A straight to it way is: change the caps on pin 3- to a .47uf and 5uf on pin 8 of the pre amp tube. You will get great tight bass at all volumes. I have done this for hundreds of Fender owner's with the same trouble. About the easiest thing to try would be to go to smaller value coupling caps. If you are using .047 try .02 or .01. this ought to give you less mud and also have the benefit of making the amp more touch responsive. We don't need no stinkin' schemtics! Try some of this: A 12AT7 driver tube or, .047uF coupling caps to the power tubes, 120K grid load resistors instead of 220K, bias those tubes to about 17 watts at idle. Try a 39K and 1k combination on the phase inverter instead of 22K or 27K and a 470 to 820ohm as seen on other amps, and use nothing more then a .01uF cap to the phase inverter but try smaller and smaller.. don't go below 500pF. DON"T get sucked into the Torres poop using big .022uF caps here unless you are going to be playing at one third to half volume all the time. There should be a wire from the output jack to the board that has a 820 ohm, them a 100 ohm to ground. the higher you go in value will = more high end. I use a .0075uF/630v polystyrene here frequently. Move over to the tone controls and install a .0047uF cap from the treble wiper to the top of the volume control instead of straight piece of wire. And finally on the cathodes of the tone recovery triode sections make sure the 12AX7 has it's cathode's split so each section has it's own cap and resistor and make it something like, 1K5 to 1K8 but no more then 5uF to 10uF cap. That will still give you a nice low freq bump. You don't have to do all of this, but pick one and start tweaking. If you increase the 47ohm to a 100 ohm resistor you will have more negative feedback available to the phase inverter driver, and the PI/driver preamp tube will start to clamp down and the bass will tighten up a bit. Smaller values of the smaller resistore (like dead short the 47 ohm to ground ) will result in No negative feedback voltage and a little more gain in the Phase inverter driver tube.That will sound very loose and flabby. Mucho fun at low volumes and pretty awfull at high volumes. If you increase the small value resistor to 330ohms, there will be even more NFB voltage, the amp will really starrt to tighten up more but, the PI/driver tube will just start losing gain a wee bit. Now, If you use a 300ohm resistor and bypass it with a .47uF to 1uF cap, you'll have what amounts to fixed presence control with a little tighter bass response still pretty good gain. My feeling is to use a 1000 to 1500ohm feedback resistor and a 47 to 100ohm grounded one. Go back to to the preamp and limit how much gain it can produce at freqs below 100Hz. I thought I was answering a question about what "part" was in a post ( cap / res.), not givin the "why" but yes you are right about the function in the feed back loop. The orig ? was about lose bass... and there are many ways to get the job done. But if all you do is to kill the low's at the tail... what about were the problem started. The pre amp tubes will distort before the power section does, where bass can kill you. If you run the bias hotter ( 40ma or so) this would not be the case. but for the tube sake ( and your pocket book) I would vote for a tighter pre amp section and keep the power tubes closer to A/B IMO. Its all about chose ! taste in tone ! Thanks for your answer Bruce, and all who give options. Mr. Hudsons point of lowering the preamp's cathode bypass caps is a good way to drop some unneeded bass energy. I use 2.2uF, 5uF and 10uF quite a bit. The metalized poly .0047uF cap on the treble wiper to volume pot rolls off some more below 90Hz. These need to be a very good cap if you try it. Don't use junk here. Don't underestimate the effect of making the coupling cap smaller on the triode after the tone stack. I can see dropping from .022uF down to .01uF as being helpful too. The .047uF coupling caps on the PI to the new 120K grid load resistors of the power tubes will help too if the problem is another kind of bass fart from what I call stiction! Sticky bass sound from grid conduction caused by way too much drive voltage at some freqs. Randall Aiken, and other EEs who know what they're talking about, call it grid conduction. If it sounds like the bass tones gets stuck momentarily and blap, there is probably also some "stiction". I believe you would divide dissipation by B+ voltage to get you bias setting. I'm not sure what your Bandmaster and Twin run, but these are the voltages that my BF amps have. If the plate voltage is around 460V then 17Watts/460v = 37ma. At440v it would be 17Watts/440v = 39ma. Keep in mind that what you read across the resistor includes a few ma of screen current. Right, 36ma to 40ma is about right for me and that 17 watts included the current through the screens. Drop 3 or 4 ma from this if you measure with the OT shunt method. Part 2 Most of you regulars here probably know that the Fender reverb circuit and the "fender farty bass" are somewhat of an obsession for me. Anyway, armed with ideas from the aforementioned thread, a soldering iron, and a few cold Miller brewery products, I assaulted my ever-tormented silverface Twin. Here's what I came up with: I changed the ( Vib channel) cathode bypass caps to 1.0 uF (1st stage), and 5 uF (2nd stage). I changed the input cap to the PI to a 680 pF dipped micah. The slope resistor was held constant at 120k, and I had previously removed the 10pF reverb mixing cap. This amp also has been blackfaced, and has a post-PI 4 cap master volume ( .047's and 1.0's). Two power tubes have also been removed. The test speaker was a single Vintage 30. The guitar used was a Gibson ES-345. The flatulence is, for the most part, gone. While I was at it, I