Fender twinreverb guitaramp electro caps replacement

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andre tchmil

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I have an old Fender twin reverb here that died last night.
I opened up the chassis and discovered that two of the 5 big caps under that metal covered exploded and were leaky.
Caps are Mallory 500VDC 20MFD.
2 of them were already replacedabout 10 years ago by someone with 47mf/350V types.(they seem to be fine )
My question is : were they originally all 5 the same Mallory 20MFD types?
and can I also replace the broken ones with 350VDC types.Or do I really need to use 500V ones?
 
There were many variations on the Twin Reverb, so "old" isn't specific enough, but... Generally, the first filter was two 70uF caps "stacked" (in series) with two 220K bleeder/charge-equalizer resistors, one across each cap. The rest were 20uF, 525V. Do NOT use caps rated less than 500V in those positions. But it is OK to use slightly higher capacitance if you like.

The usual replacements used nowadays are 100uF, 350V for the first filter and 22uF, 500V for the rest.
 
Stay away from the IC brand ones. Try to find Atoms or Mallorys.

NYDave I forget is the twin one of the fenders that Overvoltages the first filter caps in standby?
 
Gus, what's wrong with the Illinois caps? I've installed some in Fenders as long as ten years ago and they're still doing fine. I prefer to use 105 degree-C caps when possible. When the customer was willing to spend the money, I'd use Sprague, but the IC caps worked fine for the routine repairs. Or, I should say that I never had a problem with 'em.
 
A story

A fender twin is the start of why I do not like IC brand caps. A friend recapped a twin with IC brand caps and the amp ghost noted like crazy and hummed. He then increased the cap value and it still ghosted. One day talking about this amp I asked him what caps he used? He told me they were the ICs.

I asked him for a few to test on a LC102 I also bought 10, 22uf 500V ICs for the test. The 12 or so ICs measured like crap 16 to 18 uf and ESR of 5 to 10 ohms.

A 22uf atom will often measure under <1.5 ohms ESR and >22uf

He installed atoms at the standard value no ghost or hum.

The IC brand 20uf 500V stuff physically small for the ratings IMO.
 
It's been a while since someone made good caps for those high voltage Fenders. You might want to email Gerald Weber and see what the brand of the week is.

OT:

The Fender Twin dosen't use a pwr supply choke. I was told a long time ago (read:might be full of it) that the output x-former doubles as a choke in this push pull circuit. If that is true, can someone un-ravel this question of how that is? Been bugging me for 35 years?

Thanks!

cj
 
Antique Electronic Supply has Spraque Atoms 20uF up to 600v. These are the standard replacements for Fenders.
 
Sprague TVA-1966 20uf/600V will NOT fit under the doghouse.

Run two in series like NYD says.

I used 2 x 80uF/350WVDC in my '62 Bandmaster, which will give you 40uF/700WVDC AND will fit under the doghouse. I believe the doghouse in the Pro is the same size...
 
Solen makes polypropylenes in the right voltages and values, though I'm not sure of the physical size. Would there be any advantage or disadvantage to using those?
 
I have installed solens in my laney vc30. I used 1/2 the rated cap. The ESR is so low (measures 0 ohms ) on a sencore LC102 that surge currents in the rectifier solid state or tube can cause problems.

with the Laney I needed to add a 1 ohm resistor after the diodes and before the first cap to stop the torriod from humming, this was due to the diodes not matching at high current surges and torriods not liking any DC caused by the unmatched diodes in the rectifer section.

A tube rec might/will need some current limiting with solens, Solens make the most difference at the first cap(output B+) and 2nd cap (output screen if wired that way)
 
[quote author="CJ"]

OT:

The Fender Twin dosen't use a pwr supply choke. I was told a long time ago (read:might be full of it) that the output x-former doubles as a choke in this push pull circuit. If that is true, can someone un-ravel this question of how that is? Been bugging me for 35 years?

Thanks!

cj[/quote]

I think this is misinformation. One of the properties of a choke is a field large enough that doesn't collapse quickly when used in power supply applications. A transformer is the opposite; the field has to collapse according to whatever the output devices are feeding it, otherwise it adds distortion to the audio. How could the transformer decide what is a 60 Hz note being played on a bass guitar or 60 Hz power supply hum. Both signals will show up on the output. There is a certain level of cancellation in a push-pull output stage if it is balanced well due to 60 Hz noise being in phase on opposite ends of the windings, but I wouldn't want to rely on this too heavily.
 
[quote author="andre tchmil"]Just received my caps :
General purpose 22mfd 450V from Farnell.
Are these ok for an emergency repair ?
they look so small.[/quote]

FWIW, equivalent newer caps are generally smaller than what used to be manufactured 50 years ago.

[quote author="jrmintz"]Solen makes polypropylenes in the right voltages and values, though I'm not sure of the physical size. Would there be any advantage or disadvantage to using those?[/quote]

We're talking about electrolytics in the power supply, right? Aren't polypropylene Solens bypass or coupling caps? :?:
 
I've recapped dozens of Fender amps with Sprague Atoms and I've never had one fail. I usually get them from Mouser.

Cheers,
--
Don
 
ok, I will, but can you explain why ?
I need to get 20Mf / 500 V .
So if I take , let's say 2 X10mf/350v caps in series+ the resistors , I'm there , right ?
 
No, you'll end up with 5uF, not 20uF. Capacitance in series divides, it does not add. For 20uF, you'd have to use two 40uF in series.

It's time to study the basics...

Allied's Electronics Databook

The resistors that Alex recommends are to help equalize the charge between the two caps. They also act as bleeders when the amp is turned off.
 


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