Russian K40Y-9 started leaking and caused bias problem

I don't believe you can just change capacitor value. Grid resistor value will need to be changed to maintain time constant for the stage.

Correct. People do only one or the other all the time to either accommodate a less hardy output, or to add an extra bottom octave to the frequency response that the original designer somehow forgot. And make a mess.

The K40 in the top picture is physically much smaller than the ones I've used there. Then again, I tend to use ones that are one to two voltage ratings higher than original spec; 630v and 1kv. Never had a problem with these little Soviet cans, ever.
 
Correct. People do only one or the other all the time to either accommodate a less hardy output, or to add an extra bottom octave to the frequency response that the original designer somehow forgot. And make a mess.

The K40 in the top picture is physically much smaller than the ones I've used there. Then again, I tend to use ones that are one to two voltage ratings higher than original spec; 630v and 1kv. Never had a problem with these little Soviet cans, ever.

Not to hijack the thread but can you share your opinion on whether those caps actually sound better after they break in? Since you used them so much, you'd probably done some A-B comparison tests, freshly installed vs. aged ones in the similar units?
 
I like them where I want a smoother pio sound. They sound natural and smooth, with good tonal balance and no loss of musical nuance or 'detail'.

I like midrange for voices, and they do that particularly well.

Usually an upgrade, but a minor one. It does depend on the equipment and other more impactful elements. A K40 will not necessarily transform an etchy Sylvania preamp tube into a Blackburn Mullard, for instance. But it might take the very worst edge off a current production tube.

Consider them just a dash of seasoning. Otherwise, everything else being equal, I like polyester in oils for coupling in many of these, which sound most like the original ones used, except 10x more refined. Unfortunately, they are an arm and a leg if you can even find them, and have been unobtanium for a number of years. I think I used my last pair several years ago.

But the K40 series, and the y-9s in particular, are a terrific value. All of mine have been bulletproof, which you cannot say about all PIOs. It is one of the few I trust in an output bias circuit.
 
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