also changed out the tone stack bass and mid cap values, and found that using .022's increased the fartiness significantly - however, the standard tone stack values ( .1, .047 ) tend to be a bit "nasal" sounding, but all "intestinal gas" was gone with those values ! I am not sold on the stock Fender tone stack cap values, but so far, the other commonly suggested values ( 2 .022's) seem to promote the fartiness dramatically, and sound like shit because of this. Bruce: I tried the .0047 cap from the treble pot wiper to the volume pot, but I couldn't really notice any difference. The cathode bypass caps and the PI input cap did it all. These seem to be the real culprits, at least from what I tried tonight. I didn't do any other PI circuit mods. I couldn't really try out these changes at high volume, since the wife and kids were asleep upstairs, but, with all the control settings held constant, the above changes really seemed to do the trick, so far. I even ramped up the bass knob to "10", and at that level there was a hint of fartiness, but at "10", OK, what do you expect ? I must also say that there seems to be plenty of bass available on demand, and that it sounds warm enough without going into flatulence until about 7 or so on the bass pot. The results are very encouraging ! The .47uF 1st stage bypass cap change that R. Hudson suggested seemed to cut too much low end; the 1.0 uF seems "just right" for the 1st stage. It's funny - you seem to want to hear some "fart" somewhere in the bass pot on a Fender ! Anyway, this seems to have worked so far. I kinda hope to start up a big long thread like "talked with Ken" or "signs of aging" , with this much more interesting subject. So ....... all comments welcome and eagerly anticipated ! It seems to me that with all of the collective wisdom and experience we have assembled here we can eradicate the " Fender fart plague" once and for all ! Regarding your mods, I couldn't agree more with the comment about the .022uF making the problem so much worse. I wonder why people like Ken Fisher (and consequently Dave Funk) suggest that mod specifically for Fender amps. To me, perhaps the frequency mix might be better with the .022uFs, but something else has to give otherwise the fartiness is unbearable. The cathode bypass caps are a bit of a radical change soemtimes (like using .47uF or .68uFs), so sometimes a combination of keeping the cathode bypass caps lo (like 5uF) and using smaller coupling caps tackles the problem nicely. Also, using smaller impedance circuits in the coupling stages (where applicable) forms a higehr knee HPF with the coupling caps, and everything together results in less of the muddy bottom. Still, in the end, to me the Fender tone stack is too radical. The bass control, in the stock circuit, is as you already pointed out, usable within a limited range. But so is thre treble pot: too little of it and the tone goes out the window, and too much of it and the sound is shrill. IMHO, there are other tone stacks that seem to be better suited for guitar applications, yielding a wider range of settings that seem to work well with both single coils and humbuckers. Schematics of various tone stacks are out there, and without mentioning any names, let me tell you that some of them -- which recived wide coverage on this BB a while ago -- are very musical sounding to me. Fred, don't know what else to say... If you want me to spellit out... I would give the Dumble tone stack a shot, that's what I was referring to. THe schematics still live at www.deltabluesman.com While the older type tone stack sounds dark to me, changing the slope to 150K (as shown in the later schematic) seems to be the ticket. Also note that Dumble always used 5uF for cathode bypass caps, except one stage was 1 uF. So see? You are not alone in your practices. :) Fred: its good to hear the news. I use 1uf on the first stage in Blues Pearl amps. The .47 is for those who run it "on 10" all the time. A cool tone stack for you to try is 422 pf , 2x .022 and a 56K slope. The 100K with the .022's is a taste thing ( as all is). Tube gain has alot to do with it as well ( as stated before ) If you push the power tubes to hard and have a long swing ( 220K's ) The power section goes to hell quick. Most people want less fart, but at what point? Happy amping to you. Glad to hear you tred some of these things. I like the .0047uF cap fromthe treble wiper to the volume pot because it doesn't kill the bass response too much but it does roll off below 100hz, and at 80Hz, it starts to dip a few dB. A .0033uF works too but the bass starts slipping and .0022uF kills the girth of the amp and I hate that. I think if you pop over to the PI/driver and drop those 220Ks down to 120K and use .047uF coupling caps, you'll like that too becasue then you should be able to crank it. Personally, I don't like going below 2.2uF for the first stage cathode bypass and my tone recovery stage is always 10uF. I also think the 38K source Zed of the BF tone circuit sounds better with nothing less then 82K slope resistor and nothing more then 110K. The tweed and Marshall tone stack is so much lower in Zed that you can get away with 25k!! My preference in the tone stack is mid pot 25K, mid cap .022uF, bass pot 250K, bass cap .033uF or .047uF, treble pot 250K, treble cap 270pF. And I really like the phase inverter/driver tube at .0075uF using a polystyrene cap. Try the Torres thing of .033uF mid, .033uF bass and 330pF treble cap. I'm VERY interested in trying the grid load resistor mod you mentioned, but I avoided it last night mostly because, with the 4-cap post-PI master volume I have, mods in that area of the amp can have somewhat unpredictable results. I noticed that the 120k values are close to silverface specs ..... interesting. Another area