Name: Carl Z E-Mail: summit@theramp.net Date: 12/5/99 9:06 AM Subject: Re: "other" Fender mods Hi woofmain; 1) input jack resistor values: The 68k/1 meg setup is probably about as good as you're going to get here. 2) Plate load resistors: Changing the plate load from say 100k to 220k is going to reduce the tube current somewhat and give you a little more negative bias. This is actually going to make the stage run cleaner but the net gain will increase. This can allow you to hit the driver a little harder. 3) Cathode resistors: Decreasing the size of the cathode resistor is going to give you more current through the tube and thus a little less headroom. Because this resistor is typically bypassed with a cap you won't see any substantial gain increase in the stage. You may also run the risk of having the amp fart out on you. 4) Other suggestions: One easier way to get a whopping gain increase is to move the tone stack from after the first gain stage to after the second stage. This is pretty easy to do in a Fender by just swapping the stages that the volume pot goes to. For a few ideas along these lines go to Steve A's site, http://www.techaccessinc.com/blueguitar/ and check out the schematic of the Blues Junior. You can also try knocking back the grid resistors to the power tubes from 220k to about 100k. This has several effects. First, it changes the time constant of the RC network and gives you a little faster response. Second, the gain of the driver will be reduced letting you drive the driver a little harder( sheesh, now that's reduntant for you!) before you start to clip the power tubes. Last suggestion; play around with the feedback network a little. I emphasize the word little here! You can easily turn the power amp into an oscillating nightmare if you get too wild with the changes. I'd suggest starting by changing the 820 ohm feedback resistor to something in the neighborhood of 33k. Temporarily wiring in a 50k pot as a rheostat here until you find a value that you like is a lot easier than swapping resistors in and out. Hope these suggestions point you in the right direction. Regards; Carl Z Summit Amps http://www.geocities.com/summitamps * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Name: Bruce E-Mail: Missionamp@aol.com Date: 12/5/99 6:25 PM Subject: Re: "other" Fender mods: My two cents worth... 1) input jack resistor values: The 68k/1 meg setup is probably about as good as you're going to get here. Maybe, but there are some subtle tweaks here that can make a flat guitar pickup sound better or an overly hot pickup work better with farty sounding Fender amps. 2) Plate load resistors: Changing the plate load from say 100k to 220k is going to reduce the tube current somewhat and give you a little more negative bias. This is actually going to make the stage run cleaner but the net gain will increase. This can allow you to hit the driver a little harder. You'll have to explain this one a little better Carl. I agree with parts but, that isn't "quite" what my experience has been... :>) 3) Cathode resistors: Decreasing the size of the cathode resistor is going to give you more current through the tube and thus a little less headroom. Because this resistor is typically bypassed with a cap you won't see any substantial gain increase in the stage. You may also run the risk of having the amp fart out on you. To a point, but when the current through the cathode resistor is enough to drop the plate voltage to a value that is bit higher then half of the B+ applied, I think you'll find that is where the tube is actually the sweetest and still has excellent headroom. I prefer to keep the "standard" 12AX7 triode idling around 1ma to 1.2ma using other parameter values like, Ep=200vdc with an Rp of about 91K to 120K, Rk 1K to 1K5 and Ck of about 10uF to 22uF. Keep in mind, the 12AX7 triodes are normally used as class A amps and need the proper bias, current and plate voltage to keep them running in their linear curve with the max amount of gain desired. 