Express Cut Control ala Vox

[donzoid]

In another thread, Martin suggested I try a Vox-style cut control along with my PPIMV (LarMar) and I know there are thousands of references in here about either (yes I used the ol' search function and it worked REAL well). Sorry in advance for any duplication!

I took my Express out for some troubleshooting and I am reasonably certain I have it back in order. I use 6v6 "most" of the time to lower the overall volume, and in the smallest clubs I even trim that a tad with the MV and often an Airbrake (both together) - each just a little.

I've been playing on it for a while today, and before re-crating her into the headbox. I think I want to try that cut control to get a certain "ice-picky"-ness out. Nice word eh? I always use the bright switch "up" (except with single coils) and it seems like when I switch it to the down position, its just a little less bright than I like it, and maybe this will get me there.

My MV is on the back panel, and I am going to put the CC next to it (there is a hole there for the second speaker jack, it's non-invasive so I can back it out without any more holes in chassis) so my proposition is, why couldn't I run the 250K (cut control pot) CCW wire (1 in my REALLY bad drawing) to one of the CW MV lugs, and the wiper with the .0047 cap (2) to the other CW MV lug? Seems like this would work with no more than 1.5 to 2" of wire between 'em. If I am reading the layouts right, they tie to the same points at the turret board anyway...correct??? Any concerns? Thanks in advance...you might have to zoom the pic to see those 3 year-old style graphics...in stick-figure fashion...

[martin manning]

In the gut shot shown in the first post in this thread you can see the cut control next to the dual MV pot. You are correct, it only needs the pot, the cap, and a bit of wire: http://ampgarage.com/forum/viewtopic.ph ... highlight=

[Teleguy61]

I'm a big fan of the Cut control, it can really work wonders on the bright amps.

[donzoid]

Martin - thanks, I knew I had seen this thread before...

one follow up though it looks like you have your CCW lug tied to the wiper (on the cap side). I need to get a better camera and improve my graphics skills...or find out where my wife hid our 10mp one! At first it looked like you have that side tied to the bias V feed, but zooming in, I see its passing underneath the lugs.

Tele...I wasn't all that familiar with cut control until I built my 18w Rocket, but I am really liking these a lot now too. I hope it's effective here. I might try clipping in a few different values to see (hear) what I get.

I'll try it today, if it does what I think it will, it will stay in for a few gigs to seriously test it. Then I'm taking the 18w out and wire in a switched 220K dropper resistor as it has way more gain and volume than I anticipated.

EDIT: One more follow-up: Can I get away with a 300 or 400v cap here, or am I only safe using a 630v?

[martin manning]

You are probably ok with 300V. The most it will see is the peak-to-peak out of the PI.

[donzoid]

Thanks Martin. I'll put it in tonight. I gotta LOT of stuff to try to get done before the next two gigs they are both very large outdoor parties and I want "everything" in order...

[donzoid]

OK, cool! I tried it out last night for a few and it was late so I had to turn the master to 12 o'clock and there, it seems better with, than without. So I will leave it in there and perhaps experiment with the value a little (I have a .0061 and a .0033 in the drawer) during the evening.

My true test will be at my normal playing volume because it's all about usefulness at a gigging level, but my initial impression is "I shoulda done this before". Martin, you are a genius among mortal men!

[martin manning]

I don't think I was the first to put both the PPIMV and a cut control on an Express, but I know a good thing when I hear it!

[donzoid]

OK well the follow-up so far is....

With the master all the way up, I noticed almost immediately that I had less overall volume from the amp at the same settings as I usually use. So I reached back and realized the cut was on abt 25% and as I turned the control fwd and back, it acts as a volume control... I snipped the wire I had between the wiper and the "unused" CW leg of the pot. That reduced a lot of the volume going up and down with the cut knob, but I'm not sure this isn't affecting the tone by just being in the circuit as it is (even all way open). So, it seems at higher levels it's different, grainier is perhaps the word.

Tomorrow I'll try a couple other values to see what that does. If it still exhibits the same tonal differences (or similar) I'm going to take it back out, and I may even take out the MV too (return it to stock config).

[martin manning]

Unlike the PPIMV, the cut is still altering the circuit slightly even when the pot is at its maximum resistance, and I've seen schematics where a switch is added to the pot to take it out entirely. Something sounds funny re the wire you clipped, as that shouldn't have made any difference. It should be nothing more than the series combination of the cap and a variable resistor across the PI outputs, after the PI coupling caps. I am using 250k and 0.002uF.

[donzoid]

That seemed "odd" to me also Martin, I am also using a 250KA pot. Last night I went back to my original reason for pulling the amp out and re-testing and ended up re-touching all my solder points on V1 and V2 due to noise coming from wiggling V1.

So I am going to re-test that part at lunch, then maybe return the cut to the "unclipped" state, I have a switch so I can try that too. Will post results.

[donzoid]

Moral of the story..."Take NOTHING for granted"...

Seems the majority of the issues I was encountering in the first place are /were improved and hopefully stay cleared up by...cleaning the tube sockets.

Why did I take that for granted? They are brand new Beltons. 95% of the crappy crackling went away with some DeOxit on the pins and some insert/remove action. I'll add that to my checklist even on new builds.

So I went back to the cut control this time with a switch.

The first hookup was the .0047 (I believe that's the Rocket value). Not bad.
Then I tried a .0022 - also not bad, sure went "underwater" fast though as only about the first 25% of the control was useful. Ended up w/.0056, very close. Tomorrow I'll continue the experiment if I can find an even slightly higher value. Volume no longer decreases (much) and the cut acts as it should, just need to find the "perfect" value, especially now that all the tube pins are making good contact! Possibly even adding a silver mica along with this .0056 (which measures closer to .0061 cause its realllly old like ME).

[donzoid]

I ended up piggybacking two silver mica's together in parallel...a 4700p and a 1300p for an actual of 6.5n or if'n my math is currekt .0065uF. I had other types of caps in same value but the ice pick was most affected by this value AND type of cap (smoothed, shall we say...without taming the beast completely) so...in she went. With a bypass switch. Thanks!

[martin manning]

Great! You have 4700p and 1300p in parallel, which is 6000p, 6n, 0.006u.

[rp]

Unlike the PPIMV, the cut is still altering the circuit slightly even when the pot is at its maximum resistance, and I've seen schematics where a switch is added to the pot to take it out entirely.

Reading up on cut controls and reviving this old post with a question: what's the best way to switch the pot out? Short it leaving the .0047 across the PI plates? Lift it and take the pot and cap completely out? Or switch the pot to a fixed resistor, say 220k in line with the .0047?

Is this purely academic and in playing there's no real reason to bypass the cut?

BTW any advantage to using a polystyrene here? I have one but I'd rather not waste it and save it for a tone circuit.