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Re-capped my '72 Vibro Champ. Weird CAP found?

Last

Member
Messages
4,279
Was doing a re-cap and found this on the first half of the V1.



What the heck? I know it's original to the amp.

Amp sounds much better now after Sprague 25uf 25v swap from these Mallory's. Also replaced the Can as well.

Anyone with a better understanding explain what affect this incorrect CAP had on this amp?

Before the re-cap the amp sounded okay though a little tired and I needed a boost pedal to get her to overdrive.

Much better now. Really woke up and breaks up smoothly @ around 7.

Gotta wonder.
 

Last

Member
Messages
4,279
If I'm reading it right that's a 2pf CAP. Isn't that a huge difference?

Thanks for the reply.
 

zenas

Member
Messages
8,859
We're just used to the standard 25uf caps in Fenders but you'll see anything from .68uf to like 330uf.
 

Darkness

Member
Messages
2,252
Interesting. That's the cap used for the second half of V1 cathode bias on a regular champ.
 

swiveltung

Senior Member
Messages
14,486
Be careful, some Vibrolux and Pros's etc have one 5uf cap in there that looks just like that in lieu of the 25/25's. In the vibrato section if I remember right.
 

Last

Member
Messages
4,279
Yeah, before I changed it it crossed my mind that it was the right value for the term/vibe. Though, I thought Fender used the Disc type CAPS for the Vibe/Trem part of this circuit (which are present in this amp further down the input path).

Amp sounds great and vibe works fine with the 25uf replacement CAP installed in its place.

It does leave me scratching my head though why it was there
 

tmac

Goldmember
Gold Supporting Member
Messages
2,236
One of the schems show a 10uf. Should be fine using a 25uf. Maybe the SF used a 2uf I can't find all schems right now but have a Fender book somewhere.
 

Last

Member
Messages
4,279
This one is a a Silverface Vibro Champ. I used the AA764 (Blackface), schematic as my reference.

There isn't anything (that I could find), specifically for the Silverfaces.

If anyone has a link to one I'd really appreciate it.

I'm still left wondering what affect this 2pf CAP had on this amp.

Only real differences after my re-cap is the ability for smooth break-up @ 7 and definitely more volume on tap now.
 

J M Fahey

Member
Messages
2,683
Not 2 pF but 2uF :)

As to correctly guessing the effect on sound, we'd need the standard schematic and knowing where was this one.
 

mark norwine

Member
Messages
17,163
Is there really any mystery here?

The bypass cap sets the -3dB rolloff of the tube. With a 1K5 resistor on a 12AX7A, a 25uF has a rolloff WAY below the lowest freq that a guitar can possibly create. Having a 2uF (it's not pF) would only serve to raise the rolloff a little, but still below what a guitar can create.....since you can't "pass" a frequency which doesn't exist, the idea that you'd notice any difference is silly.

The old caps...2 or 25....were most likely shot, so you weren't getting any bypass at all; the "new cap" makes the amp come alive because it's functioning....not because "it's the right value".

As to why Fender used the "wrong" cap.....most likely either an error or they ran out of 25uF and figured another value really didn't matter in this little practice amp. (And they're right....it doesn't really matter!)



EDIT: let's see....lowest note on a guitar is 82Hz. With a 100K plate, 1K5 cathode, the 3dB down point with a 2uF cap would be still below a guitar's lowest note, but there would be some rolloff in the audible guitar frequencies......still, I suspect any real difference in feel is because you have new caps, not just "a different value cap".

Fun little calculator here, if you're interested: http://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/
 
Last edited:

Last

Member
Messages
4,279
Oh yeah I agree completely. Though I was unsure what difference that value in that particular spot had, if any.

Thank you for clearing it up.

I did measure all the caps I took out and replaced with an LCR & a Blue ESR. Some were double (50uf), than what they should have been. The 2uf CAP in particular did measure out at 2pf on my LCR meter however.

I'm still learning obviously and truly do appreciate all the insight I've gotten here.

Again thank you for the replies and help.
 

J M Fahey

Member
Messages
2,683
Just as a side comment, maybe you read the wrong scale or, more probably, wrong multiplier :(
It is 2uF no doubt.
1000000 times larger than 2 pF :eek:

FWIW just the LCR tester *terminals* , no wire attached, will probably show way more than 2 pF .

Just the wires (if you use crocodile clips or something) , connected to no cap at all, might show 15 or 30 pF .

1 foot of cheap guitar cable is around 100pF on itself, unconnected to anything else; a good one might be 30pF per foot.
 

mark norwine

Member
Messages
17,163
agreed.

Besides, a 2pF cap isn't a polarized electrolytic.

If the meter read 2pF, then the cap was dead.

The cap in the picture is 2uF.....for certain.
 




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