4) Other suggestions: One easier way to get a whopping gain increase is to move the tone stack from after the first gain stage to after the second stage. This is pretty easy to do in a Fender by just swapping the stages that the volume pot goes to. For a few ideas along these lines go to Steve A's site, http://www.techaccessinc.com/blueguitar/ and check out the schematic of the Blues Junior. True, the typical Fender tone stack, shoved in between the two halves of a 12AX7, does use up quite a bit of signal. But I think it has the best bang for the buck in terms of overall tonal effect vs cost, parts count and ease of construction, and as long as the next triode stage has good gain... (called the tone recovery stage for a reason)... it works wonderfully right where it is. There are other places you can get more drive, if you want or need it, other then shifting the tone stack to after the traditional tones stack recovery section. Maybe Randall A. can add something here too. I know Randall has a particular interest in low Zed cathode driven tone stacks and the effect they have on the driver. You can also try knocking back the grid resistors to the power tubes from 220k to about 100k. This has several effects. First, it changes the time constant of the RC network and gives you a little faster response. Second, the gain of the driver will be reduced letting you drive the driver a little harder( sheesh, now that's redundant for you!) before you start to clip the power tubes. I agree! This is a good mod for many Fender amps that can benefit from some additional preamp "tone" before the power tube fart out. But, this is not quite so when using cathode biased power tubes. And dropping the coupling caps value a notch or so, from say, .1uF tp .047uF to even .033uF, can go a long way in controlling the woofie nasties too. Last suggestion; play around with the feedback network a little. I emphasize the word little here! You can easily turn the power amp into an oscillating nightmare if you get too wild with the changes. I'd suggest starting by changing the 820 ohm feedback resistor to something in the neighborhood of 33k. Temporarily wiring in a 50k pot as a rheostat here until you find a value that you like is a lot easier than swapping resistors in and out. I agree again! Changing the feedback loop voltage lower by a slight amount, but staying reasonable, is a great way to get a more "fun" sounding amp. It will really loosen things up. But, going from 820ohms to 33K-50K, is too much for me in an otherwise normal good sounding Fender amp. OK if you play only mournfully bluesy slide guitar... Did I say woofie? Use small incremental amounts when doing this or use a small 25k-50k trimpot in place of the 820 ohm resistor on the eyelet board. You'll HAVE to go back to the PI/driver and shut it down a little or into the preamp stages and reduce their outputs a a bit here and there to get really good results if you try and use somethng as large as 33K to 50K. Personally, when playing at any decent volume, without redoing the PI/driver, I don't like the sound at all when using anything past about 3K for the FB resistor. You can also yank off the grounded 47 to 100 ohm resistor and use a 200ohm to 500ohm 1/2w resistor. By using a 500ohm to 1K resistor you can bypass it with a .22uF to 1uF cap to get a cranked with presence control sound too. Good post addressing some solid points Carl! Anyone else? Bruce * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Name: Carl Z E-Mail: summit@theramp.net Date: 12/6/99 3:21 AM Subject: Re: "other" Fender mods: A little long winded Bruce; Lets see if I can tighten these explainations up a bit. First I was running under the assumption that woof was looking for more distortion in his rig. As I'm sure you're aware tone is mighty subjective so what's pleasing to one person may not be pleasing to another. Case in point...I've got a customer that loves his marshall biased like an ice cube. He loves the tone of his EL34's running at about 20mA/tube. To each his own. 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 little 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. Definitely 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) Feel free to comment...I'm always open to new ideas Regards; Carl Z Summit Amps * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Name: Bruce E-Mail: Missionamp@aol.com Date: 12/7/99 6:54 AM Subject: Re: "other" Fender mods: A little long winded I think the biggest problem I see with using a 220K plate load resistor, is that if the cathode bias resistor is too low there will be a MUCH larger voltage drop across the plate load resistor and the headroom will probably be reduced, not increased. Typically, you might have a proper triode setup with a 220K plate load and a much larger Rk then you're use to seeing. This limits the current and the voltage drop. The tiny little signals an average guitar pickup puts out, would hardly have any effect on driving the tube out of it's linear curve or out of class A. With the volume maxed a strat will give you a little over a volt but by then you'll have power amp clipping. Did you mean out of the pickup, or AC out of the plate? My best sounding amps never have first preamp sections that output more then a few volts of AC. On the feedback loop I like the sound of 22k/4.7k combination. To me it's a nice compromise. OH...OK! I didn't see where you mentioned you were upping the grounded resistor by a factor of 47!!! That makes a big difference, doesn't it? :>) 22K/4K7...Check that ratio of 4.7:1, an average Marshall using a 56K and 2K5 presence control (abt 22.5:1), 100K and a 2K5 Presence control (40:1) and the stock Fender of abt 8.1 and you can see where your choice of the 4.7:1 lies. 