Coupling Cap Substitutions?

Anyone played with swapping coupling caps in an ST-120 or M-125 willing to share thoughts?  

I have only used the stock Russian PIO KY40 the came with the M-125 kits I built 6 years ago.  Doing a mini rebuild right now and I have the driver boards pulled so would be easy to swap something in.  Not unhappy with how things sound but have been surprised in past with changes to other gear from component upgrades.  (real or imagined)

Any impressions from swaps people have made appreciated.

Thanks

The only way to know for sure if there are changes with a cap swap would be to build two identical amps with identical parts. The only difference, of course, would be the disparate caps. And then hook them up to a comparator.

C5-C8 are the main caps to look at for upgrades. C1 and C2 might be worth playing with as well.

Here are the caps I’m familiar with. These are my observations and opinions.

Sonicap Gen 1 - Neutral on the side of cool. A solid choice for detail and affordable.

Clarity Cap
CMR - A very transparent, detailed, and dynamic cap. Similar to V-Cap TFTF and less cost.
CSA - Similar to the CMR but with somewhat less extension in the treble. Very good midrange and affordable.
MR - Similar to CMR but slightly less transparent.
ESA - Similar to CSA but slightly less transparent and also more muted in the upper treble. Not recommended.

Dayton Audio - Fairly generic and kind of dull sounding. Not recommended.

Audyn Q4 - Another generic cap that lacks transparency. Not recommended.

Erse Pulse X - A little dark sounding over all but ok detail. Just needs to be turned up. Cheap!

Solen Fast Cap - Not the ultimate in resolution but actually very good detail. Cheap!

Russian
K40Y - A very nice detailed and warm cap though not the most transparent.
K42Y - Similar to K40Y.

V-Cap
TFTF - Pricey but an excellent cap. Very spacious, detailed and neutral.
OIMP - Slightly warmer sounding than TFTF. Very good detail but not sure if as transparent as TFTF.
CUTF - Another pricey and excellent choice. Better than TFTF.

Miflex
KFPM - An inexpensive and very good sounding cap. Recommended.
KPCU - Very similar to the Jupiter copper foil. Excellent detail and transparency and still reasonably priced.

Jupiter JCW-060 copper foil, paper and wax - Pricey and excellent cap. Vey transparent, detailed with just a hint of warmth.

I do not have the VTA but the original vintage amps. As such they still have the normal output section coupling caps but no C1,C2 inter stage caps. Regardless I have tried a number of different caps on my restored amps. A few opinions of mine to add to Corndog71…

Russian K40/K42 in ST70 and MK IV – both similar and a real value. Smooth mids are their best quality. Modest cost and recommended as such.

Sprague Vit Q in MK III - similar to the Russian K40/K42 type but getting a little pricey. Nice

Jantzen Suprior Z Cap in MK IV – A little bright on top but maybe still braking in. More detailed top end than the Vit Q.

Audio Cap (by Rel cap) in MK III – very detailed and clean top to bottom. Nice match to the amp. Highest rating my me and worth the higher cost.

Solon Fast Cap in ST 35 – to bright and sibilant. Will be trying something else.

JFX Premium in SCA 35 - typical MKP with bright top end. Nice for economical project.

Obbligato Gold Ultra Premium -in MK VI. Smooth and extended. not harsh at all. A good value

The next few caps were tried in crossovers not coupling caps so opinion may not be of value.

Clarity Cap CSA -  agree with corndog71 on sound and cost

Erse Pulse X – seemed fine with good and not overly bright top end. Cheap.

Corndog71 – thanks for the info. I may try your Miflex recommendations.

Thanks to both of you, this is a good start to what ultimately will be throwing a (possibly semi-costly) dart at what is pretty much a whim project.  My general go to caps are Cornell Dublier 940-C for "nice but affordable" and Mundorf Silver Oil (not gold) if I'm feeling "urge-to-splurge" but they have vague reputation of not holding up particularly well in higher heat so hadn't really considered them for this use.  I doubt the 940's will improve over the Russian PIO.

Corn Dog:
The Miflex look interesting, but appears they are rated at 70c which might be of concern mounted inside chassis near a quad of KT-120s.  Are you using them in the coupling cap position and if so how long have they been installed?
(besides the temp rating they are right at limit dimensionally and their thick leads will require enlarging through holes .... Hmm, multiple flags going up, I think the world is speaking to me)

The V-cap TFTF's are being discontinued and available at 40% discount which is very tempting, going to look at those a bit closer.  Correct value might be sold out, will check with vendor this week.

Oh yeah, I forgot to mention some of those caps are big.  Always double check dimensions before ordering.  The Miflex leads shouldn’t be an issue but Clarity and V-Caps do have thicker leads.  On other kits I’ve used a 1/16” bit to open up the pcb hole which usually works.

