Testing caps?

[rymac]

How can you test a coupling cap in a tube amp circuit?

[PRNDL]

The simplest way is to measure DC voltage on each side.
It should go from 100's of volts to near zero.

It may be possible to use an ESR meter.

[tubeswell]

disconnect (i.e. unsolder) the 'tail-end' of the coupling cap (the end that is closest to the grid of the next stage) from the signal path and lift it away from the circuit, clip your DCVmeter from the unconnected end through your meter to the chassis. Switch the amp on and put a signal into the input. You should not be getting much DC through the cap. Most coupling caps leak a tiny bit of DCV, but it DC is there in any amount above single digit mVs, its probably haddit.

[rymac]

What is the best brand or type to use for coupling caps??

[Danglin' Fury]

Get a good multimeter, preferably a Fluke, set it to AC voltage and go across the leads of the electrolytics. If you read an AC voltage and it is fairly high, it is bad. Caps pass DC and block AC.

[Jag]

Caps pass DC and block AC.

NO!

[rymac]

What do coupling caps block?

Who brand of caps work best for the coupling cap?
I read CARR amps use Mallory 150 caps.
I have some orange drop caps that have been unused for a few years.
Can they go bad?

[tubeswell]

Coupling caps block DC

Any modern film caps of the right minimum voltage rating (~400V+) will work, although higher voltage ones (500V - 630V) are more robust. Different brands and compositions can have subtle impacts on tone in my view (based on my own experience of modding and building) - Other folks will tell you that the only thing that impacts on tone is the capacitance value. Who is right? Let your own ears be the judge.

Orange drops will be sufficient for most coupling cap scenarios, so will mallory 150s.

For smaller value caps I find polystryene caps too dull - is this because the ones I have tried weren't high tolerance? maybe? - they also can be problematic for soldering as they melt easily. Disc ceramic caps can go microphonic more easily, and they generally don't last as well, although they can give quite a bright edge when used in treble parts of tone stacks - but whether you like them or not is a matter of taste. I prefer silver micas for smaller values (1000pF and smaller). FWIW Cornell Dublier makes them up to 4700pF.

As to their age, it shouldn't matter how old film caps are if they are unused. Electrolytic caps are the ones that have a limited shelf life.

[Danglin' Fury]

NO!

In power supply applications, caps act as a reserve for DC voltage, and bypass AC from the power supply circuitry.

[Gtr_tech]

In power supply applications, caps act as a reserve for DC voltage, and bypass AC from the power supply circuitry.

The OP was talking coupling caps....and they don't pass DC...at least the ones in proper operating condition don't.