Question about grid bias resistors

[dude]

Typical power tube grid resistors for most Marshalls are 220ks, what effect does the value have on the tone? JTM-45 with 6v6s calls for 100k, just because tube data says so? Leo used 220k on almost all his amps. The 6v6 Plexi I have with 100k grids, I want to try a PPIMV and only have a 1 M dual pot that’s accurate, so in parallel with the 100k, I get 149k max. I assume that means wide open I have 149k not 100k, will this change the tone? What effect do it these R values have on tone?   al





B

[shooter]

values have on tone?

tone is like color, each of us "knows" what we like, the only way to know is to play with it, I know, your mom said not to, but it's ok 

make something that works, then experiment, when I started painting I used black for places I want black, now I mostly use phthalo blue mixed with carmine red.  Tone is JUST like that

[dude]

values have on tone?

tone is like color, each of us "knows" what we like, the only way to know is to play with it, I know, your mom said not to, but it's ok 

make something that works, then experiment, when I started painting I used black for places I want black, now I mostly use phthalo blue mixed with carmine red.  Tone is JUST like that

Shooter, I've been playing with it all my life, pun intended, 


I know tone is subjective, I probably asked this question the wrong way, should not have said tone. Rather, going from 470K as seen when using EL84's, 220K as with 6L6's & EL34s, and etc., are these resistor values selected for each kind power tube used or for the circuit used? What determines the value? And how does that value effect the circuit?

Form experience tweaking an 18 watt Marshall TMB circuit (EL84s), changing the standard power tube grids from 470K to say 330K brings a little more headroom, cleaner, less distortion. Would this make sense with most other amps. My 6V6 Plexi (100k grids) is very clean with a lot of headroom, I have to rattle the windows to get distortion. Maybe if I changed these grids from 100k to say 220k, I'd get sooner distortion...? Am I thinking in the right direction?


al


 

[sluckey]

Just do it! Let us know what you think.

[shooter]

 After you've taken Sluckey's suggestions and solved "that one"

is very clean with a lot of headroom

drop the PA plates voltage by 30%
now you should be able to get grungy and keep the peace @ home, within limits
I think 2 amps are called for in most cases, one that pays the bills on Friday night, one to tinker with till domestic balance can be achieved

[pompeiisneaks]

I'm more asking a follow on question, or to see if I'm getting something...

I thought I'd read that often power tubes need them, but that dialing them back enough gives a bit more raw grit/power to them, somewhat like the trainwrecks preamp phases being non grid stopped, but there's a balance too, where you can cause oscillations etc, and power tubes often need a minimum amount period. 

Or have I misunderstood the way that works? 

Therefore, the more grid stopper you have, the more 'tame' the circuit will be, but you lose some 'edge' but the lower it is, the more crazy it gets, both in a good, and if not well balanced a bad way?

I'm going more for the deeper understanding of what's at play with those resistors.   :D

~Phil

[dude]

Changed the 6V6 grids to the bias to 220K from 100K, big differences.  Here they are:


1.   The overly bright treble knob response has much more range, from low treble to bright maxed, I really like this, I've been changing the mica tone caps on the bright channel for a while without much luck, even changed the coupling cap to the bright channel up to .01uf from .0022uf. Now I might have to go back to .0047uf. The 220Ks grids took that overly brightness away, sure didn't expect that.


2.   The distortion is much, much better, now I get some crunch at a lower volume and still get cleans at a decent volume too. I'm happy about that too, pegged with a strat (singles, neck and bridge together) I get that Marshall grind with the crunch.


3.   Now I'm not sure if I want to put the PPIMV on the back panel with the pre-master vol on the front...?  I guess I'll try it after I get this tone in my ears for a while.


My pre-amp voltages are relatively high, 230v on bright channel, I'm sure if I lower them like Shooter mentioned, especially the bright side, I'll get a browner tone and even more distortion at higher volumes but the 220Ks might have done the trick.


So, if I try the 1 Meg dual pot (PPIMV), I only have 270Ks and 330Ks R on hand which in parallel gives me either 208k or 242K, I'm guessing since I like the 220ks, I don't want to go lower and cleaner sounding, so the 330K seems like it. I can always turn the PPIMV down a hair to get back to 220k. I think a pair of 290Ks would be on the money, not even sure if they are made?


Anyway, I did all this years ago with my 18 watt Marshall to get cleaner headroom, now I'm going the other way. After 6 or 7 years I forget what I did and the results, I need to write this stuff down, take notes. I'm not getting any younger either, seem to forget where I put my keys a lot, at least there not   in the refrigerator...

