TMB + Negative Feedback Loop = Soiled Underwear.

jersey_aaron · Post by **jersey_aaron** » Mon 07/05/04 3:53 pm

JA here to praise the negative feedback loop in the 18 Watt TMB. Big thanks to Richies "crude drawing" and explanation of the NFB Loop found here:

http://www.18watt.com/modules.php?name= ... sc&start=0

The only problem with the post above is that if you use anywhere near the values that Richie lists, the mod WILL NOT WORK. Through trial and error (and a little research), I found that the TMB only works with Fender NFB loop values, which are a 820 ohm Feedback Resistor and 100 ohm tail resistor (parallelled with a 1uf cap). I used the 16ohm tap. The PI tail resistor should also be bumped down from 56k to 47k for a more midrangy tone.

There has been alot of speculation as to what a NFB loop would sound like in a TMB. Does it change the character of the amp? The answer is NO! What it does, it moves the amp a couple of notches closer to plexi and fixes some major problems with the TMB design.

First and foremost, on a TMB past 7 or so, the amp will be wailing away in high-gain oversaturated flabbiness. Add the negative feeback loop and the low-end flabbiness now becomes a tight, bassy karate chop in the neck. Like a Plexi. Also, the amp gains more volume and usability as 7 isn't the distortion point any longer. You now have to kick it up to 8 to get the distortion. The distortion is also more complex and just much better sounding. The NFB loop adds a high-end sparkle when you hit 9 (as opposed to the "blanket over your speaker" effect it used to have), 10 is quite plexi-ish and addictive. The NFB controls the insane amounts of gain thats inherent in the TMB design.

In my opinion, THIS MOD IS ESSENTIAL for anyone who owns a TMB. In fact, it should be added to the official design. Its just that damn good. If y'all need a diagram, I'll see if I can get one up soon.

loverocker · Post by **loverocker** » Mon 07/05/04 4:01 pm

Sounds great - any chance of before/after clips? Er... though I'll understand if you don't want to undo the mod just to record an MP3 to satisfy my idle curiousity:)

jersey_aaron · Post by **jersey_aaron** » Mon 07/05/04 6:01 pm

Well, its going to be the same guitar/speaker setup as my jersey aaron clips in the zounds folder. The mic placement may be slightly different, but I can record the same riffs. However, the mod is easy enough to do, just takes a few minutes. Its also easy to undo, but after you do it, trust me, you will never go back.

Post by **Plexi** » Mon 07/05/04 7:56 pm

Glad to see you liked it. Its funny i just finished my 36 watter trainwreck/18. And i was telling bnwitt about useing a feedback switch,Which the TW has to have. But i told him it would be good to add this to the TMB. Its very easy. With the switch,you can filp it for the TMB,or switch it to use the 18 channel.
the feedback helps tame the TMB. I too thionk its worth it to use it.

The values i gave were pretty stock,they should have worked. But the ones you used will also work fine. The 100ohm you have,is llike the presence set at about 1[if you had a control] changing the value of that resistor,will change the voltage. You can measure the voltage there,and it will be more neg voltage,if you bump up that resistor. Which i don't use the presence much anyway..so it being set to zero to one,would be fine for me..lol But the thing is,not only does it help contrl some of the gain,useing the 820ohm will also give you more clean volume control use.
If you change the 820ohm..you start losing the clean on the volume control. As you can see some anps use 820,some use 22k 27k 56k or 100k..the higher ,up to 100k you have,you loose some of the clean.What i mean is..its more like its on full,even when you move the volume to 1 or 2. The 820 will give more clean on the volume control..but will still sound almost the same when cranked.. Yeah i think it is good to have it for a TMB. The switch makes it possible,to use the feedback circut..or just to ground for the 18 stck channel. HAHAH and if you have the switch,you can sure tell when you forget to flip it to the channel your useing..

Richie

Tpetty5 · Post by **Tpetty5** » Tue 07/06/04 6:50 am

Hey JA, good to hear you are pleased. If you get a chance I would like a diagram of the mod. I think I may like to try this in my JA TMB.

jersey_aaron · Post by **jersey_aaron** » Tue 07/06/04 7:01 am

Rich, one think I did notice is that if I "up" that 100 ohm resistor I get this low hum, even with all the volume controls turned down and nothing plugged in. What do you think?

dansamp · Post by **dansamp** » Tue 07/06/04 10:42 am

hey plexi sorry to intrude
what schematic did you use for your trainwreck????
dan.

