Negative Feedback Loops and the Fender Presence Control 

Wow. In 9 minutes, you completely explained the function and basis of the Fender Presence control. I have been twisting that knob for a long time, and could tell that it brightened the sound, but now I understand why and therefore, how to use it better. I know that these are now very old videos, but thank you for taking the time to make them and for your incredibly clear explanations.

Once again Uncle Doug has led me out of darkness into the “presence" of the….anyway, thank you, and keep ‘em coming!

I forgot to mention, your videos remind me of the training I use to get when I spent a lot up at McIntosh Laboratory in Binghamton New York. I would go up there quite a bit over a 25 year period and learn from the engineers up there. One of my favorite teachers, was my uncle Gordon J Gow, the president and co founder of McIntosh back in 1949. Thanks again, Bob

Finally I can start to understand how the NFB works. Thanks. Great videos I have learned a lot.

You make it all so simple and clear. I have a multipurpose Newcomb amp that could possibly benefit from this mod. As always, Thanks! BTW, you've got your 100'th video anniversary just around the corner. I'll have to open a bottle of champagne or at least a cold beer to watch that one :)

More great info. I am really enjoying going over many of your older videos. Such a great teacher. Thanks so much.

Although i am not a "tech," i truly enjoy your videos. I play and like vintage amps especially fender. I own a 63 super bf and a 64 tuxedo bassman as well as many others. I started watching your videos several months ago and admire how you walk us through the finer points and explanations. This has opened up a new interest for me in discovering and enjoying the vintage technology. When i meet a fellow gearhead there is now so much more information to parlay. Kind of like "car guys" i guess. Looking forward to more videos and again, thank you for your channel. "Toh"

Plain and simple. Your are an amazing teacher Uncle Doug.

You´ve nailed it again! Thanx for another well explained basic Amp-lesson :) It is so easy to follow, as you are always make clear explanations on how things work! If you go on like this, some day every viewer could be able to do his/her very own Amp just because you have been a good teacher!

Another great video and a great mod! Once in a while we see people who say to completely remove the NFB loop, and that is not a good idea. At low gain levels, it's not that apparent, but when you start pushing an amp, you can quickly turn the amp into mud. Low frequency overload. Removing the NFB loop makes an amp sound terrible and it's counterproductive. Leo used NFB in order to control the gain so that we could hear all that wonderful chime. I've been using an adjustable NFB for quite some time after I came to appreciate its purpose. I typically use a 3k resistor with a b50k linear pot. With this setup you are able to carefully dial in some great breakup. I was pleased to see that you use it, too! I also add the Presence Control. Try auditioning a few different cap values or use a switch to select from several different values so that you can dial in the low frequency control. This really comes into play if you're a Blues player. In a Triode, the electrons flow from the Grid to the Plate and from the Cathode to the Plate. When you use NFB, the triode acts like a mixer. The grid contains your guitar signal and the cathode "mixes" a low amplitude guitar signal from the output transformer; just like you said, the cathode signal subtracts, (not adds) the opposite of amplifies, the incoming signal from the grid. The Plate in the triode sends this mixed tamed signal on to the power tube(s). NFB subdues the signal in that triode. The neat part about the Presence Control is that you can subdue the lower frequencies, that causes farting and blocking distortion, which sounds terrible, and allow the (lower amplitude) higher treble frequencies to run wild. Again, selecting from various Presence capacitors to dial in the low frequency cutoff is recommended. Furthermore, guitar pickups will reach all the way down into an good tube amp. Use a higher value presence capacitor for a humbucker equipped guitar - 0.1uf, 0.047, because they produce more low end harmonics. Use a lower value capacitor for a single coil equipped guitar - 0.047, 0.022, 0.01 because their low frequency output is lower. Adding a 3 or 4 pole rotary switch that allows you to select the Presence capacitor is a great mod. Try it and while you're strumming a chord, select different cap values. I think you'll be pleasantly surprised. Thank you Doug for providing us a forum to share our experiences! We are all blessed to have you around...

Thanks Doug That was very informative

Uncle Doug- I am a recent subscriber. You do an excellent job explaining how things work. I look forward to more of your instructional videos.

Excellent video. Thank you very much for making these. I feel like my understanding of my amp has increased 10 fold

Thanks for another clear explanation. I appreciate the way you break down the function of each portion of the circuit during the explanation, including both the electrical consequences and the effect on the sound.

