Alpha pots and Marshall knobs

[alerich]

The Marshall knobs that Doug sells do not fit on the shafts of the Alpha pots (the knob opening is slightly too small). Does anyone have a suggestion (or maybe has been down that road) on how to enlarge the opening of the knobs?

[FYL]

There are two types of Marshall knobs: metric, designed for a 6 mm dia shaft, and SAE, designed for a 1/4" dia (6.35 mm).

There's no easy way to fit a 6 mm to a pot with a 1/4" shaft: you need to remove a .35 mm ring from the brass collet inside the pot (possible with a step drill) or from the pot shaft (possible with a lathe). Or use the right pot/knob combination...

[jeff]

 I just had the same problem. Don't force them on. I did and when I went to take one off it ripped the shaft out of my pot. To get the others off I put a drop of oil in the set screw hole. Mine were all plastic(no metal insert) and I just removed the set screw and hand turned a drill bit. Then I had to poke the set screw hole out with a small screwdriver to remove buildup.

[alerich]

Mine were all plastic(no metal insert) and I just removed the set screw and hand turned a drill bit. Then I had to poke the set screw hole out with a small screwdriver to remove buildup.

That was my plan but I thought I'd toss it out and see if there is another way. Thanks - I'll give it a whirl when I am finished building.

[mcrracer]

Is it considered bad taste or wrong to offer up another companies parts on this forum if they work? I don't want to step on any toes as I buy from Doug occasionally but I build a lot of amps so I buy a lot of my stuff wholesale , probably from the same places Doug get's his. Mods let me know and I will not say another word if it is wrong.

[theundeadelvis]

I bought a huge lot of knobs once, and they were all slightly too small for the pots I had. I ended up drilling a hole in a 2x4 just big enough so I could press the knob into it (tight fit so it wouldn't spin). Then I was able to drill the brass insert out enough to work with my pots. Was a slight pain the rear, but it was that or waste a lot of knobs.

[Tiny_Daddy]

I have drilled lots of knobs but in this case, the correct knobs are out there.

[sluckey]

Is it considered bad taste or wrong to offer up another companies parts on this forum if they work?

I link to other venders all the time, but not for components that Doug offers (at least not knowingly). IMO this is a case where a pm to Doug would be proper.

[stingray_65]

There are two types of Marshall knobs: metric, designed for a 6 mm dia shaft, and SAE, designed for a 1/4" dia (6.35 mm).

There's no easy way to fit a 6 mm to a pot with a 1/4" shaft: you need to remove a .35 mm ring from the brass collet inside the pot (possible with a step drill) or from the pot shaft (possible with a lathe). Or use the right pot/knob combination...

There are also 2 kinds of alpha pots too! 3/8thread have 1/4" shafts and 10mm threads have 6 mm shafts.

ALSO the better way to enlarge that 6mm brass barrel is with a chucking reamer. a drill grabs in brass easily and can force it's way down and through the front of the knob. this doesn't happen with a chucking reamer as it cuts are only tangental . it also will give a much cleaner finish. I would suggest a .251" reamer. they can be had around $15.

Best way is to be informed and buy matched hardware for the components.

Metric pot = metric mounting hole and hardware
imperial pot = fractional holes and hardware.

I did not figure all this out until I had metric pots with fractional knobs. they would fit on VERY easy, but when you spun the knob it would move like a cam, off center.

Ray

>> Now that I'm thinking of it alpha has pots with 8mm threads that wobble so bad in a hole drilled for 3/8 mounting that they are difficult to center and can look way off  when you're finished

all stuff I've learned the hard way

[FYL]

There are also 2 kinds of alpha pots too! 3/8thread have 1/4" shafts and 10mm threads have 6 mm shafts.

You must mean 8 mm threads. If so the pots are available with either 1/4" (standard) or 6 mm (M series) shafts. Quite confusing, and this gets worse if you take into account the various entities trading under the Alpha brand.

[EL34]

The Marshall knobs will fit on the Alpha pots, I have used them on a bunch of projects.

It may be the batch of knobs changes from time to time, hard to say.
If you want to open up the knob a bit, use a .25" drill bit on a drill press.

Take the set screw out first.
I measured an Alpha pot shaft at .248", so  1/4" bit should do the trick.

The last project I did was this one
All the pots are Alpha and the same Marshall knobs
http://www.el34world.com/projects/StereoPreAmp1.htm

Had a guy email me last week saying none of the Knobs he got fit on the pots.
I asked if he backed out the set screws?

He emailed back and said no. 

[alerich]

When I stated "The Marshall knobs that Doug sells" I should have said "A few Marshall knobs I purchased from Doug" since they obviously aren't all too small. After further review it is more likely that this was just as Doug suggested - a small number that are a hair smaller in internal diameter than the standard knobs. I want to make that correction for the benefit of any potential customers reading this thread since the quality of  every item I have ever purchased from Hoffman Amps is excellent and as has been reported time and again here in these forums his customer service is almost without peer.

I don't want anyone to get the wrong idea. This is a great place to buy your stuff. Period.

[EL34]

Some are tighter than others.
Must be different molds used by the manufactuer???

They are real Marshall knobs that come in a Marshall peg board hang up bag, so everyone on earth gets the same knobs.