# So I removed the negative feedback on my amp and . . .



## traynor_garnet (Feb 22, 2006)

I like it! It's a Garnet Gnome and I wasn't getting enough dirt out of it. I did a little reading and decide to do my first "mod" by myself. The amp now has more dirt, a fuller low end, and the distortion has become more "musical" for lack of a better term (it used to kind of sound like the dirt was on top of the clean sound). It wasn't a night and day difference, but more of an afternoon and evening difference. 

I've owned a bunch of these amps over the years and I am thinking that my favourite one had been modded in this way. My current amp now sounds like that one. I could easily jam with this if I didn't need a ton of clean headroom. I have something I may try to solve this potential problem . . . 

Now I am wondering if I should bypass the tone stack. With that, I would basically be converting the amp to a Tweed Champ rather than it stock BF/SF Champ like circuit.

TG


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## mhammer (Nov 30, 2007)

I've experimented with lifting the negative feedback path on my tweed Princeton in past, and the treble is instant icepick-through-the-head. I liked it but sometimes one longs for smoothness.

The tonestack can be bypassed by lifting one end (either) of the 10k resistor that goes to ground from the bass control. I did this to a BF Tremolux I used to own 30 years back, and the difference in drive was noticeable. The treble control still works, but adjustments to bass do not. The increase in signal level at the volume pot is probably around 3db, maybe a little more. because it is essentially a mids-on-max the tone is "rounder" and more vocal. Good for some kinds of solos, but maybe too claustrophobic for some guitars and styles. But I can confirm the mod "works".

Alternatively, consider that this is normally the path that constitutes the midrange control in many Fenders. Replacing it with a 25k pot would give you an exceptionally wide-range mid control in addition to a bit of boost when maxed. If you have a jack or space for one in back, you can replace the 10k with 47k-100k, and use a footswitch to put another fixed resistor in parallel to restore normal tone-control functioning.


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## traynor_garnet (Feb 22, 2006)

Thanks so much for the info. Here is a pic of my bass pot. Which one is the 10 k resistor I should lift? I want to make sure I lift the right one.


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## jb welder (Sep 14, 2010)

The resistor right in the middle of your pic, with green-blue-red colour code. Looks like it's 5K6 instead of 10K, but that's the one.


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## mhammer (Nov 30, 2007)

The higher the value, the less midrange scoop. And clearly you can't get any higher resistance than open circuit.

Get the Duncan tone stack calculator - TSC - install it on your machine, and input the current values of your Gnome. Play with the controls and see how the frequency response of the stack changes. Then, sub other resistor values in for that ground resistor and see what that yields. It will let you identify omponent values that let you create the sort of tone shaping and boost function you want.

One of the handiest little applets out there. It provides a variety of tonestack configurations including Fender, Marshall, Vox,and many others.


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## dradlin (Feb 27, 2010)

mhammer said:


> I've experimented with lifting the negative feedback path on my tweed Princeton in past, and the treble is instant icepick-through-the-head. I liked it but sometimes one longs for smoothness.


I did the same on a 5f2a based amp that I own... similar results being brighter and more breakup.

The mod is useful to me to brighten up the amp when using humbucker based guitars, though too bright for single coils.

The bypass on mine is wired to a toggle switch so I can turn bypass on and off.


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## traynor_garnet (Feb 22, 2006)

Thanks for all the help and responses guys. I got it and . . .

HOLY CRAP! This thing is a fire breathing monster. I cannot believe the difference these two mods made. An amp that once barely broke up with my Strat (low output Lollar pickups) now just rips. It is astonishing. I am running it through two 2x12 cabs (essentially a 4x12) and it is a freaking massive sound. I had a huge smile across my face while playing, and for some reason I know a treble booster would just kill for a solo boost.

Two thoughts:
1.) I could see replacing the resistor with a smaller value rather than leaving it completely wide open even though I love the sound. There is almost no clean headroom now.

2.) With both these mods there isn't enough clean headroom to jam, but it would be a perfect volume for incredible dirt. Pairing this with another amp set clean would be amazing (a stock Gnome perhaps?).