Thanks Uncle Doug, that was an interesting vid, I have been looking for ways to adjust tone in my circuits without having to include an additional tube for a tonestack. Using caps and pots and NFB's and treble and bass boosts are pretty cool. Cheers

Thanks for the simple synopsis of these circuits and controls. I'll be adding both to my next design.

Doug. I've watched SO many of your videos in the last days, and I must say: You Sir, are the single most effective teacher or professor I've ever had. And I've got two degrees in Education, so I know a thing or two of what I'm talking about! -Jonah H.

Excellent video. This is dead on with how my Louis Electric KR12 is affected by each of these controls. Very fun and effective for shaping tones and bloom with each of these pots. Keep up the great work! TB

Thank you again Uncle Doug and Rusty...learning so much.

Subscribed! This video was great. It's great how you explain what effect each component etc will have on the tone of an amp rather than just talking electronics like most tutorials. Thank you. I'll watch them all now. :)

Thanks for these great simple explanations, Finally i understand this.

Thanks, good uncle! This is hands down the best explanation of both negative feedback and presence control I've ever seen. I'm seriously considering building this into my 5E3. Switchable of course :-)

Thank you, Doug; keep up the great work.

Thanks to Doug and Rusty for a very informative explanation of NFB loops and presence controls

Uncle Doug I added the variable NFB loop to my 69 Princeton. It has really opened up my amp and made it so much more versatile. Before the modification, to get the sound I wanted, I was either too loud for the audience in the front row or (if I was in a small room) playing so quite I couldn't hear my amp on stage. Now, after the mod, I can adjust the amps volume to the level I need, then adjust the NFB. I also play with a pedalboard and they sound even better now. Thank you Uncle Doug for another great video

You are an excellent tutor - many thanks for your videos.

Thank you for the Presence explanation; it is counterintuitive and I did not understand it before.

Thanks Doug. I finally know what the presence control actually does! I have heard countless explanations over the years but none of them imparted any actual knowledge.

I'd shock myself right off the planet, but I love learning what you teach.

These are the most informative videos about tube amps on YT. .

I love your videos. My 16 year old son and I watch them together. We have been building guitar amps together. Thanks for the very clear and entertaining tutorials.

Great video! Thank you so much for sharing! You've cleared up many things I had questions about..

Great teachers can change the world, you sir are great teacher...with a great sideman. Rusty, king loves your video's as well.

Thanks Uncle Doug, I followed your instructions and what do you know,, it works.. A 5k ohm in series with the 20k pot and now I have an adjustable negative feedback loop.. Thank again Scott

Great video Doug. Subscribed!! Say G'day to Rusty! I had one just like him! (18yrs) Thanks for making even less time in my day now I've discovered your vids! :)

I'm green on building amps but being a mechanic this makes sense to me - looked at the other video to the end and I'm adding it to my amp I'm building. Thank you!!

Magnificent upload sir,God bless you a lot.

Outstanding Explanation. Love your content! Thanks For your Time and Effort. I am a Component Level Repair trained Electrician, Retired. These explanations are huge as a refresher for skills that I haven't used in decades. I am also a Owner of Three Awesome Vintage Amps from the Sixties and Early Seventies. Looking for a Dual Trace and a Variac. Just for entertainment sake. I always enjoyed Electronic repair. Quite satisfying to properly troubleshoot and repair something. 🏴‍☠🎸😎

Excellent video as always! You do it up with class! I think you're on target with adding a feedback pot on the Twin; you'll have the best of both worlds and have a little more control over the tone...Looks like Rusty didn't make it onto the set in time :)

I appreciate you teaching things like this as I am about to build an amp like this...good to get this information so I won't have to go back and mod ..I would like to install a reverb unit in mine...I have plenty of room in my chassis for tubes caps wiring pots and the extra transformer plus I have room in the cabinet for the spring reverb unit. .again I do appreciate what you do for us

