Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on December 12, 2010, 03:28:15 am
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It has happened twice lately with Silver Mica caps that a cap
behaves badly maybe allowing DC through it which has caused bad noise in the signal way.
One of these problems I have handled here. Have you had similar problems with SM?
/Leevi
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I don't recall ever having a problem with silver mica caps failing, but that's not to say that they don't ever fail. Try number 3?
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Yes. I have had that happen. But it is really rare. Got a bad batch?
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Old Silver Mica is 500V. Doesn't break down in audio gear.
I see that new Silver Mica is sometimes 50V. Which will break-down at audio amp plate voltages, including the treble cap in the Fender Tonestack.
Read the specs. Get 500V unless you KNOW you can go lower.
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I have not had any problems with the SM caps. But this week i was swaping between ceramic and SM on my amp fine tuning it. There is a difference. Ceramic is more edgy and brittle which is cool sometimes. SM is smoother sounding
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It's one of the derivitives of Murphy's Law things. You make enough of something, everything that can go wrong WILL go wrong. So, when they make them, how many caps are in a batch, 10,000? 100,000? Even if they get their failure rate down to .1%, that's still as many as 100 bad caps in a batch. (I doubt the failure rates are even that high, but the numbers are easy to calculate.) All you can do is say, "Murphy's a bitch." My sister once had the rear axel fall off of her car while she was driving on the freeway. Be glad your run in with Murphy didn't involve flames shooting out the back of your car and a three day stay in a small town in Wisconsin!
Gabriel
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In my case the both caps were connected to the treble pot. Both caps were specified for 500V. The first one (47pF) was connected from a plate to the treble pot, where the voltage is more than 200V. The second one (500p) was connected from the cathode (follower) to the treble pot where
the voltage is more than 100V. What is interesting the problem started after the amp was warm i.e. after 15-30 minutes. Normally the noise caused by this is connected to a bad tube pin contact or microphonical issue and at least for me it was new that a bad cap can cause it as well.
/Leevi
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And you are certain it was the cap(s) and not something else?
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And you are certain it was the cap(s) and not something else?
Yes 100% sure, I used lot of hours to localize it. I made only one change at a time
and after I had changed the cap the problem disappeared. The second case was easier
to troubleshoot since I had a clue where the problem could be.
/Leevi
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I've heard about this happening in the past with silver mica caps....for some reason they are more susceptible than other types. What people have done is stick a .1uf 630v cap in front of the whole tone stack, so that cap will take the voltage stress, leaving the picky silver mica cap to just work. I've never had one fail myself though.
Greg
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Have you had similar problems with SM?
Some silver mica caps can deteriorate if overheated during assembly. Using a thermal shunt while soldering them is a good building practice.
Edit: typo korectaid
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Have you had similar problems with SM?
Some silver mica caps can deteriorate if overheated during assembly. Using a thermal shunt while soldering thems is a good building practice.
Fixed that for you.
Gabriel
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if overheated during assembly
This could of course be one reason. I normally use a quite hot soldering iron in order to avoid
cold solder.
/Leevi