[dennyg]

I'm surprised (and curious) that you hear that much difference between 220K and 100K grid leaks (not grid stoppers) - which impact the effect of grid current which increases with input signal.  While the data sheets state that many fixed bias tubes have a limit of grid impedance of 100K, keep in mind that the effective grid impedance includes not only the MV in parallel with the grid leak, but also the tone stack (assuming it immediately precedes the MV).  Nonetheless those specs are conservative anyway.  Also curious with your interest in PPIMV.  My experience is that it works great on clean preamps with only a couple of stages, since you're overdriving the PI to get distortion, but for high gain designs you want to maximize PI headroom as an overdriven PI just super-saturates and compresses the already-squared wave going into it. 
But per above, tone is what sounds best to you!

[pompeiisneaks]

I'm more asking a follow on question, or to see if I'm getting something...

I thought I'd read that often power tubes need them, but that dialing them back enough gives a bit more raw grit/power to them, somewhat like the trainwrecks preamp phases being non grid stopped, but there's a balance too, where you can cause oscillations etc, and power tubes often need a minimum amount period. 

Or have I misunderstood the way that works? 

Therefore, the more grid stopper you have, the more 'tame' the circuit will be, but you lose some 'edge' but the lower it is, the more crazy it gets, both in a good, and if not well balanced a bad way?

I'm going more for the deeper understanding of what's at play with those resistors.   :D

~Phil

Now that I just saw the last replies, I realize I'm talking apples to oranges lol, nothing to see here, move along

 

[dude]

dennyg, are in Philadelphia? Noticed "Hillbilly from Philly"? 
The Plexi 6V6 is not a hi-gain amp, it's a Marshall JTM-45 with 6V6's, basically the same circuit as the original EL-34 or KT-66 JTM-45. IMO, the grid leaks do affect the tone a great deal. The master volume I now have is after the tone stack and just before the input to the PI. The JTM-45 Plexi 6V6, guess it's really a JTM-22 as it puts out a loud 22 watts with 6V6s but mine can take 6L6s or EL34s too. This amp is very clean with a lot of head room, one input into two channels with separate volumes to mix. One channel bright and the other normal or more bass driven. Changing these power tube bias grid leak resistors to 220K from a 100K in this amp made the tone stack much more pronounced, mids, trebles and bass more responsive, along with less headroom and a nice crunch distortion at full tilt or 3/4 way up.


The pre-PI master volume is nice, lowering this MV doesn't really produce distortion but just lowers the overall volume (turning up both volumes with this MV down doesn't produce the Marshall Crunchy distortion, it's ok but changing the grid leaks a much better sounding distortion).  Cleans are still clean that's why I was going to add the PPIMV to get that power tube distortion with a lower volume. But changing the grids to 220K cut down the ton of headroom I had and give a nice Marshall crunch that  I couldn't get with the 100K's. Most people say the PPIMV works great on these amps, so I might install it with the pre-PI MV too, should make this amp very versatile.


al 

[dennyg]

yeah man, i'm in bucks county - if you're in the area send me a pm.
so i also mixed apples and oranges in my previous reply - mv & tone stack are only in parallel with grid leaks on single ended amps which i think is why larger values are allowed vs fixed bias leaks.  sorry about that - i was writing that post while on a conf call at work - oops. 
so i'm thinking about your observation of less headroom with larger grid leaks, and seems to make sense because grid current thru 220k would produce a larger voltage drop across it vs 100k, resulting in greater grid limiting distortion on the positive side of the input signal.  i'm gonna have to try that on a few amps.  if your observation holds true, i'll be using 100k's on my higher-gain designs as i prefer to maximize headroom in the PA for those amps.  I put a PPIMV on my 18watt marshall and the crunch is just perfect - I'd put that on a jtm45 in a heartbeat.  I built a 20W version of Tubenit's TOS and to my ears, the overdriven PI was way too compressed so I wired in a standard MV and the OD channel on that amp remains my favorite lead tone EVER.
 

[dude]

Try 330k grid leaks in place of the 470ks you have in the 18 watt. We came up with this on 18 watt.com years ago. Takes that overdriven frizzy distortion out, cleans up the saturation. Also, did the Paul Ruby mod, a couple of diodes or zeners, on the grids to ground. This took the infamous ratty distortion you get sometimes with 18 watt TMB. It’s on the web.               I’m located in Media but have a lot of friends up your way, we use to have big jams but more often it’s hard to find old classics rock players any more.                                                                 al