Greetings Uncle Doug! First, thank you very much for your series of videos that have helped many musicians and electronics aficionados understand how this beautiful gift of science sweets our lives! I have a PRS MT15 tube amp, a modern amp but like you’ve said “they all work on the same principles”. Preamp works as follows: V1 - input 12ax7 -> to V2 - lead gain 12ax7 V3 - lead gain 12ax7 OR V4 - clean gain 12ax7 -> to V6 - effects loop 12ax7 (always on) -> to Power Amp The power amp It has a match pair of 6L6WGC tubes with a 12ax7 phase inverter, and corresponding Presence potentiometer. Also a switch that controls the output with 15 or 7 Watt options. Output transformer has an 8 Ohm and 16 Ohm option. The problem is: While amp is powered on and standby off, a subtle noise is present at the speaker (30 watt 8 ohm 10 inch) with a 50 Hz frequency (Germany : 240 AC 50 Hz). Manipulating the Presence knob affects the hum sound, while still at standby off. Your experience and vast knowledge is requested: Is this something common in tube amps? If not, could you point me out to a circuit phase where I could test and possibly find an issue? In advance, thank you very much! Best regards, Gus P.S. Clean channel is very low compared to Lead channel (which volume potentiometer seems useless to control such beast).

Guitars and Amps Hola from San Diego Ca, Another fine video UD, here is an old video of my collection, a 66,67 and 72 princeton included.Original tune with my wife on vocals and me on guitar.The collection has grown since, you know, G.A.S.

Great explanation. Thank you Doug

Very informative and a really interesting modification to the NF loop. I've been focusing on your 5F1 scratch build video series. I've manage to build my chassis, although my design will be a 5F2-A Princeton. I will be posting updates on my Facebook page. Thank you for the Harbor Freight tip on the shears! I ended up purchasing the pistol grip pneumatic with a 20% off coupon which really takes you for a ride! Thank you for all you do Doug.

Thanks again for the great video. Your video convinced me to try out a "presence" addition to my champ 5f1. But I didn't have a 10k resistor handy, so I put in a 4.7k to the feedback loop from a 50k pot, the middle pot connection to the speaker jack...and then I put a 0.05 uf cap from the pot to ground (I didn't have a 0.1uf cap handy). I really like the control, and up until a volume at 7, it is wonderful. Past 7, it fattens up, and the control works nicely. After a volume of about 9, the volume doesn't increase any more, but is quite loud!, and has some neat variation in growl with the "presence" controller changing/cleaning the sound nicely. At 11, the with NFB at it lowest, it distorts in a unique way, the peaks are compressed in an exaggerated way, and almost sounds like it's cut out at the beginning of the strum: with NFB at its highest, it cleans up noticeably, but it still has a very compressed sound at the initial moment of strum...All the while, no longer increasing my volume since 9. I find it very interesting, and pleasing, but should I bother putting in that 10k instead of 4.7k, and the 0.1uf instead of the 0.05uf? Your opinion matters to me. Thank you.

Good explanation. The presence control is similar to the tone control on most guitars You have a variable pot (250 or 500K) and a capacitor in series (0.01uf, 0,022uf or 0.047uf to rolloff the high frequencies to ground. This circuit with the inductance of the pickup is a LRC (Inductance/resistive/capacitive) frequency filter.

thank you again, Uncle Doug, this is exacly where I'm at on my fathers APS-4 amp from a Gulbransen 1960 organ.I'm useing the leslie and amp with my guitar. I won't do any thing to the amp just yet, till i learn more!

A musician friend asked me to convert his Great Grandmother's early 1940s Westinghouse tube console radio to a guitar amp. I tried the "It's easy to use an old tube radio as a guitar amp" route. The radio was not working, so I trouble shot it, recapped it and replaced three defective tubes. With the radio now working, I wasted a bunch of time trying to use the "phonograph" input for guitar. After much trial and error I came to the conclusion that the idea is largely bunk. Instead, I gutted the chassis, saved only the new can capacitor which was a 20/10/10 @450V and told my friend I would build him a Tweed Champ circuit in the chassis, retaining the same knobs and controls arrangement, with the dial lamps as pilot lamps. He picked out a new 8 inch 4 ohm speaker and away I went on my first amp build. I decided to include a tone control and when looking at late 50s schematics, I realized I was building a 5F2 Princeton! I referred to a number of your videos for instruction and decided to include a Negative Feedback control as I had a spare knob available on the radio panel. I fired up the amp yesterday, corrected a couple of my mistakes and tried it out. Whoa, that thing sounds just like it rolled right out of 1956. The NFB control was the most amazing thing about the whole project. With the control, and a Stratocaster, I can seemingly vary the gain from pretty clean (6K resistance) to very pleasing overdrive at 56K resistance. With a humbucker guitar, I can get over the top overdrive! The tone also changes subtly. I am really happy with the amp and am excited to let my friend (who is a professional musician) try it out. Without all the help from your videos, this project would have been a non-starter. I am pleased to show my gratitude with a Patreon subscription. Thanks again Uncle Doug and Rusty! Scott President, CEO and Janitor at Scotty's Workshop.

