In an earlier post on Ampage ju said: Here's what I like to do to my Fenders.... 1) change the input grid stoppers to 33k 2) change the cathode bypass caps to 5uF. 3) change the power tube grid resistors to 100k and the grid stoppers to 4.7k 4) change the driver coupling caps to 0.047uF 5) change the driver input cap to 0.01uF 6) change the feedback loop values to 22k/4.7k 7) add a 120pF cap across the plates of the driver to supress oscillation. (good insurance if you don't have a scope) The grid stoppers are located on the power tube sockets. Usually 1.5K in value. If you look at the power tube socket on a fender amp you'll see two resistors soldered across the socket. The top one is the screen resistor. Usually 470 ohms, and underneath that resistor you'll see the grid stopper, usually 1.5K in value. 1) change the input grid stoppers to 33k Not worth the hassle to change them, plus, you'll alter the response of the lower gain If you decide to try this, replace the two jacks and install the combination you want on the new jacks. Install a 27k to 33k resistor right to the tube's socket and leave the 68K resistors where they are. 2) change the cathode bypass caps to 5uF. OK but maybe try 5uF on the first triode and 10uF on the second after the tone stack. Or, you might like it the other way around. 3) change the power tube grid resistors to 100k and the grid stoppers to 4.7k OK, you understand the difference between grid stopper resistors and grid load resistors, right? If you're playing the amp loud enough to cause bass farting, reducing the PI/driver couping caps to a lower value, like .022uF-.047uF and changing the 220K grid load to 100k-150k is a decent way to get some of that in control. I think you'd find that using a grid stopper resistor of 47K instead of 1k5 might start to roll off the high freqs but I doubt many could prove to me they can hear the difference between 1k5 and 4k7. 4) change the driver coupling caps to 0.047uF If you mean replace the .1uF caps to .047uF, yes, see above. 5) change the driver input cap to 0.01uF I don't care for this mod too much. I've had amps come in for repairs where someone had used a .022uF there. More urban legend crap... actually suburban basement band legend... ha ha . Unless you play in your bedroom at low volume 95% of the time you might find the lower frequencies of the amp (at higher volumes) caving in the power section. I seldom use anything larger then .0047uF here and I try to use the best cap I can find for this job. 6) change the feedback loop values to 22k/4.7k The 4k7 resistor will slightly lessen the overall gain of the PI driver tube and that 22k/4k7 combination actually allows a little more NF then a stock Fender and lowers the gain of the PI/driver so more. So, IMO, that one's no good either... well, that was a little harsh... maybe it was a suggestion from somone who did it once to one amp and they like it based on their personal taste. If you want to loosen up the NFB loop a little, try changing the 820 ohm resistor to 1k2 to 1k5. Too much resistance here will create a very mushy/woofy tone and I wouldn't think of using anyting larger then 4k7. OK if you like bluesy slide guitar at reasonable volume levels. 7) add a 120pF cap across the plates of the driver to supress oscillation. (good insurance if you don't have a scope) OK. I've gotten away with using as high as 500pF here with virtually no major difference in tone. If you notice the amp has lost it's top end "air", then reduce it's value. As a matter of course I use a small pF 500v silver mica cap on all my amps in this position. Wow Bruce, that was harsh! Guess this calls for a rebuttal on my part huh? First let me preface this by saying that it always comes down to personal taste. 1) On this one I must agree with you to a certain extent. That being that the resistor really should be on the tube socket. However I seldom see anybody that actually uses the low gain jack so it's kind of a wash. 2) I've never seen the point of using a larger bypass cap on a stage after a stage using a smaller one. If you've already rolled off the lows on the first stage a larger one here won't make any difference. The frequencies just aren't there for it to have any effect. Changing the value of the cathode resistor of course will void this argument. 3) I never said to use a 47k resistor as a grid stopper! Also, the reason I up this value is NOT to stop farting but to roll off some of the highs to assist in suppressing oscillation. I much prefer this method above shunt capacitance to ground. I also said that the sonic effect of this change was subtle and that it is much more noticable at higher playing levels. 4) Here's one that's grounded in personal taste. I've never used an 0.022 there either. It seems to be just a little too much. Fender was all over the map with this value. They've used as small as 0.001uf all the way up to 0.022uf For the record, Marshall uses 0.022uf caps here so use what you like. 5) Yet another one that's a matter of personal taste. Some people like virtually no NFB. The driver has more than enough gain to hard clip a pair of 6L6s so sacrificing a little gain here isn't any big deal. Another thing I've done on occasion is to bypass the feedback resistor with a small cap. This one is tricky because you can cause some serious oscillations if your phase relationship isn't just right. These recordings were run into a semi-sealed enclosure using a single Celestion G12T-75. The riffs were played on a Lonestar strat and the amp was running at about 3/4 full power. Actually Mark Cameron posted a pretty cool post some time ago about killing oscillations. I guess I'm just not a fan on the "cap to ground" on the power tubes. It takes time to correct the lead dress in those Fender's but it's sounds better in the long run. Of course it depends on how much time you have and if the customer is has the money to pay for it comparing carbon comp to metal film, I COULD hwoever notice very mnor differences in tone and major differences in noise levels..,. the metal films were dead quiet while the carbon comps were quite noisy... the tonal diffferences would be like comparing brand new strings to strings that have been layed for an hour... think you'd find that using a grid stopper resistor of 47K instead of 1k5 might start to roll off the high freqs but I doubt many could prove to me they can hear the difference between 1k5 and 4k7. I'm with you on this one Bruce! :) As Carl Z mentioned you'll hear subtle differences but I think you have to make some large leaps in values in order to really hear a big difference. I think most people use the grid stoppers because everyone else does. I've not used grid stoppers on some amps but usually it's when the plate voltage is lower. The higher voltages pretty much demand you need grid stoppers to at least stop any oscillations as Carl mentioned. This is open to debate as this is of course all very subjective. "Grid stoppers or no grid stoppers...that is the question" (OK, that was a bad joke--ha, ha)