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Post subject: Power supply cap value for 5E8
Posted: Fri May 29, 2015 12:22 pm
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I have an older narrow panel 5E8 Twin. Very likely pre '57. It works but has "barking" problems at higher volume settings.
I checked the electrolytic caps, and sure enough they are leaking/need replacement. So, I theorize there are supply voltage issues, possibly drops outs under stress.
I see the 4 caps clearly marked 8MF 500WV. However, all the 5E8 (and 5E8-A) schematics/layout diagrams, show these caps as 16MF/ 450WV.

It's not my first rodeo, been and electronic tech then Engineer for 30 years. So, I have some training and experience with electronics. I know that bigger caps in a power supply generally provide less ripple in the supply output. Should be a good thing to double the capacitance, and probably Fender though so, too. I am concerned they were chosen to coordinate with the choke parameters installed on this particular example. The schematics don't list a value for the choke in any of the schematics I've seen. So, I can't do any math calculations to reverse engineer the circuit. Even if I tested the choke to derive a value, I don't know what value is found in the amps that were supplied with 16MF caps.

So, the ultimate question is...
Do I replace the caps with 8MF or 16MF values?
Or, what is the value of the choke in examples of amps that originally had 16MF caps?

Yes, I know I could be over-thinking this. But, I also want to be confident of not doing further harm to the amp. I ready know that electronic smoke smells really bad, from prior experience. I don't need or want a repeat lesson for that.

Thanks for any info you may provide.


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Post subject: Re: Power supply cap value for 5E8
Posted: Fri May 29, 2015 12:50 pm
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Are the caps that are currently installed original to the amp? Check the date codes and see if they jive with other component dates found within the chassis. Personally, I would follow the schematic but as we all know, Leo was fond of deviating from the drawings whenever the mood suited him. Doubling the storage specs of those caps will likely not harm the amp but you might find that the bass response is a bit tighter than what you're accustomed to.

Arjay

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Post subject: Re: Power supply cap value for 5E8
Posted: Fri May 29, 2015 6:46 pm
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The caps are Astron Mini Mite with date code of 518. I'd be mighty stunned if they aren't original.

That said, the amp has been worked on before. There is a newer mallory cap added to the selenium rectifier. And it has 12AX7 in all four positions. But, I've owned the amp since 1974, and I know nothing's been done to it since then.

It plays VERY clean until the volume is turned up. Then it barks so bad no one wants to be in the same neighborhood with it.

I guess I'll order the 16 MF and some 12AY7s.

Thanks,


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Post subject: Re: Power supply cap value for 5E8
Posted: Fri May 29, 2015 8:10 pm
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Whatever you do, make damn sure the "magic smoke" does not escape from that selenium rectifier......it's a smell you'll likely never forget and one which is extremely difficult to eradicate.

I'd recommend F&T electrolytics.

Arjay

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Post subject: Re: Power supply cap value for 5E8
Posted: Fri May 29, 2015 10:00 pm
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Retroverbial wrote:
Whatever you do, make damn sure the "magic smoke" does not escape from that selenium rectifier......it's a smell you'll likely never forget and one which is extremely difficult to eradicate.

I'd recommend F&T electrolytics.

Arjay


Second all the above! Get rid of the selenium diode in the bias supply. Go with a 1N4004 to 1N4007. 16mfd to 20mfd at 500VDC is the caps to go to. It's the higher voltage rating that is important, to compensate for the higher outlet voltage --- compared to when the amp was made.


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Post subject: Re: Power supply cap value for 5E8
Posted: Sat May 30, 2015 12:08 am
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Retroverbial wrote:
I'd recommend F&T electrolytics.

Arjay


Ok... Why that brand over Sprague Atom or other brands?


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Post subject: Re: Power supply cap value for 5E8
Posted: Sat May 30, 2015 2:17 am
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Sprague has suffered some QC/QA problems of late -- principally, caps whose values were incorrectly marked. Those made prior to 2009 are fine.

As for "other brands", there aren't any worthy of use in a Fender amp. Older Sprague Atoms and F&Ts are all I'll ever use.

Arjay

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Post subject: Re: Power supply cap value for 5E8
Posted: Sat May 30, 2015 2:52 am
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Retroverbial wrote:


I'd recommend F&T electrolytics.

Arjay


+1


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