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. Hmmm...I not sure you'd hear the 250P cap across the 22K FB resistor. The -3dB point would be about 29KHz. It might be OK for a little ultra high freq oscillation control though. But it also might start one! If you wanted a bass or mid boost (FB high or mid cut), the idea discussed hear frequently of using a big resistance in the FB loop, and then bypassing it with a small cap, is pretty cool, and to a point, works well. But, when using such a small 22K FB resistor, I think you'd do better to try something like bigger, like a .0022uF to .0047uF cap, and, if you were looking for a big boomey bottom, using a 22K FB resistor, I think you'd have to go way up there like around .022uF! By that time you may as well not have any FB voltage! Resonant FB loops are tricky and can create horrible PI/driver distortions and oscillations when the amp is cranked and there's a chance it will be driven to clipping! anway, to make a long story short here's what I like to do to my Fenders.... 1) change the input grid stoppers to 33k If by that you mean the top 68K resistor, as seen in the scheamtic drawings, OK. But not both 68Ks replaced with 33K, right? I prefer to use a 22K-33K and the 68K on the middle resistor, and then move to the tube socket and install a small 1/4w to 1/2 watt 20K to 33K on the tube socket's grid lug. 2) change the cathode bypass caps to 5uF. I like this too... I use a bunch of 5uFs and 10uFs. The -3dB point of a 12AX7 with a 100K plate load resistor, a 1K5 Rk and a 5uF Ck is about 40Hz... 10uF is about 20Hz. 3) change the power tube grid resistors to 100k and the grid stoppers to 4.7k I tend to drop these too. Sometimes as low as 100K... it depends on if the amp is cathode biased or fixed bias and what amp it is and also what year it is too. Grid stoppers can be about anything up to 33K to 47K with nearly no difference in sizzle tone to my old man ears. 4) change the driver coupling caps to 0.047uF I'm OK with that too and think, for fun, everyone should experiment with going down to .022uF and back up to .1uF. 5) change the driver input cap to 0.01uF Oops... it depends on the amp because, you'll start getting into murky waters if you over do it... I wish I had a dollar for every "Torres" modded amp I've pulled the stupid .022uF coupling cap off! Sure it sounds great at low bedroom volumes but wait until you go out and play it louder and with a bass player. Woof! I usually find, for my taste, most blackface or silverface Fenders need nothing higher then .0068uF, if the rest of the amp is set up properly and in balance. Your choice of .01uF is still pretty close to that and should work fine if the other stuff is addressed too. This is a pretty important cap and I'm always surprised to see so many cheap ceramic .02uF caps in Fender amps here. Some of the best, killer sounding Fender amps I've worked on had small, cheap chicklet sized poly whatever plastic PI/driver coupling caps! 6) change the feedback loop values to 22k/4.7k I guess I already replied to this....22k/4.7= abt 4.7:1 ratio... stock Fender 820\100= about 8:1 ratio. blah blah... 7) add a 120pF cap across the plates of the driver to supress oscillation. (good insurance if you don't have a scope) No guarantee it will stop all oscillations, but I always make sure there is some snubber cap in there too. I've tried everything from 47pF to 470pF and I can't really hear anything bad happening with the 470pF caps so it looks like for the most part, anything works in that range. Feel free to comment...I'm always open to new ideas Me too... I'm considered by many to be a pretty big bag of hot air but I'm always learning new things from a variety of different sources and willing to be "corrected" :>) Bruce * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Name: Carl Z E-Mail: summit@theramp.net Date: 12/7/99 4:45 PM Subject: Re: "other" Fender mods: A little long winded Sheesh...I've really got to stop posting at the crack of dawn! Ok....let's try this again! "Did you mean out of the pickup, or AC out of the plate?" What I