3.) I am glad the Gnome doesn't have a tube rectifier (like a Champ) as I could see things getting too loose. The bottom end is certainly big, but it is staying fairly tight.

I am still blown away. It is going to be hard to work today because I just want to keep playing!

Thanks again,
TG


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## mhammer (Nov 30, 2007)

When I modded my Tremolux way back when (jeez, over 30 years ago, now), I not only installed a tonestack-defeat, but I also installed a post-inverter master volume, so there was some serious burn available. Kinda miss it, but had to sell. I wonder who has that amp now.

Incidentally, the schematic I was looking at shows a single 6L6 in the Gnome and a plate voltage well under 400v. As long as the plate voltage is low enough, I am given to understand that 6V6 and 6L6 can be interchanged in such single-ended amps. That does *NOT* mean one can stick a pair of 6V6 tubes in a Super or Bassman and get away with it, because the plate voltage in those two amps is too high and will kill the tubes pronto. But you CAN stick a pair of 6L6 tubes in, say, a Deluxe, because the plate voltage is low enough. Doesn't mean it will necessarily sound _better_, but it's an experiment some might want to try. I know I have put a 6L6 in my 5F2A on several occasions with no ill effects, and there was a slight increment to output, though it didn't sound quite as warm. In any event, this is the long way of saying that if you have a 6V6 lying around, consider plugging it into the 6L6 socket.....that is, assuming the plate voltages shown here are accurate.


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## PTWamps (Aug 5, 2016)

Glad you found a way to pull more breakup out of that amp. Some people who cut the negative feedback opt for a switch, concealed often on the underside or back of the chassis, where you can choose your flavour. The same slider switches used as bright switches on old Fenders (I think it would be a DPDT) would do this job rather well.

Is your V2 a 12AT7? I seem to recall that. Try throwing a 12AX7 (higher gain) tube in there... you might also get a bit more kick that way.


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## traynor_garnet (Feb 22, 2006)

Thanks MHammer. You will see different Gnome schematics showing either a 6V6 or 6L6. I've owned different Gnomes that have come stock with either tube, but my current one uses a 6V6 (chassis is stamped as such too). I'll see if I have a 6L6 sitting around  

TG


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## traynor_garnet (Feb 22, 2006)

They are both 12ax7s and there is a ton of dirt now! It would be interesting to hear the amp with the negative feedback enable while the tone stack is bypassed (I clipped the NFB first). I'm starting to see that you could go nuts experimenting with this stuff!

TG




PTWamps said:


> Glad you found a way to pull more breakup out of that amp. Some people who cut the negative feedback opt for a switch, concealed often on the underside or back of the chassis, where you can choose your flavour. The same slider switches used as bright switches on old Fenders (I think it would be a DPDT) would do this job rather well.
> 
> Is your V2 a 12AT7? I seem to recall that. Try throwing a 12AX7 (higher gain) tube in there... you might also get a bit more kick that way.


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## traynor_garnet (Feb 22, 2006)

What resistor value would you recommend for taming the amp a bit without losing all the dirt. If the stock value is 10k (or was it 5k as Jake, the man with a snake on face, suggested?) I'm assuming I want higher than 10k so I scoop out less mids (re: mhammer's post).

TG


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## mhammer (Nov 30, 2007)

Here is where I again recommend the Duncan applet. You can plug in different values for that resistor, and not only will it show you the frequency response curve, but will also show you the overall resulting level. It doesn't _tell_ you explicitly that it will be 3, 4, 6 db higher. It's up to you to keep tabs on the graph and note the location of the curves.

Note that retaining those lower mids will add to the overall signal amplitude, even if the treble and/or bass curves seem to be in the same place. My sense is that, given the other component values, a 25k-50k mids pot, in series with a 3k3 fixed resistor, will be sufficient. The 3k3 will assure that at max mid scoop you'll still have some signal left.


