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Bad Taper On A Vibrolux Clone Volume Pot - Help Please.

hardys

Member
Messages
1,738
I have an almost new, Clark Belmont Vibrolux clone. However, the volume pot on the vibrato channel has a bad taper. From 0 to 3.5, there is very little increase in volume, but from 3.5 to 4.5, the volume jumps dramatically. Within that range (3.4 to 4.5), it's very hard to dial in, because just moving it a hair causes a big jump in volume. It acts similar to the volume pot in a 5E3 where it's like all or nothing with minor movement.

Could this simply be a bad pot causing the issue, or possibly something else altogether? If it's just a matter of replacing the pot, can some please recommend a proper replacement (taper type, value & brand)? I haven't pulled the chassis to look yet, but from schematics/layouts I've found on the web, it looks like it should be a 1 meg pot with a 47 pf resistor. Thanks for any help on this.

FYI...the volume pot on the normal channel works fine.
 
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xtian

Gold Supporting Member
Messages
2,493
Certainly sounds like a bad pot. You can reuse the bright cap.
 

Jeff Gehring

Gold Supporting Member
Messages
6,351
Most likely just the pot. I assume this is basically a Vibrolux Reverb; both channels should behave similarly volume control wise, though. You could swap the two volume pots to satisfy yourself it is the pot's fault. One thing though, measure for DC voltage at the top (high side) of the volume pot, referenced to chassis ground on both channels. There should be none, or perhaps a few tenths of a volt. If there is anything beyond that, the likely culprit would be the 250pF cap in the tone stack, I'm guessing on a Clark that's a silver mica?
 

J M Fahey

Member
Messages
2,650
Agree on the possible faulty pot.

Check it with a multimeter: disconnected from everything, set knob to "5" (halfway rotation) and measure:
* end to end resistance
* center to ground
* center to hot

I expect around 1M end to end (it might be anything from 800k to 1M2 though, that's normal tolerance for a pot) ; 10% that center to ground, 90% that fron center to hot.

That's a normal "10%" Audio taper pot.

There's a few 15% ones and Fender has even used 30% ones sometimes, but the general rule applies.

For reference, measure also the other Volume pot if possible, then compare results.

Won't generalize, but a few days ago I helped a musician who got a batch of 1M log Alpha pots, *all* (brand new and unused) scratching between 8 and 9 .
 

swiveltung

Senior Member
Messages
14,486
Fenders typically are about off at 2 and very on at 3.... so it may be just a marginal pot in combo with the basic circuit... assuming that circuit is pretty much Fender. Both my BFDR and my Vlux are unplayable at 2 and I normally play them fine at 3.
 

pdf64

Member
Messages
7,623
CTS pull switch type; they look to have bright caps on their switches. Should be available from most amp bits suppliers.
There's room for improvement in how the bright cap leads have been formed; good practice is to use fine tipped pliers to grip the lead wire immediately by the cap body and then form the desired lead shape.
The idea being to try to minimise stress on the joint between the cap and lead wire.
Using the cap body to support the lead as it's being shaped can result in damage to the cap, which may take time to manifest itself, eg humid air can get into the joint and cause oxidation / breakdown.
 
Last edited:

hardys

Member
Messages
1,738
CTS pull switch type; they look to have bright caps on their switches. Should be available from most amp bits suppliers.
There's room for improvement in how the bright cap leads have been formed; good practice is to use fine tipped pliers to grip the lead wire immediately by the cap body and then form the desired lead shape.
The idea being to try to minimise stress on the joint between the cap and lead wire.
Using the cap body to support the lead as it's being shaped can result in damage to the cap, which may take time to manifest itself, eg humid air can get into the joint and cause oxidation / breakdown.
Oh man...my bad. I rarely use the bright switches on Fender amps, so I forgot about that pot being push/pull for the bright switch. Thanks.
 

hardys

Member
Messages
1,738
I tried to find a 1 meg push/pull pot to replace the one in my amp. However, the only one I can find is the one shown below. The problem is that the shaft is longer ( 1/2" pushed in, 5/8" pulled out) than the original pot's shaft and all of the other pots shafts in the amp as well. Does anybody have a source for these with a shorter shaft? BTW...this is an SPST pot. The original one functions as a volume control & bright switch when pulled out. An SPST is what I should be using, correct?

If I can't find one with a shorter shaft, is it acceptable to mount a pot using washers and nuts as spacers internally?

View media item 3488
 
Last edited:

ylo

Member
Messages
785
If too long, you can carefully cut the shaft shorter with a hacksaw, smooth the edges with a file. Holding the end of the shaft in a vice makes this easy. Just don't cut too much off or you are back to square one.
 

wookiefoot

Platinum Supporting Member
Messages
651
I've had the same issue on an amp before. Turned out it was an audio vs linear taper accidentally put in.
 




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