I have used the the Miflex caps in my custom ST35 for over a year with no issues.  I have also used the Jupiter’s in my custom M125 amps for a couple of years and again no issue.  I don’t think the inside of the tube amp chassis should be a problem if it’s vented well enough.  Most of the heat comes from the tubes and the PT which are outside of the chassis.

I have been running a "full" set of Audience Auricap capacitors on a pair of Mark 3s (with VTA driver boards) I built some years ago. I love the detail it adds. Been running without any issues for about 10 years now, however at the time the added cost did hurt a bit (nowhere near the $$$ I have seen for some of the "boutique" couplings caps that seem to be popular):

Has anyone ever tried Kimber Kaps?
I have a set leftover from another amp. I might install them in the MarkIIIs.

E.

Oh yeah, I’ve tried Kimber Kaps too. Still have a couple pairs in a box. I didn’t mention them since they’re discontinued. They were ok as far as can remember but it’s been a long time since I’ve heard them. Try them and report back.

OK, I got sidetracked a bit and thought to post an update almost 3 months after the fact.  On WLT's recommend I installed the AudioCap RT series in my M125's.  I made a number of other changes to the amps as built 6 years ago, and the cap sub was side mod when my real quest was to remove a low level 60hz hum present from day one.

Well, after getting everything rebuilt and doing a quick bench test I proceeded to leave the amps on my workbench for a couple months while indulging in other projects, a couple trips and, well, life in general.   I just got them hooked up in my main system a couple weeks ago and initially everything sounded, umm, fine.  Hummmmmm was pretty much gone, and they're solid but not particularly knocking my socks off.

Fast forward a few dozen hours of play time and now they have really opened up.  I've always been dubious of "burn in" claims, but damned if all of the sudden they sound really good, retrieving details I've not heard in some of my recordings before.  Big smile and happy.  Will post some photos and rebuild notes below.

The vintage design multi point chassis based grounding scheme on the M125's will have noise floor limitations but I was looking for low hanging fruit without making major revamps requiring reinventing the wheel (beyond my pay grade anyhow).  Replacing the stock PCB alloy standoffs with nylon was easy pickings save the fact that I built my amps with a custom glass cover and my mountings were semi messy to replace.  I ended up fabricating these standoffs with a stainless threaded post that anchored the standoff to chassis and extended up into the glass cover and used the molded male threads to mount and isolate PCB from chassis



Examining my PCBs once removed from amp revealed an area where lock nuts scraped away solder mask and exposed copper from PCB ground plane - might have been source of my hummmmmm issue




As long as I'm in there might as well replace some caps.  With the glass cover, all my electrolytics are mounted on bottom of PCB and the new standoff height required a search for a short replacement for C14.  Not a lot of choices but found a Wurth brand that fit and had appropriate specs.  Also replaced the stock Jantzen Poly in C2 with my favored Cornell Dublier 940.  It's a much larger cap and it had to be mounted on bottom as well.




The stock driver/splitter tube sockets needed to go too, despite constant retensioning they always seemed sloppy so replaced with Beltons that have really secure grip

Now on to replacing the coupling caps.  Corndog and WLT both had excellent suggestions and I was tempted to try and make the Miflex fit but there was no reasonable way to stuff them in the space available and I ended up with RelCap AudioCap RT series suggested and highly recommended by WLT.  These are a fair bit more compact than the Miflex but still ended up a cozy fit with all the 'lytics mounted on bottom of board







All done and back together








As mentioned in my earlier post, hummmmmmm is now gone and after about 12-15 hours of sounding pretty good they have come alive and sound wonderful.  I changed a few things at once so hard to say if the coupling cap is what I'm hearing, it's certainly the most likely candidate but I was not seeking to narrow down an incremental change to a single upgrade and just shotgunned in a bunch of parts and hoped for the best.  Fun little project with excellent results.

Thanks to all who offered input

The Miflex KFPM 0.22uF are only 23mm wide. They should fit just fine.

I've had 35mm V-Caps fit inside of an old Mk IV.

corndog71 wrote:The Miflex KFPM 0.22uF are only 23mm wide.  They should fit just fine.  

I've had 35mm V-Caps fit inside of an old Mk IV.

Oh,  was looking at the KPCU copper series at about 30mm diameter not the KFPM.

Tried my best to fit the copper series in and went to the effort of finding a couple small cylindrical bottles of similar dimensions to do a dummy fit and see if 30mm would work next to my electrolytics but it was too crowded.

I just bought some Solen Pure Silver caps.

And some Mundorf Gold/Silver caps...

WOWZER~!

Mundorf's in my tube pre and Solen silver in my Acurus P10 SS phono preamp.

I am SO impressed~!

It's not the first thing I would do, But have already maxed out performance in every other way.

Buck