Large Main Cap discharge?????


2006-10-23 10:38 am
Hi Everyone,
I have 4 large capacitors ( Electrolytic ) in my tube amp, they are Mallory 800 Uf @ 450V,
when the amp is powered up they all read 435v in the DC setting on my trusted tester,
but as soon as the amp is turned off it drops dramatically down to 9 v in a matter of 20 seconds
is this normal? The amp is 12 years old , and I figure the caps are as well. Can anyone shed
some light on this matter please? Thanks.




2006-09-08 2:04 pm

Based on your data (435-->9V, 20s), the equivalent time constant is 5.16s. Assuming a linear resistance load, this translates into 1611ohm, which means an operating current of 270mA, and 117W dissipation. I find this pretty high: the cathodes must quickly cool down after the power is removed, and all that's left to draw current are the bias circuits.
Anyway, something dissipating in excess of 100W must be easy to locate? Is there a big bleeder resistor somewhere? Are the caps themeselves getting hot?
If it is not the case, you may suspect the capacitors: do they still retain their nominal capacitance?
Hi Slate,

I see you are still at it and that is good!

Not to change topic but how is the Sansui Tuner project you were working on going? Last time you posted, you were considering IC filter changes and maybe then an alignment and also now that you have several months on the caps, how has it settled? Even better?

I hope all is well!