1M Reverse Audio Pot

[brewdude]

Does anyone know where to buy a 1MRA pot?
I want to try a Matchless Chieftain style pre-amp/tone stack.

[Andy Le Blanc]

Use a Mfg. link from a product page from say Mouser, see if their in somebodies
line card, you might be faced with a minimum order, or you might have to ask.

Which product did matchless use?

[jmohr58]

I am not sure about a 1MRA pot, but Tube Depot has a 3MRA pot and if my math is correct
R=((R1XR2)/(R1+R2))
1E6=((3E6X1.5E6)/(3E6+1.5E6))
Than adding a 1.5M resistor in parallel with a 3M pot shoud give you a 1MRA pot.
http://www.tubedepot.com/r-cts-a.html
This is a CTS for $3.95
So if you can't find a just a 1MRA, this is an alternative.
Hope this helps.
Jerry

[brewdude]

Thanks for the suggestions.

[jmohr58]

No problem,
Looked at about 15 different amp part websites and the CTS 3MRA was the only reverse above a 50k I could find. The only reverse I found at Mousers website is a PCB mount minature on web catalog page 725 sytle H. But they do have it in a 1M values. I don't think you have to order quantaties, but you would have to check.
http://www.mouser.com/catalogviewer.asp ... atalog=643
At Tube Depot you can order by peice and shipping is $4.95 FedEx ground for under $99
They also have the 1.5M resistor in 1 Watt Carbon or 1/2 Watt Metal Film Precision. http://www.tubedepot.com/resistors.html
Jerry

[Deric]

Smallbear has them in the smaller 16mm pot.

http://www.smallbearelec.com/Detail.bok?no=116

[jmohr58]

Thanks Deric,
I can add another site to source parts.
Jerry

[brewdude]

Thans again for all the help.

I tend to avoid the smaller pots like the one at Small Bear. So, at this point I am leaning towards the 3MRA from AES with a 1M5 strapped across the pot.

Will this change the taper?

Also, this pot is utilized as a variable resistor controlling bass within the Chieftain circuit. Will this be an issue?

[Deric]

Will this change the taper?

Unfortunately, yes.

[Kagliostro]

Can this help ?

http://www.geofex.com/article_folders/p ... tscret.htm

Kagliostro

[brewdude]

Thanks for the link.

O.K., if I understand "The Secret Life of Pots" correctly, a linear pot can be converted into a reverse audio voltage divider. Mouser sells a 5M linear pot by Alpha. If a strap 1.25M of resistance across to the wiper I should end up with a total pot resistance of 1M with a reverse audio taper yielding about 85% resistance at 50% rotation .

Am I correct?

[Kagliostro]

Try it with a pot and a resistor you have and measure the results

you'll find that you can obtain a very nice log pot from a linear one

Commercial log pots often has two sometime three different portion of resistive material and what you obtain is near to a log pot

see here

http://sound.westhost.com/pots.htm

and, yes a 5M + 1.25M will give you a 1M pot

Kagliostro

[brewdude]

Thanks for the link. It is quite informative.

My plan is to get a 5M linear pot with my next Mouser order, and experiment with strapping different resistor values from 750k to 1.5M and see what works best for this application.

[ApexJr.]

I have some Allen Bradley house numbered measuring a little
over 1 Meg (1.25M - 1.5M) Reverse Audo RV 4 type pots
for $3.50 each in stock.

Steve @ Apex Jr.
steve.apexjr@prodigy.net

[martin manning]

Thanks for the link. It is quite informative.
My plan is to get a 5M linear pot with my next Mouser order, and experiment with strapping different resistor values from 750k to 1.5M and see what works best for this application.

Steve's (Apex Jr.) pot would probably be a better way to go. If you create a "RA" pot with a shunt, the resistance across the element will change with the setting, so you are likely to get some other changes to the circuit that you don't want. In your example using 5M with a 750k shunt, it'll go from 650k to 5M from 0 to 100% rotation.