Hi Uncle Doug. Once again I find your videos useful. Had missed this point about a presence control with a capacitor to the variable negative feedback (which I allready have). Just tested a 0.1uF in there (a 5f1) to get a sense of it, and yes it tightened up bass and got me intrugued. So this will be implemented. I always scroll the comments section of your videos to see if some people have some bright ideas or experiences and often there are. Gonna do more testings. Autumn Greets. Jon

Bah, Uncle Doug! I've just buttoned my Princeton back up having installed the variable bias pot and added a 1M resistor to the tremolo circuit (best mod ever!), and now I'm going to have to open her up again to do this! As with all the other people who have commented, I've found your videos invaluable. My amp is an A1164 kit from Tube Amp Doctor and shortly after building it with no confidence about what was going on inside she blew her power transformer. This forced me to learn what was going on inside the amp so that I could diagnose the issue. (I think it was ultimately caused by a combination of excessive plate current and voltage, and using the 230v tap instead of the 240v tap. In Australia 230v should be fine buy my mains varies from 230v to almost 250v.) Thanks to your videos I know feel way more confident about the amp I built :)

Put a nfb in the last amp. nice addition when you wire it up correctly. Thanks. Happy Thanksgiving.

Thanks Doug and Rusty for the video. Always baffled me what a presence control did in reality on a circuit level. Really enjoyed your recent tone control video too. I will post comments there soon. Take care . I think you mentioned you once were a teacher, you still are. What got you into tube amps and electronics.

This is the best explanation I've seen for these amp circuits. You're really helping a lot of people that want to better understand their amps. Maybe you could use a oscilloscope to show the signal as it passes through the amp? Show how the filter capacitors shape the signal. Just a suggestion.

Uncle Doug, you rock.

Very , very clear and excellent information . Thanks !

You are a super explainer.

Awesome, very clearly explained and easy to understand. Is this the same general principle by which all presence controls work on all amps? Seems like it would be, why reinvent the wheel? Thanks!

Thank You for your diligent response..the cap of .1uf is one end to ground the1st & 2nd lug are connected together as in a boost or cut but on the 3rd lug there is a 4.7k resistor connected to the wiper and the other end to ground. The potentiometer is a 5k which is average on Marshalls but this addition started to show up on the later models. The .68uf cap was usually the cap used on the 200 watt Majors (1968) and this is where the ''One Wire Mod'' was introduced, consisting of a 1 Meg Ohm resistor on the Volume1 (Normal Channel) output was rewired into cascading Volume2's (Bright Channel) input resistor or so I believe in theory these respond as a Volume and Gain when plugged into the Bright but if you plug into the Normal only that channel responds breaking the Gain's chain but very Loud and Bold which I use as such and call it the''Loud & Bold Mod'' on the 2 channel amps which is minus the Master!! I rebuilt so many amps in my day and a number of Marshall's but I was not privileged until today to be able to ask anyone or thought to do so. Thanks again Uncle Doug!

As usual uncle doug your vids are very good Without boredom just Very infomative. You teach with excellence. Love Your sarcasm to about all the bad workmanship out there. Thanks again. Between you and D Lab on UTube The differences in thaught on cirquit Designs is interesting Stay well. Hope to see more of your vids in the future I'll be implementing the info into the repair of my old 60s blackface champ amp I baught for 100 $ back in 1996 at a little music store Back when I lived in the state of KY. The man offered it to me so I baught it. Still use it today. I didnt know it was worth 800$ at the time I dont care. I wanted it for practice and other uses like in church so it is now modified for more utility I added a tremolo cirquit And some other things But no modern solid state junk,all tube I like the sound of tube Electronics full warm Tones. See you again On utube Dwight

simply awesome..thanks uncle!!

It was very interesting, thank you very much!

me inscrevi recentemente no canal do Uncle Doug e virei fã... obrigado pelas informações Doug!

Great info. This should help me reducing the excesive low frequencies when i push my princeton to maximun volume

Excellent! Thanks Doug!

Awesome again. Would love to see your explanation about the how and why of amp stage coupling whether reistance, capacitance, inductance or a transformer is used. I understand it in a basic way. But why its used when its used in a guitar tube amp not really sure.

