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my st70 has developed a loud humm. i was running SS rectification for a while maybe that damaged the tank caps. how much humm should the st70 have?

is it possilbe that SS rectification damaged the caps?

thanks

A typical 5AR4 rectifier drops about 17 volts wereas a SS diode drops about 0.7 volts. By replacing the 5AR4 you increase the B+ by 16 volts. If you dont have an inrush current limiter installed in the PT primary circuit you have near instantaneous voltage applied to the filter cap before the power tubes warm up enough to drop the B+. It is very likely therefore that the first or second stage of your filter cap is kaput.

i installed a CL 90 on the primary of the PT between the fuse holder and the power cord. i have been using the gz34 rect with the diode mod to protect the tube.

i guess ill have to buy some new caps just to to eliminate that possibility.

could it be the signal tubes? i read on a forum that one person had a noise problem and solved it by installing new signal tubes.

anyways.

thanks for the reply

Well if you kept the rectifier tube in place then just adding diodes in series won't change the B+ at all. Especially with a ICL in the primary.
Short the inputs to ground directly at the amp RCAs. If the noise diminishes significantly the problem isn't the amp. If the noise is still there remove the 7199s, power up and see what happens. If the noise is gone then chances are its in the 7199s or the last two cap sections.
If the noise is still there then its in the first two cap sections or in the bias supply.

thanks a bunch, it's quiet with the inputs shorted! this is good news because l like the amp.

where the noise is coming from i haven't found out yet.

thanks again

chopchip wrote:where the noise is coming from i haven't found out yet.

Did you recently connect anything new to your system?
Anything that connects to TV Cable or a PC tends to
create a ground loop and hummmmmm.