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1978 Vibro Champ - Need Knobs

Blue97FXSTC

Member
Joined
Jan 11, 2005
Messages
657
My 1978 Vibro Champ is missing the Intensity knob.
I ordered a set of Fender replacement knobs P/N 0990930000, but these are for 7/8" shaft pots.
This amp has 1/4" knurled plastic shaft pots, so the knob just slides on with no set screw.
I've searched the web and come up blank.
Does anyone know where to find these for sale, please?
Thanks.


Vibro-Champ.jpg
 

PaulD

Active member
Joined
Jun 25, 2007
Messages
673
7/8" shaft pots? That must be one hell of a big pot! The vast majority of pots will have either a 1/4" (6.35mm) diameter shaft or a 6mm diameter shaft. Sorry I can't help with where to source the correct splined knobs but a grub screw knob will usually work just as well on a splined shaft, have you tried to see if they will fit?

If the shafts are 1/4" and the knobs are for made for 6mm shafts it may be possible to enlarge the hole in the knob with a 1/4" drill (take the grub screw out before drilling to prevent damaging the screw)
 

Blue97FXSTC

Member
Joined
Jan 11, 2005
Messages
657
I'm mistaken (as usual) about that 7/8".
Not sure where I read that.
The Fender website says these knobs are direct replacement, compatible with 1/4" shaft pots.
That's just not what I'm finding.
The new knobs are loose.
The original knobs on this amp do not have set screws, and they measure 0.231" ID:
The new replacement knobs have a set screw and measure 0.256" ID:
Not much difference, but enough that they don't have a proper friction fit on the pot shaft that measures 0.251" OD.
I'm reluctant to use a the set screw on a plastic shaft, but it looks like I have little choice.
Original on the left in each picture:

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PaulD

Active member
Joined
Jun 25, 2007
Messages
673
0.231" is near enough 6mm so I imagine the pots are metric 6mm ones and the knobs are for imperial 1/4" (6.35mm) shafts. There are a couple of ways round this, the simple (less elegant) way would be to wrap a few turns of PTFE tape around the pot shaft. The slightly more elegant way would be to use some of these https://www.thonk.co.uk/shop/brass-pot-adapters-x20/ (this is a UK supplier but I would imagine similar things are available elsewhere).

Using the set screw on plastic shafts shouldn't be a problem, many manufacturers do this and the shafts are usually made from fairly tough nylon or ABS type plastic. There is no need to over tighten, it only needs to nip the shaft enough to prevent the knob slipping.
 
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