Cut brightness on 6V6 plexi

[dude]

Which cap would lower the "treble knob's" brightness best, turning down the Presence does lower the range but lowering too much to get wanted treble takes fullness away. Ceramic caps seem to be a little harsh too, I assume mica are best.


Would lowering the 470pf (really 510p)to the treble knob be the best way?

[tubenit]

I would go about this another way.   Here is a link to a thread about an "enhance cap".

http://el34world.com/Forum/index.php?topic=12723.msg153636#msg153636

You can safely use insulated alligator clipped wires to parallel a 220p cap across the 82k LTPI entrance plate resistor to experiment. 

I have found that sometimes it's not needing less bright, but instead less harshly bright.  A smoother bright tone can be very useful.

With respect, Tubenit

[dude]

Tubenit, thanks for the link and info.


I tried the smoothing cap across the PI resistor, didn't have correct values of mica caps, only had 250p and 500p but tried ceramic caps 221p, way too harsh and changed the tone in a bad way. But lowering the bright cap to the treble didn't sound good at all, took away the cut, made the treble almost useless. Also, I learned that different brands of mica caps had various results in treble tone. A few mica's  sounded weak, don't know the brand  but they were cheap. A good cornell cap is $3 and up for a single.


I ended up with a 500p that hoffman carries, not too bad, a little too bright but the amp really cuts through the mix so I left it there. I think I'll try getting a 470p and a 390p, Cornell Dubilier, as the Mica I have now reads 520p. Speakers play such an important role, A Celeston GH1230 and a Weber Blue Dog 30 watt sound incredible together. One cancels the heavy lows and the other keeps the beef.


I'll get a few better brand mica caps with the correct values to try over the PI plate R. I did notice some 12AX7's will cut that brightness a bit, too. So many ways to skin a cat, don't know where that saying came from sounds terrible,       

al 

[sluckey]

Referring to Hoffman's schematic... remove C5 (500pF).

[Big_Mike]

Did you ever install a PPIMV on this amp?  I am curious of your wiring layout for that.  Did you use a Lar/Mar?

[dude]

Did you ever install a PPIMV on this amp?  I am curious of your wiring layout for that.  Did you use a Lar/Mar?

I used a different PPIMV, didn't have the correct Pot. The Plexi uses 100K grid leaks, so I'd need a 100K dual pot, but the 220K with the two 2.2M R would work just as well most likely but might take away a little headroom. The PPIMV I used was what I posted above, just used another set of coupling caps, fooled with values but totally sounded bad, took it out in minutes. When I get the right pot, i"ll try it, most of the dual pots are way off, unless you get lucky  or spend the money.


Actually Sluckeys MV works very well, can't put it on full as the OT distorts too much (18 watt OT) but the amp is so loud 3/4 up is way too loud. I have high plate voltage, 418v on a set of old GE 6V6 GTA's, biased at 75% D. I might swap out the OT for an Allen Deluxe upgrade I have in a 5E3. I think it's 25 watts. 6L6's sound great but again the OT saturates and distortion gets a little flabby. This is the loudest 6V6  I ever played, probably do to the high plate voltage. I'll probably get a big 5K resistor just before the choke like Huss used, the AO43 PT is 340-0-340, high for old stock 6V6s', IMO, great for 6L6s and bigger OT.
 

[dude]

Referring to Hoffman's schematic... remove C5 (500pF).

Lifted that cap on the bright volume, I had a 390p there that measured around 400p, not 470p. The results surprised me.
I thought I'd lose some of the bright volume's cut, but no, still cuts well as I turn that vol up. The treble is the same, still have a 500P there now (actually 522p). But the OT (18 watt OT) saturation, I thought I was having at higher volumes, seems to have disappeared..., when I removed that cap? I'm playing with Weber Blue Dog and Celeston GH1230 (good sounding speakers, IMO).


Bottom line, the treble issues didn't change at all but the overdrive distortion did, in a good way! The amp was a little floppy on the bass side when I turn up the MV (your master) and both volumes but taking out that cap it's gone or much better...?


The presence control works the best I ever played in an amp. I can control the brightness, (kind of close to ice picking but not in a real bad way) on treble knob where I want with the presence. I'll have to put a switch on that cap to really know, my ears are kind of shot. I should have used ear plugs but never did. I have this constant buzz in my head like a swarm of locust, ha. Drives me crazy, and if I have a few beers it gets louder. Anyway, I'll AB that cap, so I can really hear the effect.


So I'm wondering why the flabby distortion at high volumes kind of cleared up just from removing that cap...? I was thinking about using a bigger OT but not sure I need it now. The amp has a ton of head room, maybe going to 220K on the grid leaks would cut some of that headroom...?


Anyway, from what I see with that cap out the two channels are almost wired the same, only difference is the biasing of the two sides of the 12ax7 and the two coupling caps, .02 and .0022. Tone is so subjective, what one hears someone else doesn't. 


By removing that cap, I'm no longer dumping the highs pass 400p...? Not looking for a detailed explanation but what exactly is  that cap doing? BTW, I did read up on Ohm's law and I have a good understanding now, thanks for the tip guys.


I tell you Sluckey, every amp I copied from you sounds killer, your single channel deluxe lite is an awesome amp. I have friends that are great players and tone freaks, remarking how awesome that deluxe lite is. All I did different was remove the cathode cap on the second stage and lower the cathode cap on the first stage to 5.6uf from 22uf. We're blues players and classic rock, we don't use a lot of pedals, just compressor and a tube screamer when needed.


Also, would there be any benefit to putting a 250p there..?


al