm

    Bob, thanks for the quick reply!

    Have you seen a similar modification with the two chokes?

    I haven't really been able to enjoy the amp yet because of the 60Hz hum that I guessed is from failed power supply caps. Finally I have found some time to actually do some soldering, so I hope to keep plugging away. Once I get it working well with the stock Dyanco board I hope to then get the VTA board to compare with.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Power supply resistor question

    Post by Bob Latino on Sun Jan 04, 2015 6:09 pm

    The VTA ST-120 uses TWO CHOKES because it draws noticeably more current than the ST-70. IMHO, you don't really need two chokes in any ST-70 .. Dynaco shipped out about 350,000 ST-70's with one choke and those chokes rarely ever go bad. The original C-354 was rated at 200 milliamps and evidently those chokes were not stressed out much. I use the Triad C24-X chokes in all the VTA amps. They are the same size as a C-354 choke but are rated at 240 milliamps. The VTA ST-70 uses ONE choke and the VTA ST-120 and M-125's use TWO chokes. The use of two C24-X chokes would raise the dual chokes rating to 480 milliamps current draw. 480 milliamps is far beyond what either a VTA ST-120 or a VTA M-125 can draw during normal use. As a test one time I ran a pair of M-125's at 75 milliamps per tube current flow for 5 hours. Four tubes (4 X 75 milliamps) would equal 300 milliamps and the three driver tubes use maybe 10 milliamps each. So I am guessing about 330 to maybe 350 (when the music is playing loudly) milliamp current draw for those 5 hours. The amp got a little warmer than normal (checked with an infrared thermometer) but nothing undesirable happened ..

    Bob

    colin86325

    Posts : 40
    Join date : 2014-04-01

    Re: Power supply resistor question

    Post by colin86325 on Sun Jan 04, 2015 8:24 pm

    Ok, thanks for clearing that up. I think for now I will keep the second choke in place as it probably can't hurt anything.

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