CTS p/p pots

surfsup · Post by **surfsup** » Tue May 17, 2011 12:50 pm

This pot has two lugs on back (besides the three normal lugs). I've never used a p/p pot before. I assume when you push/pull the shaft, it disconnects/connects the two lugs on back?

[img:500:326]http://www.tubesandmore.com/cemirror/inv/R-VC1MA-PP.GIF[/img]

M Fowler · Post by **M Fowler** » Tue May 17, 2011 1:11 pm

It is a on off switch.

Lonely Raven · Post by **Lonely Raven** » Tue May 17, 2011 1:25 pm

Just test it with a meter, see if it connects or opens when you pull it.

surfsup · Post by **surfsup** » Tue May 17, 2011 3:39 pm

Ok yea that's what I figured. I was going to dmm it to see which was which. Thx

Now I need to come up with ideas to use them. I have a couple...

rp · Post by rp » Tue May 17, 2011 4:17 pm

Pulling it is on. BTW if you ever need to cut one back a bit make sure it's pushed in before you measure and cut. I'm too embarrassed to say how I know that.

dynaman · Post by **dynaman** » Tue May 17, 2011 5:23 pm

rp wrote:Pulling it is on. BTW if you ever need to cut one back a bit make sure it's pushed in before you measure and cut. I'm too embarrassed to say how I know that.

Lol. Go ahead and tell us! Not that I've ever done a boneheaded move like that before. Oh, except for that one time I laid out and drilled a new blank chassis COMPLETELY backwards. And then did the same thing with board for a different amp. Of course I only realized my mistake after swaging the turrets.

Yeah, the switch is "on" when it's pulled out.

Firestorm · Post by **Firestorm** » Tue May 17, 2011 10:07 pm

Those things are great for switching all sorts of stuff. Mesa Mk IIs are full of 'em. You used to be able to get DPDT switches like that as well (still can actually, if you need a lifetime supply all at once). I think they're all 1M pots nowadays, but since they're CTS, you can swap the resistance "wafer" with any (available) value and taper you want.

surfsup · Post by **surfsup** » Tue May 17, 2011 11:15 pm

One idea is i've got a 220k/100k voltage divider/gridleak (b4 3rd gain stage) and thinking to put a parallel resistor on the pot to the 100k to lower the value a bit depending on if i have a HB or SC plugged in. Would that be a good way to adjust for the pickup type?

Also maybe switch in a parallel R to the Rk to up the overall value but lead dress might be am issue with cathode runs?

rp · Post by rp » Tue May 17, 2011 11:42 pm

Firestorm wrote:but since they're CTS, you can swap the resistance "wafer" with any (available) value and taper you want.

It never occurred to me, what a great tip, thanks

Firestorm · Post by **Firestorm** » Wed May 18, 2011 6:21 pm

surfsup wrote:One idea is i've got a 220k/100k voltage divider/gridleak (b4 3rd gain stage) and thinking to put a parallel resistor on the pot to the 100k to lower the value a bit depending on if i have a HB or SC plugged in. Would that be a good way to adjust for the pickup type?

Also maybe switch in a parallel R to the Rk to up the overall value but lead dress might be am issue with cathode runs?

On the 220K/100K voltage divider, you just have to be careful about what part of the divider is connected to the switch (probably not good to have an extraneous wire connected to the grid itself). You could parallel the whole divider with a 100K (switching the ground connection on the additional 100K) and wind up with something like 110K/80K, or build half of the divider out of two series resistors and short one of them via the switch: 100K+100K/100K -- short the first 100K and it becomes 100K/100K (louder); 220K/47K+47K -- short the last 47K and it becomes 220K/47K (quieter). Same thing with the Rk: series resistors and short the "bottom" one. Lead length should be tiny that way, so less worries about lead dress. If Rk is bypassed, you could switch the ground side of the cap so it bypasses part/all of Rk.

Oh: if you significantly change the total resistance to ground of the divider, it will have an impact on frequency (filter formed with the preceding coupling cap) so consider that, too.

surfsup · Post by **surfsup** » Wed May 18, 2011 7:21 pm

Firestorm thanks for your reply:

On the 220K/100K voltage divider, you just have to be careful about what part of the divider is connected to the switch (probably not good to have an extraneous wire connected to the grid itself).

On V2, in this pic you can see the green lead feeding the screen with the 220k on the terminal strip. I have a black lead that runs under the board to the red resistor turret at the bottom of the board, to the red R, to the centerturret that has a ground lead that runs under the board to the filter cap's ground just to the right. What I was doing was thinking of running a lead from the two ends of the red 100k R to the back of the pot with one leg of the new R on the pot lug. This R, larger or smaller, would increase or reduce the gridleak resistance but leave the 220k alone. So another parallel 100k would be 50k overall, as an example (220/50).

So I have a lead (black) off the grid directly to the 100k red R on the board.

[img

2000]http://chicagocadcam.com/ChrisHahn/xen1 ... 1small.jpg[/img]

Firestorm · Post by **Firestorm** » Thu May 19, 2011 4:28 am

That would work. But I see an awful lot of wire hanging off that V2 grid: the green wire, the black wire (and technically the white and blue wires from the tone caps, too).

I'd move the 220K right between the middle pot lug and one of the switch lugs and run a 47K between the two switch lugs. Then run the 220K/47K junction to the V2 grid. Get rid of the black wire. Replace the red 100K with another 47K and run a lead between the new 47K and the other end of the 47K on the switch. This gets rid of the extra black wire to the grid and works the same. Switch off: 100K (94K) to ground; Switch on: 47K to ground. If you still feel you need a grid stopper, you can add it on the strip where you have the 220K now.

surfsup · Post by **surfsup** » Thu May 19, 2011 11:59 am

No Rgrid at the tube = bad karma? I put it there for that purpose originally.

What is the primary reason for the shorter overall leads (besides the switching)? Is removing the black going to be less hiss or less RF pickup iyo, or something else?

Firestorm · Post by **Firestorm** » Thu May 19, 2011 2:47 pm

Try it your way and see if it's quiet enough. I was just concerned that you've got about two feet of unshielded grid wire to pick up stray signals and thought maybe you could reduce it a little. Using the big grid stop will kill off higher than audio frequencies, but will couple audio frequencies. BTW, BF/SF Fenders had even longer wires on their second stages and were mostly okay, so you could be fine.

Structo · Post by **Structo** » Thu May 19, 2011 4:54 pm

If you end up getting some interference I would use some small Teflon shielded cable to and from V1 grids.
Ground the source end of the shield and trim the shield off the other end and heat shrink it so only the middle conductor is exposed.

Good looking build, hope it sounds great!

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