Very good video

Thanks Doug!

thx Uncle Doug! i watch your videos again and again as my knowledge of tube-amps increases over time :-). 2 questions : is it wise to use a bypass cap (lets say a low value to enhance the treble on the plate output) in combination with a precense control in the nfb loop? or is this a not common thing to do (using a bypass cap in the cathode in combination with a presence control)?

Thank you for your videos. ....

Hi Uncle Doug, as always thank you for the great explanation and everything you do to explain things in detail. I don't know if this has been addressed yet, but I am a little perplexed. If you have a NFB resistor value of 2700 ohms or 2.7k like on an AA764, and you change the value to 10K in series with a 50k pot. How does that go below the original value when it is already 3-4 times higher on the lowest setting? Am I missing something? I would think that a 1k or 1.5k resistor in series with the 50k pot would allow adjustment above and below the original values. I am just wondering, since I am in the process of building my first amp, an AA764 with both NFB and a presence control. Any response would be greatly appreciated 🙏 Take care!

Great stuff as usual! Maybe do some more videos explaining how the different circuit building blocks work? (EQ, fx loop, etc) Rusty is a classic btw :)

Inside my amp, I removed the Presence control and replaced it with a another potentiometer (250 Ohms, after few experimentations) which allows me to play with midrange frequencies as well as different other punchy sounds. It also controls the amount of the overall volume. But in my case, it is connected between one of the grids of the Phase Inverter and the 8 Ohms ouput jack.

HI Doug, TOP video really enjoyed it, hope you are still making more video"s and are all in good feckle. All the best for the new year 2018. wigan John.

Excellent video, as always! Question: why is the signal for negative feedback typically (always?) taken from an OT tap (as opposed to a pre-OT signal, for example, the connection between the anode of the 6v6 and the OT)?

Excellent explanation!! So clear, I can probably guess how a Resonance control works. Just like Presence in the feedback circuit, only with a capacitor that filters low frequencies?

As always great video. you lost me at the end. but at least i have something to consider.(voltage spliter in a NBL) i'm new to this and i'll figure it out. thanks for the homework.

excellent, thank you very much 👌🏼🙏🏼 It could both be added as modifications (variable NFB and presence ctl.) to an old Champ, later Champ/VibroChamp or even "recent" Champion 600. Nice, interesting and enlightening.

Thanks Uncle Doug! I always thought that I was creating a "presence" control that was just pegged in perpetuity when I DC'ed the NFB loop of an old Hammond (AO-44, AO-35, AO-fill-in-the-blank), but since an actual presence control is only rolling off very high frequencies from the loop with a high value cap, I suppose I'm sort of making a presence/resonance/gain/boost control that is, of course, maxed out! My AO-44 is like that, having pulled the input transistor and the low-pass filter stuff it was driving, I also pulled the NFB loop. I use it mostly as an output section behind my Frenzel (Jim Frenzel is a guy in Texas who builds and sells a lot of good PTP-wired stuff at really good prices, has an ebay store and frenzeltubeamps.com) preamp, or sometimes I use it for bass, with a little compression up front, and drive a single 15" black widow in a dead 1x15 solid state peavey combo. It works really well for both, and if I plug a guitar straight in, it also does an incredible almost-clean that I like for reggae-plucking with four fingers and then digging in with a thumb-pick. I am completely in control of the dynamics without touching the volume knob on my guitar. I do believe I will be subscribing now, this is a class I should definitely be taking.=)

hi i appreciate yours vidéos and you speak very clearly and youre a good teacher for a french like me ,id like to know how about what voltage of power supply and plate voltage on tubes and cathode voltage cause shematic dont mention nothing about this 5f1 .thanks

great video! on the twin it has double rectifier tubes thats interesting

Thanks Doug for doing this video. I found it informative, concise and to-the-point. It was well explained. I am wondering if you could say something about the "resonance control" and how it could be mixed with the presence control circuitry you described in your video. I could not find any schematics on the topic...

Very cool. I know this is an old video but I wonder if you could change the presence control to cancel some of the highs in the negative feed back. Instead of running a capacitor to ground run a resistor to ground after your negative feed back potentiometer? Very interesting stuff.

I have achieved enlightenment. :)) Thank you for the clear explanation. I think Rusty says it's time to play ball now.

