Smoke is never a good sign, WA 88VA

[MarkL]

Will do Richard - just the thing for a rainy Saturday!

[MarkL]

OK, all good. Resistor replaced. Coupling caps replaced.
Saw a Youtube video of a guy demonstrating a failing coupling capacitor and showed why it was significant, even if it measured ok in the first instance.
Now I know a little more, and the amp is fixed!
Many thanks guys
Mark

[John Caswell]

Hi Mark,
Nice to hear the amp is fixed, have to admit I would not have been suspicious of the Clarity caps - extremely reliable generally. The things I would have been most suspicious of are the valves.
Be that as it may it will be worth keeping your eye on it for a little while.

John

[Chris]

I recently sold some Clarity Caps to person who designed them and had since left the company, small world.

Chris.

[Richard]

Originally Posted by John Caswell

Hi Mark,
Nice to hear the amp is fixed, have to admit I would not have been suspicious of the Clarity caps - extremely reliable generally. The things I would have been most suspicious of are the valves.
Be that as it may it will be worth keeping your eye on it for a little while.

John

Yes I agree with John about reliability but a conversation I had a few years ago with a very well known maker/seller of paper in oil caps opened my eyes a bit.

He said pio caps often leaked a small amount of voltage when used as blocking caps. He reckoned a few 10's of mV when blocking 3-400V was to be expected but towards 100mV was not acceptable. I have absolutely no idea if that's true but he'd sold hundreds and built similar numbers of amps. In bias terms that is only a fraction of a volt and would probably go un-noticed without a meter.

That did set me thinking that we would check a cap for capacity value uF, and maybe for esr if we had a meter, but few of us actually stick 400V on one end and measure what comes out of the other. We can do this easily in situe in a cathode bias amp where it should be 0V or near but how many of us have done this simple check using the mVdc range?

Back at that time I had Kit88 which used a compound bias with some negative being fed to the grid and some being derived at the cathode. That complicated things a little, but I checked first that all were receiving the same amount of neg bias, then again the figures at the grids running, and the 4 valves with pio caps all varied a measurable amount so I accepted he was correct and probably like everyone here I have never remembered/bothered to check again since unless there was some obvious problem.

Poly caps may be more reliable and/or leak less voltage but I don't know that they don't leak any at all and for how long.

[bob orbell]

Interesting Richard, I know in my Phono III there is 0 volts DC passing the polyester capacitors, but as for 400 plus volts, I will post next time I am in a power amp, some go over board on PIO, no time for them myself. Bob

[MarkL]

Thanks for the comments everyone. I am keeping a close eye on the amp. Definitely not leaving it unattended!

This was the demo I looked at https://www.youtube.com/watch?v=adhUM3sDbno[/url]

Didn't take much leakage to have a big effect.

But so far, so good