bypass cap on Silverface Fender Twin Reverb

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bypass cap on Silverface Fender Twin Reverb

Postby Jonhy_Greenwood on Sun Dec 18, 2016 3:53 pm

Hi guys!
first post here..

I have a Silverface Twin Reverb which I did a cap job.

So, when I oppened the amp I saw three rarely caps. Two of them are the filter caps of the balance-bias circuitry. The other is a 5uF bypass cap of one of the stages of the oscilator circuit.

Those three caps looks like a poly cap; axial leads, white sealed plastic cover with blue lettering on them. Also they have the postive markings on them. Really I dont know if they are electrollytic type or not..

The two of the balance-bias circuit I've replaced with new electrolityc.

But the bypass cap I'm not sure if is worth to replace or not..
What do you think?

Also.. I do not have a 5uF direct replace.. Maybe two 10uF in series will do the job?
http://www.subeimagenes.com/img/20161218-123714-1686199.jpg
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Re: bypass cap on Silverface Fender Twin Reverb

Postby eolon on Mon Jan 23, 2017 12:17 pm

The white/blue capacitors you mention are Mallory electrolytics. If the 5µF cap is good, then I would not replace it, unless you just want to spend money. You can check the ESR/DCR with an in-circuit tester, which aren't too expensive; and of course you can un-solder one end of the capacitor and measure the capacitance.

If your tremolo is working correctly, and sounds good, then I would just leave it alone. A lot of people recommend changing all electrolytic caps as a matter of course, but this doesn't make sense to me, as I have seen plenty of 50+ year old electrolytic Astron Minimite capacitors that are still good and in specification. Just because an electrolytic capacitor is old does not mean it has failed.

Those white Mallory caps were used extensively in the Silverface era, and were good quality capacitors. Of course there are always those people who will argue that their "tone" is inferior to some other capacitor that they prefer, but I always am skeptical about these subjective opinions.

Best Regards,

e
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Re: bypass cap on Silverface Fender Twin Reverb

Postby Unit_1 on Mon Jan 23, 2017 6:03 pm

Power filter caps that leak b+ have to be replaced

But there shouldn't be any b+ in an RC on the preamp, since it should have been filtered out by the coupling caps.

As long as it has capacitance, it is good.

I think. Pretty sure that's the way it was explained to me, lol... :mrgreen:
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Re: bypass cap on Silverface Fender Twin Reverb

Postby stratele52 on Mon Jan 23, 2017 11:00 pm

Jonhy_Greenwood wrote:
But the bypass cap I'm not sure if is worth to replace or not..
What do you think?

Also.. I do not have a 5uF direct replace.. Maybe two 10uF in series will do the job?
http://www.subeimagenes.com/img/20161218-123714-1686199.jpg


By pass caps, most of the time they are good. Sometimes they are working but out of specs, I replace them with Sprague Atom. Would I hear any difference ?

Two 10 mfd in series are good =5 mfd
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Re: bypass cap on Silverface Fender Twin Reverb

Postby pdf64 on Tue Jan 24, 2017 10:28 am

Unit_1 wrote:...As long as it has capacitance, it is good...

Electrolytic caps can have capacitance but exhibit excessive ESR, preventing them doing their job, eg cathode no longer adequately bypassed.
Old electrolytic caps aren't necessarily bad, but they are manufactured with an operational life expectancy, which will by now have been long passed.
My experience with the white ecaps in SF amps is that failures are more than just 'one offs', so for a working amp it's best to regard them as a life expired consumable and replace them as part of normal service.
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Re: bypass cap on Silverface Fender Twin Reverb

Postby stratele52 on Tue Jan 24, 2017 10:38 am

pdf64 wrote: , so for a working amp it's best to regard them as a life expired consumable and replace them as part of normal service.



+1
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Re: bypass cap on Silverface Fender Twin Reverb

Postby Unit_1 on Tue Jan 24, 2017 6:34 pm

Found this while looking at ESR on wiki, something I don't often consider is ESR since I build new amps and have only worked on mine and friends amps. My 68 sfdr is still working *fantastic*, although I have changed the power caps only. If I develop (or should I say WHEN I develop) any problems I'll probably change those out just as a first check.

ESR may depend upon operating conditions (mainly applied voltage, temperature); a capacitor which has excessive ESR at operating temperature and voltage may test as good if measured cold and unpowered. Some circuit faults due to such intermittent capacitors can be identified by using freeze spray; if cooling the capacitor restores correct operation, it is faulty.
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Re: bypass cap on Silverface Fender Twin Reverb

Postby stratele52 on Tue Jan 24, 2017 7:48 pm

I have a Vibrolux Reverb Silverface in the shop.
All by pass caps read 32 to 36 mfd not 25mfd and most of them are leaky .

I test them with a Heatkit Capacitor Checker IT-28, a kit I build when I study electronic in school in the '70.
Great test equipment, a hidden secret.

I have also a Meterman capacitor checker and the Blue ESR Tester
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Re: bypass cap on Silverface Fender Twin Reverb

Postby BetterOffShred on Thu Mar 30, 2017 8:33 pm

Not to confuse the subject with yet another opinion, but if I had it open, and I was "Freshening" it up.. I'd replace the cathode electrolytics too. That's just me though. They are so cheap!
-Brett

P.S. January.. Yikes! I haven't been around here in a while, sorry to dredge a dead thread. The amp probably isn't even opened up anymore!
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Re: bypass cap on Silverface Fender Twin Reverb

Postby stratele52 on Fri Mar 31, 2017 8:24 am

BetterOffShred wrote:Not to confuse the subject with yet another opinion, but if I had it open, and I was "Freshening" it up.. I'd replace the cathode electrolytics too. That's just me though. They are so cheap!
-Brett

P.S. January.. Yikes! I haven't been around here in a while, sorry to dredge a dead thread. The amp probably isn't even opened up anymore!


These are the caps I'm talking about.
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Re: bypass cap on Silverface Fender Twin Reverb

Postby BetterOffShred on Fri Mar 31, 2017 2:57 pm

Yeah I'd replace them for sure if it were my amp. Electro's are just suspect when they get old, and if you don't know the story on them, might as well. I think they are a cap that has a lot of unwarranted "Mojo" assigned to them as far as brand and size and color and taste etc.. I've done a couple tests with 4 or 5 sizes and voltage ratings, and honestly you can't tell much difference until you really go outside the window on size. Just my opinion!

-Brett
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