that intrigues me is the phase inverter - I have messed around extensively with the values there in the past, but didn't approach it very intelligently ( i.e., no organized procedure, no notes taken) , so I don't have any real personal knowledge base to plan mods there. I would like to learn more about the sonic effects of changing component values in this area of the amp ! The .0047uF didn't seem to work much for me, but at the low volumes I was testing it may not have been very noticeable. I also definitely want to try different values in the tone stack, as the .1, .047, 270 pF sound "nasal", but I love the high-end clarity, and the "farts" seem to be reduced the most by these values. The 2 .022 combination sounded TERRIBLE - unless I turned the bass and mids below 3, then it actually sounded pretty good. But the amp becomes even more of a one-trick pony with those tone cap values. The 120k slope helps clear up the bottom a little bit, as well. I changed the (Vib channel) 1st cathode bypass cap to 1.0 uF, 2nd one to 5 uF, slope resistor to 120k, and PI input cap to 680 pF. Removed the 10pF reverb mix cap altogether. The bass control has become useable now, the mid control has become less "puffy". I can actually turn all controls ( treble, mid, bass) up all to 10, and still have a useable tone (even with the neck pickup on an ES-345) ! The interaction of the treble pot with the other two seems much greater, but in a positive kind of way. The amp is , of course, brighter, but not "shriller". Seems like upper mids are more prominent now. The setting of the treble pot seems to really more prominently "run the show" as to how the bass and mid controls will react. VERY INTERESTING - I recommend these changes to anyone interested. Changed the vibrato channel's personality quite significantly ! Now, if I can just get around the slight "nasal tightness" that the .047 and .1 mid and bass caps impart .......... OK, it's on to the Normal channel, and to the mods I have been directed to by other most generous Ampagers ! Steve, you're too kind! As far as answering your question, I'm not sure I understand it entirely: do you mean a 5F6-A/Marshall circuit with EL34's or with 6L6/KT66's? I think the plexi OT would work fine in any of those circuits with EL34's. I haven't tried it with 6L6's to be honest; the primary imp is a tad low at 3.4K. (is that what you mean?) Whenever you guys start talking about the Zed of an output transformer I keep thinking of Bobcat Goldthwait's character in Police Academy... 2 & 3? Yes, I was thinking of an amp with two 6L6's or 5881's, although I sometimes switch them over to EL34's when I feel like burning up the filament windings... < g > Would mismatching the speaker impedance help with the mismatch of the primary impedance? (You can tell I'm no tranny guru- I consider myself lucky if I get an OT wired up in phase on the first try!) Unless you definitely would not recommend using your trannies with 6L6's, you might want to see how they sound... Might be okay despite the unfavorable math! Let's start with the input jack. Hot pickups such as humbuckers or even active pickups really could use some loading before they hit that first stage so you could reduce the size of that 1 meg resistor. The 68k grid stopper can be twiddled to give you a littl more high end and clarity by reducing the value to something like 33k or 22k. You could even remove it all together. Thumb rule here is that lower values will extend the high frequency response resulting in a crisper tone while larger values tend to dull the tone somewhat. Another tweak here is to run a bypass cap across it like some of the Ampegs. Trace (where'd ya go?) had a mod for this but I never got around to trying it. Plate resistors: Without writing a book I'll try to clarify this a little. I'm ballparking the numbers here so i may be off a little. If we assume we're running with normal channel fender; gain -> tone -> recovery, changing the plate load on the second stage to 220k is going to give you an amplification factor somewhere in the neghborhood of 70 and bias current is going to be about 0.68mA. This gives you a bias voltage of just about 1 volt. I probably shouldn't have used the word headroom here, got sloppy with the terminology. What I meant to say was that the stage is going to run a little closer to pure class A and the THD is going to be much lower. I want to say it's about 1%, feel free to correct me if i'm wrong here. Unless you're running the amp flat out or again using hot pickups you probably won't drive it into distortion. With the volume maxed a strat will give you a little over a volt but by then you'll have power amp clipping. Hope I cleared that one up. On to the tone stack.... I've tried the blues junior arrangement and it's not too shabby, just different. It's a quick easy mod to that can be undone in a couple of minutes if you don't like it. From a purely engineering point of view passive tone controls really need to be placed after a low impedance source such as a CF stage. In this regard I've had some interesting results on a reverb amp by bootstrapping the first two stages and running the tone off the follower. Then tapping the tone control to send to the reverb circuit and back to the mixer stage. Definetly worth further investigation by the adventurous! As for the grid resistor RC network I tend to prefer the 047 caps. 022 and 033 caps seem a little too constricted to me. There's that personal preference thing again. On the feedback loop I like the sound of 22k/4.7k combination. To me it's a nice compromise. You could also bypass the 22k with a small cap of around 250p but I'm not a big fan of that. seems to roll of too many highs. Anyway, to make a long story short 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)