Bias Pots

[Ed_Chambley]

Does anyone know where to get a 25 or 50 ohm flathead screwdriver, thru panel.  Like the ones in fender amps, but in a larger ohm sweep.  Even better would be a multi-turn.  Looked at Mouser and Digi-key and cannot seem to locate one.  Any other solutions would be welcome as I am adding bias tip to all my amps and a lot of times 10 ohms is not enough so I end up opening up the chassis to change the resistor which defeats the purpose of the bias tip.

[Ed_Chambley]

Sorry, I meant 25 or 50K.

[Willabe]

Hi Ed,

Weber speakers has 25K and 50K locking pots for a flat blade screwdriver.

I just bought some myself. I have'nt trid them yet, but they seem pretty good. Nice small size. Good price too.

 http://tedweber.com/weberfest/weberfest.htm

Look in Pots and you can click on for a picture.

Some of the guys here have said before that you don't realy need an (expensive) multi-turn pot. It's how you set up the voltage divider for the bias supply to get the smooth swing/adjust you need, they can help you set it up right. 10K can still work too.


        
                  Brad      

[Fresh_Start]

The Vishay 43P family of trimmer pots has worked well for me in a couple of builds.  You do have to buy the adapter (without shaft) to mount it through the chassis, but it works nicely.  I actually like the fact that you need a really thin screwdriver and have to be deliberately trying to get in there to make an adjustment but you might be looking for something else.

In this pic, you can see the bias trimmer upright between the rectifier socket and the first power tube socket. Follow the blue wires from the bias board.  This location makes it relatively easy to adjust bias from outside the amp.  Other guys have mounted the pot so the adjustment is located on the back panel, but that kind of requires a cover to avoid the "Hey!  What does this do?" from users.



Cheers,

Chip

[Willabe]

Fresh_Start,

I don't recall seeing any of your work before. Very nice indeed.     

                Brad          

[Ed_Chambley]

Hi Ed,

Weber speakers has 25K and 50K locking pots for a flat blade screwdriver.

I just bought some myself. I have'nt trid them yet, but they seem pretty good. Nice small size. Good price too.

 http://tedweber.com/weberfest/weberfest.htm

Look in Pots and you can click on for a picture.

Some of the guys here have said before that you don't realy need an (expensive) multi-turn pot. It's how you set up the voltage divider for the bias supply to get the smooth swing/adjust you need, they can help you set it up right. 10K can still work too.

        
                  Brad      
This is what I was thinking, but after seeing the photo posted by Fresh_Start, I will have to give that some consideration.  I am using 10k's right now which work fine, but I have many tubes I like to swap around and 10k does not always give me enough adjustment without pulling the chassis.  Since I have installed the bias probe tips, I am just wanting to go that one step further.

Thanks for the help guys, seems like either solution will work, now it is just deciding.

[Ed_Chambley]

Doug just emailed me a tip and I never thought of it.  I just did it and it works great.  I disassembled a cts pot.  Sawed the shaft off just below the threads and slotted it with a coping saw.  Reassembled the pot.  Now I have a recessed bias pot made from one I had in my parts cabinet.  Took less time than ordering one.

[EL34]

Yes, that is an old trick, glad you liked it.

It's a bit of work, but not very hard to do

Remove the back cover of the pot by carefully bending out the tabs.
Push the shaft assembly out.
Saw the shaft off.
Use a dremel or thin saw blade to create a slot
Clean everything with tuner spray
Push the shaft assembly back in
Put the cover back on

This lets you use cheap pots and have many custom values available.
The Alpha pots have an aluminum shaft and are just as easy as a CTS plastic shaft pot
Cheaper too with many more values available.