SF Vibrolux reverb filter caps 20uF vs. 16uF

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SF Vibrolux reverb filter caps 20uF vs. 16uF

Postby suthie99 on Thu Jan 28, 2016 8:55 pm

I am in the process of restoring an early 70's fender vibrolux reverb. It is a bit rough around the edges but as far as I can tell completely original. It seem to follow the A1270 schematic. I am planning on replacing all the original electrolytics as the larger caps have signs of leakage. However the filter caps in the "dog house" have me stumped. These five large Mallory caps are 20 MFD 500VDC with date codes from 1972 (I believe, 235-7240A). All vibrolux schematics show 16 MFD 450 VDC.

What do you all think? Is this a deliberate "improvement" or had the factory run out of 16's the day my amp was built. It seems like there might be some theoretical benefit to the 20's particularly in the first two filter caps, particularly as these tend to have a higher voltage rating. A lot of folks say 16-22 caps are essentially interchangable so I'm probably overthinking it but should I restore this to the SF schematic or what the factory "intended".

Apologies if this has been tackled before. I have search pretty hard for a existing discussion of this topic. Thanks in advance for your thoughts.
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Re: SF Vibrolux reverb filter caps 20uF vs. 16uF

Postby RickJL on Fri Jan 29, 2016 12:14 pm

I've just had mine done with 20uF. It had 20uF in it when I got it. I considered going back to 16, but was advised the higher voltage rating of the 20s is beneficial, particularly if you want to experiment with a GZ34 rectifier instead of the 5U4. And when you hit that bottom E, you want things to stay together, right?
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Re: SF Vibrolux reverb filter caps 20uF vs. 16uF

Postby stratele52 on Fri Jan 29, 2016 12:55 pm

I would go for 500 volts with 20 MFD .

Shematic show 465 volts at rectifier and 450 volts at 6L6 plates

I have a 1977 Vibrolux Reverb with original filter caps which are all working good . Test with my Heatkit AT28 capacitor checker.
Most of the time these caps are good.

I mod the amp to AA270 . It play great .
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Re: SF Vibrolux reverb filter caps 20uF vs. 16uF

Postby sok66 on Sun Feb 21, 2016 12:37 am

Those caps are original. Somewhere in the transition from BF to SF Fender started using 20uf 500v caps in the doghouse and didn't update the schematic/layout. Here's a shot of the original caps I found in my early '73 Vibrolux Reverb.

Image

I replaced those with Sprague Atoms (I now use F&Ts almost exclusively). I'd add that on VRs I bump the power supply filtering to 50uf by putting 2x 100uf 350vs in series, 220UF 3w resistors across the leads as EQ resistors. To do this you have to remove the jumper between the caps on the front side. Easy mod.

Image
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Re: SF Vibrolux reverb filter caps 20uF vs. 16uF

Postby Tube Rube on Sun Feb 21, 2016 5:30 pm

Adding extra capacitance in the first node of the power supply will change the feel of that amp and exceed the recommended ratings of the rectifier tube a bit. I personally prefer the stock values in that critter. .02 YMMV :wink:
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Re: SF Vibrolux reverb filter caps 20uF vs. 16uF

Postby slider313 on Sun Feb 21, 2016 8:39 pm

I use a pair of F&T 22uf@500v in the first two spots and F&T 16uf@475v for the remaining three. A NOS GZ34 is good with up to 60uf of filtering.
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Re: SF Vibrolux reverb filter caps 20uF vs. 16uF

Postby Baron Von Machinenmann on Tue Feb 23, 2016 9:57 am

But some of these did come stock with 5U4GB's that recommend no more than 40uF max. :wink:
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Re: SF Vibrolux reverb filter caps 20uF vs. 16uF

Postby slider313 on Tue Feb 23, 2016 1:19 pm

Baron Von Machinenmann wrote:But some of these did come stock with 5U4GB's that recommend no more than 40uF max. :wink:


True. The 5Y3GT data sheet shows a max of 20uf of filtering yet they hold up just fine @ 40uf. I've seen stock mid 70's Fender Champ's with 40uf on the first node. DeArmond Rowe used a 40/20/10 can in their R15, R15T, Martin 112 and Martin 112T; which all used a 5Y3GT. :wink:

On the other side of the coin, Ampeg used a 60/20/20 on the short lived Reverberocket R-12R-A, and found out the hard way. :shock: That circuit was burning up 5Y3's and they quickly switched over to diodes and the R12-R-B
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