dismally mis-communicated here was that under "normal" circumstances the signal at the grid of the second stage probably won't see anything higher than about a volt. Of course this is assuming you're using standard single coils and no pedals. "Hmmm...I not sure you'd hear the 250P cap across the 22K FB resistor. The -3dB point would be about 29KHz. It might be OK for a little ultra high freq oscillation control though. But it also might start one!" I don't know if I'm hearing it or not...but every time i've tried putting a cap in here it seems to dull the highs just a tad. A little hard to describe. The oscillation problem is one of the primary reasons I don't like to do this. I think, or rather hope, I mentioned having a scope handy for this one. "I usually find, for my taste, most blackface or silverface Fenders need nothing higher then .0068uF, if the rest of the amp is set up properly and in balance." I dropped a cheap ass 0.001 mylar in that position a while ago and it definetly sounded great. Bright as hell though. Definetly retains the Fender vibe! FWIW, I tend to voice my amps with a good amount of bottom. My ears seem to be really sensitive to highs for some reason. As for the hot air....I don't know how some of you guys find the time to post here as much as you do. I've got a bad habit here of thinking in a few different directions at the same time and getting all tangled up in the posts. Need to proofread a little better I guess. Carl Z Summit Amps * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Name: Carl Z E-Mail: summit@theramp.net Date: 12/7/99 4:04 PM Subject: Re: Carl, Bruce - don't stop now ! Fred; Here's the short form explaination of what's going on here. First let's address the 100k grid resistors. As you may or may not know, a triode's gain is dependent primarily on the load resistance. This consists of the plate resistance parallel the load resistance parallel the grid resistance. Normally in early stages the grid resistance is ignored in calculations because it's usually a 1 meg pot and you really won't see much difference between if this value gets changed +/- a little. In the case of the driver the grid resistor is 220k and moving it down will make a HUGE difference in the output. What this results in is a smaller signal being presented to the power tube grids, as well as a slightly lower impedance, but not really enough to be significant IMHO. This means that you can turn up the wick on the preamp without driving the power tubes into clipping. Now, as for the grid stoppers in my experience I've never really heard too much difference when using different values here. These are more for reducing oscillation and rolling off some of the highs. Check out Randall Aiken's explaination of Miller capacitance to help clear this up. Now on to the capacitor. The RC combination here gives you a first order filter. Using the fender values of 0.1u/220k give you a -3dB point of about 7 Hz. If you change the size of the cap and resistor to .0.47uF/100k this value goes up to 34Hz. Much less bass that the amp has to deal with. Where you really get into trouble here is with the power supply trying to keep up with frequencies this low but that topic gets really messy really fast! Anybody want to take a shot at poles and zeros? Now as to the charging time, just like power supply caps, smaller caps charge faster than bigger ones. What this translates to is that the change in voltage is porportional to the change in the charging time of the cap. Faster charging time = faster response of the amp. That should have either cleared it up or gotten you really confused. Anybody want's to elaborate in more detail or correct a no brainer...feel free! Carl Z Summit Amps * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Name: Paul C E-Mail: p.cochrane@worldnet.att.net Date: 12/6/99 2:43 AM Subject: Re: "other" Fender mods: My two cents worth... Another plug for dropping the pwr tube grid leaks down... I've never liked the sound of the vib kill switch trick, but with the changes to the pwr tube inputs they can deal with the increase in level much smoother. Also these amps sort of have a master volume inside. The ch mix resistors/plate loads set up a divider that dumps a large amount of signal (the ratio isn't the same for each channel because of the vib 50k pot). If killing the vib pot is to large of a boost you can dink with the values of the mix resistors. For a little more gain try a 150k series resistor instead of the 220k. For a little less change the other resistor. Of course this will have the opposite effect on the other channel you're not using!