K-resistor bypass cap value

[Deke609]

In preparation for some SII-45/Kaiju comparisons and tests, I want to replace the stock 'lytic output tube k-resistor bypass caps in the SII-45 with film caps. My Kaiju already has film caps in that position, and making the same change in the SII-45 should make for a more apples-to-apples listening comparison. Stock values for the SII are 220uF/250V. I've got some 330uf/250V Solen film caps on hand.  They should be fine, right? If anything, the extra capacitance will reduce capacitive reactance a tiny bit?

Or is something other than cap reactance at play here and I need to stick to 220uF?

many thanks, Derek

[Paul Birkeland]

There's not a ton of room in the Stereomour to fit a 330uF film cap.   The 220uF caps are in that position because there's another pair of 220uF caps in the power supply, and packing and including another different part wasn't a reasonable option at the time.   If I had to revise the kit, I would take the 220uf caps out and put the axial caps in from the Kaiju (that are also used in the Crack-a-two-a).

IMO 47uF is the lowest value I would use there, and that's also a value that has been used in previous kits for the same position.  If you have 100uF in your Kaiju and want to do a consistent comparison, then I'd use that in your S2.

[Deke609]

Many thanks PB.

There's not a ton of room in the Stereomour to fit a 330uF film cap.

Yeah, not much room at all, especially with all three upgrades. I'll be taking the Mourquiet out. The amp is going to be rebuilt on a much bigger chassis, so it's slated for a full dismantling anyway. The film caps will go where the Mourquiet presently is.

If you have 100uF in your Kaiju and want to do a consistent comparison, then I'd use that in your S2.

Fair point.  I'm not sure what I put in the Kaiju b/c, foolishly, I installed the caps with their labels facing the chassis plate. But they're Solens, and judging by the size of them I'd guess they're about 200uF or so. I doubt I'd be able to hear a difference (assuming there  even is a performance difference) between a 200uF and 330uF film cap doing duty as a k-r bypass cap. But I'm pretty sure I heard a difference going from 'lytic to film in the Kaiju. I'll have a look through my posts about the Kaiju to see if I asked about bypass cap sizing - I mostly likely did as that's the kind of thing I would probably ask about (as this thread evidences). [Edit: they are 100uF - I found the emailed order confirmation from Solen. I'll use the 330uFs for now and order some 100uFs]

cheers and thanks, Derek

[Deke609]

The 330uFs wouldn't fit, even with the selector switch and MourQuiet removed. So I ordered some 130uF Solens and just installed them this afternoon. What a great mod. Sound is at once more detailed and smoother. With the new film caps and new iron, the SII-45 is now playing in the same league as my Kaiju (which also has Solen film cathode resistor bypass caps). There are still noticeable differences between the amps, but I can't say that one is better than the other. They're just different.  Cool!

cheers, Derek

[2wo]

Another option especially on the 45 is to try no cap at all. Not saying that this is the way to go but worth a listen...John

[Deke609]

Thanks John. I will try it.

When you write "especially on the 45", is this on account of the lower potential voltage swing of the tube (lower plate-to-cathode voltage as compared to 2A3 or 300B), making the presence of a bypass cap less important? Or something else? I thought I understood what the function of the bypass cap was until I did some more reading about it today online ... lots of seemingly conflicting info. I haven't had a chance to really think it through yet.

cheers and thanks, Derek

[Paul Birkeland]

The presence of the bypass cap is critical.  You would definitely learn that by listening to the amp without them.