Post by **Plexi** » Tue 07/06/04 11:51 am

"""What do you think""

I think if it sounds and works good where you have it.. leave it like it is..

When adding a feeback,you can have all kinds of weird things happen.
You then have to make sure the phase is correct on the EL84s Sometime it will howl,sometimes a small squeal. It can be tricky,and have to get things just right with the amp. What tap did you connect it to?

"""what schematic did you use for your trainwreck???? """

Thats not an easy question..lol Since i had to make it work for both the 18 and the trainwreck together and for EL84 tubes. I even changed a few things of my own layout i started with..lol
It is based on the Express preamp,and is very close. I haven't drawn up my finished amp yet. I know alot of the guys on the wreck site have built the amp and had mixed outcomes. Many say its way to bright,noisey,and alot of hiss. But i have none of those problems,its one of those amps,that make you enjoy playing. I really love the sustain of it. There are some sound files on the yahoo site of the real "wreck" and some of the Komet and real wreck on the Komet site.
Richie

jersey_aaron · Post by **jersey_aaron** » Tue 07/06/04 1:38 pm

Rich - It was out of phase at first, so I switched the OT leads and then it worked. There actually is a low, low hum without anything plugged in using the just 100ohm resistor (I'm on the 16 tap), but its so negligable, it doesn't become a problem unless you "up" the resistor.

By the way.. You know that "Tone @ zero" problem everybodys' been having? Well, when I first hooked up the NFB loop, I had a bypass cap across the middle and end lugs of the TMB preamp pot - 470pf. Sure enough, when I turned JUST the TMB preamp pot down to ZERO, the amp freaked out. Well, I got rid of the cap and that was the cause of the squeal. Those with the "tone @ zero" problem might make something of this info.

myker · Post by **myker** » Tue 07/06/04 11:42 pm

i would like to see the layout of what you have done with this nfb loop, because i too want soiled underwear tone!
hehe
i was wondering why my tmb sounds farty when i crank it...
mike..

soundguy · Post by **soundguy** » Wed 07/07/04 12:41 am

OMG, "soiled underwear tone" thats one for the books!

Question: Is soiled underwear tone **** than clapton's woman tone?

I love the woman tone, but I think I might opt for soiled underwear tone.

dave

Post by **Plexi** » Wed 07/07/04 1:54 am

now thats the real "brown"sound

jersey_aaron · Post by **jersey_aaron** » Wed 07/07/04 2:58 pm

Here ya go, fellas:

The JA/Ritchie Negative Feedback loop mod aka The "Dookie" Mod.

Post by **Plexi** » Wed 07/07/04 3:18 pm

JA you might want to swap sides for the input,and feedback.
The .1uf cap does not have to be electrolitic..

Richie

Post by **Plexi** » Wed 07/07/04 3:19 pm

oh,and make the .01 a .1uf cap

jersey_aaron · Post by **jersey_aaron** » Wed 07/07/04 3:25 pm

I didn't know you could use any kind of cap. Its at .1uf now. I guess whether its on the left or right side depends on how you wired the OT?

EBE · Post by **EBE** » Wed 07/07/04 6:56 pm

Did the mod affect the normal channel?

Norm

Post by **Plexi** » Wed 07/07/04 8:45 pm

It will unless you use a switch to take out the feedback. The switch is very easy to hookup..

Richie

Post by **Plexi** » Wed 07/07/04 9:38 pm

here is another quick crude drawing of the switch,and where to put the parts. I didn't color the values,but i think you can tell what and where things go.
JA maybe you can draw it up a little better..

Richie

Post by **Plexi** » Wed 07/07/04 9:44 pm

Oh i forgot..the switch is drawn to be used on the back..the toggle going sideways instead of up and down. Its easy to know by looking at it from the back,which direction the toggle is in..is for what channel it is..

I guess if you put it on the front of an amp,youd have to reverse the hookups so it would go to the right direction of channels..
But you could mount it so the toggle is up and down..lol

Richie

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TMB + Negative Feedback Loop = Soiled Underwear.

TMB + Negative Feedback Loop = Soiled Underwear.