Interstage cap (replacement value)

[kevin flynn]

Hello All,

I have found a great deal of info regarding changing values for the parafeed caps in these amps (quite variable), tho not much about the Interstage cap values. 
I'm wondering if there are reasons not to go with a slightly different value here, say .15 vs. the stock .10? -Inadvisable?  Implications?  -

Thanks,

Kevin
PS:  I also have some Russian K40 PIOs rated at .10uf, but only 400v.  Firecracker?  -Thanks again...K.

[Paul Joppa]

The 0.1uF value gives a bass roll-off of 6.4Hz. That's well below the usual 20Hz target, so the frequency response effect of moderate changes (half to twice the value) will be hard to hear - if you hear a difference, it is most likely to be a difference in the capacitor, not the capacitance.

[kevin flynn]

Great, thanks!

K.

[Natural Sound]

If it were me I would never consider using a capacitor rated at 400v when the circuit called for a voltage rating of 630v. Using a higher voltage is fine but not lower. Thats just a general rule of thumb and PJ, PB or Doc may shed some more light on this subject.

[Paul Birkeland]

400V for the interstage 0.1uF cap on the SEX is OK. It will see full voltage during warmup, but significantly less than that after the 6DN7's start conducting.

The parallel feed cap, OTOH, sees a lot of DC voltage with a nice helping of AC on top of it.

[Paul Joppa]

PB is probably correct, but I am a little more conservative, so I won't recommend anything less than a 600 volt rating.

It's true that in correct operation the voltage is about 70 volts. During the startup transient, and in fault conditions such as an inoperational tube, the voltage goes closer to 500v momentarily. Many 400v caps can stand 500v for a short period, but the lifetime of a cap is sensitive to both voltage and temperature - and under the hood most tube amps get pretty warm. Some cap constructions are more sensitive than others - mylar for instance has a much greater de-rating for temperature than polypropylene.