Output Cap Values

[greg788]

How low in capacitance (1uF, 1.5uF, etc.) can I safely go with the output caps as far as bass roll off? I have tons of caps in the 1-2uF range ...

[Paul Joppa]

The stock 2.2uF is down 1dB at 15Hz, -3dB at 7.5Hz. The cutoff is inversely proportional to the capacitance, so for example a 1.1uF cap would roll off -1dB at 30Hz, -3dB at 15Hz.

But there are other rolloffs in any system. Our power amps usually have a rolloff of -3dB at 5 to 7 Hz for the interstage coupling cap, and around 20Hz for the parafeed output cap, so in general I would not go below 2.2uF for a general-purpose preamp. For your particular system, you can make your own choice. It's a cheap experiment, especially if you have some suitable caps on hand, so try it and see! (Be aware that most caps need 50-200 hours of music to reveal their true sound; teflon seems to take longer.)

[greg788]

Just to confirm what's in the manual, since I have a pair of 2uF, 200V rated caps, these are safe to use for coupling since the 6V6 is being run at 125V?

[Paul Joppa]

At startup, the voltage across the output capacitor is quite a bit higher than 200v - I'm away from my notes this morning, but somewhere around 350v. I wouldn't recommend anything less than 400v unless you know the surge rating to be high enough. (The stock 250v cap has a surge rating of 375v.)

[greg788]

Paul (or whomever else has experience):

In your experience, is it worth upgrading these 2.2uF caps?

From previous personal experience, there's a lot of expensive caps out there that aren't worth the cost or trouble (a lot of these caps are really big and heavy, too).

If it's worthwhile in your experience, any recommendations? Auricaps seem like a good bet for their sonics, light weight and small size, for example. I don't mind spending decent money on, say, Vcap Odams or Jupiters (which worked well for series position on tweeters), but curious to know your experiences with tube coupling applications and what you like?

[Paul Birkeland]

V-cap ODAM, Auricap, and others will certainly sound different than the stock caps.  We generally use polypropylene coupling caps, so the differences aren't as drastic as they might be if changing over from a ceramic or mylar cap, but I believe the Moreplay cap is polyester.

Do be careful though not to buy something too enormous and/or heavy, as the terminal strips will only support so much weight.