Can you show about Phase Inverter what it does and also Solid State rectivires,love your site even I can understand some of it THANKS

great video. also love that you include your dog. I love 4 things in life. God my family rescuing dogs building amp and pedals. plus my 5th love smoking weed but I do that while doing the other 4 haha. great videos though, I love that you include your dog.

I'm so used to seeing Cathode bypass caps yet even with Fender, some do n some don't. Ha! Just noticed you have a video on this ... need to watch.

Howdy. An addition to my previous comment. Most relevant for Hi-Fi enthusiasts. Because the neg. feedback arrives delayed to the summing point using secondary (global) neg. feedback there is a possibility for blocking distortion using strong neg. feedback, and therefore, strong drive. It may be checked, with an oscilloscope, whether the end tube grids go positive feeding the amp. with a square wave signal. If that happens the driver stage Slew Rate must be reduced. The reduced driver Slew Rate provides time for the feedback to act. Regards.

Thanks for another great one! I have a PA amp with multiple microphone inputs (6SJ7) that feed into a 6sf5 then a 6n7 phase inverter to two 6l6s. The NFB goes to the grid of the SN7. If I wanted to limit the NFB to one (or two?) of the inputs, could I reroute it to the appropriate 6SJs and leave one without (for harmonica nastiness)? Or will the phase be wrong?

If you change the values on the presence control could you make a resonance control? I love the one on the 5150 But I don’t play music that needs such a high gain amp and haven’t seen one of those on any other style of amps. just curious if it is as simple as switching a cap value?

Video is greatly appreciated. Dry humor even more.

uncle doug, I am completely new to mods beyond bright caps. does the negative feedback loop connect to the positive or negative side of the output transformer tap? the bugera v5 mod I was following had the negative feedback loop connecting from the 8th connection of the 9 pin preamp to the 4 ohm ot tap, but did not show which tap. is this effective with the use of a pot to vary the feedback, and could I use a 50k and 100k pot to dial in specific frequencies?

nice and wel explained

Thank you for your very insightful videos! How come you don't you need a diode on the negative feedback loop to make sure that the signal goes FROM the output transformer TO the cathode instead of the other way around?

Thank you very much

Hi Uncle Doug: Thank you for your reply today. I built a mojotone 5e3 deluxe and it came out great. I do find it breaks up a little too much. I'm trying to decifer what pin of the 12ax7, pin 7?, of tube to come off of with a resistor to create the negative feed back loop. I also assume the other end goes to the negative speaker terminal. I have a photo of my chassis, how can I send it along for your reference. Thanks again, Bob.

Hey Uncle Doug - I've got a question for you. On some more modern amps, I've been seeing a "resonance" or "depth" control, which to my understanding is a low frequency NFB control. I assume this is done by having the pot and cap in parallel instead of in series, would that make sense?

Say I wanted to add a Presence control to a AB763 circuit. The NFB voltage divider uses awkwardly small values (820R/47R)... Would adding a typical Presence circuit work properly? By typical circuit, I mean a 25k pot + 100n cap in parallel with the 47R resistor. Or, should I increase the voltage divider values to something like 82k/4.7k and adjust the PI tail resistor to compensate? Hope my question makes sense. Glad to see you're making regular videos again! Cheers :-)

Uncle Doug, I am grateful for the knowledge you share. I perform in situations often with an orchestra where I need a clean amp, then the next day it can be classic rock and blues. I've decided to try a Tweed Super amp for both purposes. So, I believe the variable negative feedback idea may be of value to me. On this amp (5F4) the resistor in the feedback circuit is 56k. in your video, you stated that you were going for a range of about 50% to 300% of the 22k resistance in your Champ amp. So, would I perhaps use something like a 25k resistor and a 125k pot? I figure this would bring my range to 25k to 150K roughly maintaining the 50% to 300% ratio. Could I try a 250K pot? (much easier to get a matching alpha 250k pot) I've been a professional musician for over 40 years now, I guess I'm trying to recreate that magic I felt when I first cranked that tiny tube amp back in the late 60'S.... :)

I wonder, did Leo arrive at 820ohm for many of Fender amps Neg feedback resistor by ear or did he rely on a formula of sorts? I have been playing with different values but havent settled on anything. If the effect of changing the resistor is eq I would guess it could never be ideal for every guitar as guitars seem to have different natural peaks and valleys at certain frequencies.

Uncle Doug, Would the same thing work for a Marshall 1959 100watt Superlead. Marshall has a 47k or (100k) where your variable pot would go. I am not sure what size resistor and pot to install?