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Vintage threads from the first ten years
Fender Mod Questions
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 12/6/1999 5:14 PM |
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Ron Orlando |
Fender Mod Questions Two questions: 1). On the Fender Standalone Reverb there is no 68kgridstop resistor coming from the input as we see in most fender amps. Has anyone stuck one in there and what did you get? 2). On the 5E7 schematic there is a 1.5k cathode resistor on the inverter. Have any of you guys hung a 25/25 cap along side of it to juice up the gain a tad?? |
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| 12/7/1999 5:13 AM |
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| Steve A. |
Jerry! Jerry! Jerry! Jerry! Carl: Watching you two amp gurus battle it out is even better than "The Jerry Springer Show"! Keep 'em coming! Steve Ahola P.S. It's great when you guys stick your neck out with specific values because you can't hide later behind overly-generalized statements (like the published amp gurus tend to do). There is more than one way to skin a cat so I saved all of the posts on this thread from both you and Bruce. Thanks!!! |
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| 12/7/1999 6:56 AM |
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| Bruce |
Yeah, but you've caught me in bonehead statements and at least had the decency to ream me in private Email! Bruce |
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| 12/7/1999 10:18 AM |
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| Steve A. |
Bruce: In the crossfire between the expert opinions of you and Carl most of us here can find plenty of things to experiment with in the basic BF circuit, even woofmain jack! If you made a bonehead statement it certainly went over my head... Thanks! Steve Ahola |
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| 12/7/1999 6:54 AM |
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| Bruce |
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.
My best sounding amps never have first preamp sections that output more then a few volts of AC.
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.
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!
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.
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.
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.
I'm OK with that too and think, for fun, everyone should experiment with going down to .022uF and back up to .1uF.
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!
I guess I already replied to this....22k/4.7= abt 4.7:1 ratio... stock Fender 820100= about 8:1 ratio. blah blah...
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.
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 | ||||||||||||
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| 12/7/1999 3:11 PM |
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| Fred G. |
Carl, Bruce - don't stop now ! I have now seen cutting the power tube grid resistors from 220k down to 100k or so suggested as a cure for Fender flatulence, eliminating parasitics, and now for "improving" gain a bit ? Can we get a little more info on the how's and why's of the combined effect of lowering the power tube grid resistors, and bumping up the grid stopper resistors ? Thanks, Fred G. |
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| 12/7/1999 4:04 PM |
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| Carl Z |
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 http://www.geocities.com/summitamps |
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