Princeton Tweed tone cap... Ceramic or Silver Mica?

[tubedood]

Hi again!

In my quest for the lil Princeton Tweed project I have moved on to what cap to use for the 500pf to tie the tone control to the volume wiper? I don't have a 250pf-500pf so this is one to purchase. What is folks thoughts on the type to use in this application?

Dale

[Chuck H]

I've always used mica because it's "supposed" to be the best cap for the job. I wonder sometimes though. Lots of great tones from older, stock amps with ceramic caps for HF circuits. And I've seen interesting distortions on the mica treble cap in TMB tone stacks. I've never applied the same tests to ceramic caps so I don't know if that was just a mica thing or not.?. Then there was a recent post where someone was having trouble with a manufactured amp that was known to be commonly fixed by replacing the mica cap in the tone stack. The cap tested fine and I poo poo'd the idea that it was responsible for the problem. In the end the OP did replace the mica cap and the problem went away, much to my chagrin. One of the posters on that thread did mention that mica caps were a poor choice for HF circuits in amps and gave a technical reason for it, but I can't remember what it was. If I find the thread I'll link it. Not really an answer to your question I guess. Just sharing my own experience and "subscribing" to see what others have to say about it.

[Chuck H]

Ok, I looked up the thread. It's pretty windy so I decided not to link it, but I'll do a short synopsis...

The C11 mica treble cap in the Fender Prosonic is known to go leaky. This shouldn't be a big deal since it follows another cap that is already blocking DC. Myself and others tried really hard to help and put the poor guy through all sorts of paces. In the end another tech mentioned that the phenomenon was absolutely true and that he'd replaced the mica cap in three of these amps. All resulting in a happy ending. So the OP replaced C11 and the problem was fixed. And no one ever did site a technical reason not to use mica but that tech did say "I hate new mica caps in high voltage circuits" (paraphrased). Hmmmm.?.

[tubedood]

Thanks Chuck glad you replied... your findings are like mine in regards to the two. I also heard the silver micas are great for tone circuits but in certain cases tend to be microphonic or some other issue. It seems some folks are returning to the ceramic in some tone controls? This is such a simple (yet effective I guess!) tone control... with one capacitor to send certain frequencies to ground and another to allow certain frequencies to bypass the volume control... I want to make sure I get what will provide the best sounds!

[Pedro Vecino]

The Vox AC30 Korg of the 90s also had a higher than expected problem rate with silver mica capacitors. At that time I had four or five in a very short period of time. Later I did not see any with problems again. But today is no reason to distrust them. It could be that the installation system, directly on printed circuit and without any margin in the pins was related.

[Chuck H]

Yet another thread about problems with mica caps has popped up. Our resident Leo Gnardo pointed out that he's seen a lot of old silver mica caps go hinky. He replaces them with ceramic.

Mica's seem to be getting a rep as the common denominator in association with problems... Just sayin'.

[Leo_Gnardo]

Yet another thread about problems with mica caps has popped up. Our resident Leo Gnardo pointed out that he's seen a lot of old silver mica caps go hinky. He replaces them with ceramic.

Mica's seem to be getting a rep as the common denominator in association with problems... Just sayin'.

Good memory Chuck, let me clarify. Might seem that way as I've spun the yarns here more than once. Add to the one about $$$$ bootiki with the old mica, a recently made silver mica that was installed by me and started shorting less than 10 years later. Rated at 500V too, a good bit over what it was subjected to in the preamp of some ordinary Fender. I'm not swearing off silver micas entirely BUT I'd avoid old mica caps silver or not, and consider any of them with a scanny eye when troubleshooting.

[tubedood]

Good to know Leo, I was thinking of trying out an old domino mica cap either 250 or 300pf for that tone control. Looks cool but maybe not the most reliable idea.

[Leo_Gnardo]

Good to know Leo, I was thinking of trying out an old domino mica cap either 250 or 300pf for that tone control. Looks cool but maybe not the most reliable idea.

I wouldn't say never use 'em. Just be aware in case your build starts making awful noises. And place your mica so that it can be easily changed out should it start causing trouble.

[Chuck H]

I wouldn't say never use 'em. Just be aware in case your build starts making awful noises. And place your mica so that it can be easily changed out should it start causing trouble.

That's what I'm doing with my stuff.