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## bolero (Oct 11, 2006)

hey wow, great thread

I have a Garnet gnome as well, that sounds good but stays too clean for my tastes & doesn't break up. which is great if you want to use pedals etc

but I love that juicy goodness of a cranked snotty little amp. I will have to try this out!

thanks traynor_garnet!


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## PTWamps (Aug 5, 2016)

I just finished wiring my Champ (5f1 clone) chassis. Inspired by this thread, I tried clipping the negative feedback wire to the output jack. It gave the amp more grit and presence, but made it too ice-picky bright for my tastes. So resoldered it.

Another thing I found useful (I think but forget whether your Gnome has it) was to add a 25uF/ 25v bypass cap next to the 1k5 resistor on V1. The original Fender layout leaves this cap out (though it appears in many actual examples of vintage 5f1). The cap added a lot of gain and enabled earlier breakup.

Without that cap the amp sounds fine, but cleaner -- maybe better suited to jazz or country players. Along the way I also put in a .68uF/150V cap (which I believe is more of a Marshall value)... but there was little difference from no cap at all. So I went with the 25/25 and am happy.

TG: You'll get to hear this probably when we meet up. Starting on the cab tomorrow.


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## traynor_garnet (Feb 22, 2006)

Glad others are in on the fun. Today I ran the modded Gnome into my attenuator (set to load) and then from the attenuator's line out into the front of my amp (Bass Master). I am willing to bet that the Garnet Herzog is basically a Gnome with a bypassed tone section, removed NFB loop, running into a dummy load. Not sure if that is_ exactly_ what is going on in the Herzog, but boy does it sound similar.

TG


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## mhammer (Nov 30, 2007)

As with JFETs and bipolar transistor, adding a cap to that path to ground increases gain for AC, with a low-end rolloff determined by the capacitance value. Insert a smaller cap value and the increase in gain is still there, it's just applied to the mids and higher frequencies and not to the entire spectrum of the guitar.

That gain could be varied in a number of ways; e.g., by inserting a 2k trimmer in series with the 25uf cap. When the resistance through the cap is _less_ than 1k5 (or whatever the functionally equivalent resistor value happens to be in any given amp), the cap begins to matter. If the resistance in series with the cap is greater than stock resistor to ground, it's as if the cap isn't even there. If one were to dial in, say, 800 ohms on the trimmer, you'd get some boost, but not the full boost attainable.

Alternatively, the extent of boost can be frequency-shaped. So, for instance, a 1uf/25V cap to ground would provide boost at mids and highs. Running a 560R fixed resistor, in series with 25uf, to ground in parallel with the 1uf and 1k5, would provide moderate boost for the overall spectrum.

I noted that it works the same way for bipolar and FET transistors. I made myself an EHX Muff Fuzz clone. The Muff Fuzz is essentially a silicon Fuzz Face with fixed gain, given by a 2k7 emitter resistor for Q2. I connected the wiper of a 5k linear pot to the junction of R4/R5 and the Q2 emitter. Each outside lug of the pot went to a different value cap to ground. In one direction was 1uf, and in the other was 22uf. reducing the emitter-cap resistance for the 1uf path provided significant treble boost, while rotating the pot in the other direction provided substantial full-range boost and heavy fuzz.

I give a deep bow and big tip-o-the-hat to J C Maillet for the diagrams he provided in the copy of his book that he gave me: IFMTA Book (PDF) - viva Analog


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## traynor_garnet (Feb 22, 2006)

So I have been having fun swapping in different resistors. It is amazing how much difference it makes. Right now, I 've been going back and forth between the original 5.6K and an 82K resistor. WAY more distortion with the 82k but the bass pot doesn't seem to do anything (or barely until the very last part of the sweep on the bass pot).


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## Farmboyjo (Aug 26, 2016)

So can I ask... How do you remove the negative feedback loop on an Gnome? I have one with 6V6 that I would definitely like more dirt out of. I'm just set to put in a 3 prong plug, and would do this at the same time. Is the 'death cap' part of the negative feedback loop- as I'm thinking to do that too. 
Tks!


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