I get a lot of Email questions about the Russian K40-Y PIO cap upgrades for the VTA ST-70 and ST-120 amps. These caps (four to each amp - one for each output tube) are COUPLING caps. All four are directly in the signal path. What the Russian K-40Y's do is take the audio signal from the driver circuit and send the signal to each output tube. What you want to do here is send that audio signal to each output tube with as little "alteration" (compression, loss of frequency response, added distortion etc.) as possible. These caps do the job just fine and are at least comparable and in many cases BETTER than caps costing a lot more money. These caps were made in the 1980's for the Russian military during the Cold War and are hermetically sealed. I once left one of these caps in a glass of water overnight, pulled it out, wiped off the water, measured the capacitance and it was exactly the same as I measured the night before. I have used over 4000 of these caps since 2007 and (to my knowledge) only ONE has failed. That is a failure rate of .025% or a reliability rating of 99.975 %.
Through experience I have found that these caps take about 100 - 200 hours of play time to break in and sound their best. Now and then I get a customer who used solid state amps their whole life complain that these caps "seem rolled off". The top end frequency response is not rolled off but is so smooth to those use to the harsh top end of some solid state amps that something seems to be "missing" .. Yes - that solid state harshness is missing. I have heard other coupling caps like the Mundorf, Clarity, Auricap etc. and some (like the Mundorf's) were excellent sounding caps BUT - they are about $40 a cap about 5X times the cost of a Russian PIO cap.
Below is a link to a 2010 forum post that I made which shows what the inside of a Russian PIO looks like ...
Inside a Russian PIO cap
Bob
Through experience I have found that these caps take about 100 - 200 hours of play time to break in and sound their best. Now and then I get a customer who used solid state amps their whole life complain that these caps "seem rolled off". The top end frequency response is not rolled off but is so smooth to those use to the harsh top end of some solid state amps that something seems to be "missing" .. Yes - that solid state harshness is missing. I have heard other coupling caps like the Mundorf, Clarity, Auricap etc. and some (like the Mundorf's) were excellent sounding caps BUT - they are about $40 a cap about 5X times the cost of a Russian PIO cap.
Below is a link to a 2010 forum post that I made which shows what the inside of a Russian PIO looks like ...
Inside a Russian PIO cap
Bob
