How important is the coupling cap?

[rick57]

I’m about to buy parts for my first tube amp.
I’m told the coupling cap is the most important cap. I read two ‘top of the pops’ comments here for V-caps, by Trombone and serengetiplains (one of whom has tried most types).

It was suggested to me to get MutiCap film & foil RTX polystyrenes “should be 95% as good” and about $80 US the pair less.

Would correct extrapolations of the above suggestion be:

A *completed amp* *with RTX polystyrene coupling caps would be about 95% as good as one with V-caps

And therefore that a *completed amp* with V-caps coupling caps would sound about *5% better than one with RTX polystyrene?

Thanks

[rdf]

I feel building my first with premium parts was a mistake. Now I have version 4 of a different circuit and a pile of $4 resistors with short leads. My recommendation for a first cut is to not go crazy on parts quality. If you're comfortable with Ebay maybe some Russian PIOs for coupling plus non-magnetic generic carbon comp resistors, $10 ASC poly in oil (or Blackgates, still nowhere near $80) for cathode bypasses. Leave the leads long to start. Put the real money into the iron (my second mistake) and save the final voicing for when you're absolutely, positively certain it's the amp you want.

[dsavitsk]

I second this. The difference between $100 film caps and $2 film caps is subtle. Get the circuit right first with Solens or something of similar cost, then upgrade as needed, which may be never.

[jeff mai]

As I mentioned already privately, what's best for someone else may not be the best for you and your system. In fact, I'd say it's likely you wont like the same thing as someone else - no matter how many caps they tried.

I suggested the RTX polystyrenes as good candidates because they aren't particularly expensive in the value you require and lots of people seem to like them even if they're rarely judged "the best" - whatever that means. I don't really care which you choose, just don't spend a lot on caps right now. That's my advice.

And, man, you take everything far too literally... What does "sound about 5% better" mean exactly? I used 95% to talk you out of spending the money because I KNOW you are much more likely to get a sound YOU LIKE by trying several different caps than by trying only one cap no matter how much you spend on it.

When I said "most important" I didn't mean "spend heaps of money". I meant "listen to a lot of different ones and pick the one you like best." You're not building this amp for serengetiplains to listen to are you?

[analog_sa]

A *completed amp* *with RTX polystyrene coupling caps would be about 95% as good as one with V-caps

A very sensible suggestion. I have no experience with V-caps but the RTX may be 95% as good as having no cap anyway.

[arnoldc]

I have used a) Jensen Copper Foil, b) MultiCap RTX, c) Audience Auricaps and all I can say is that my wallet "hears" most of the difference than my ears

[rick57]

5 out of 5!

That’s er a misere – thanks guys!!

[planet10]

One place where you might not think of spending money on a cap -- especially if an SE -- is the last cap in the power supply. This is often ignored and an elco is used, but since it is in the current loop it is in the signal path.

I do recommend starting with an elso and then swapping in a Solen (or somesuch)... we did that a couple times, and don't bother with anything but poly there.

dave

[rick57]

Thanks dave

I'll look into that too

[tubelab.com]

The "sound" of a cap has a lot to do with the circuit that it is in. If the cap is working into a high impedance load then the differences between caps is minimal. If the cap sees a low impedance, or time varying load (the usual case) then there could be an audible difference between the various capacitors.

As stated above, the output transformer will have the greatest effect of the sound quality by far. You are not likely to change it once the amplifier is built, so get a good one from the start. Caps can be changed easily, so you have two choices. Put a cheap cap in the amp to get everything working, and plan on changing it. Or (my preference) put in a good, but not outrageous cap (I use Auricaps) and change it later if you feel the need. Leave the leads long enough to use it over on the next amp.

As also stated above, in a SE amp the last power supply cap is very important. Leave room in your design for a upgrade later. Polypropelene motor RUN (not start) caps are relatively cheap possibilities. An electrolytic bypassed with a Solen also works.