TMB + Negative Feedback Loop = Soiled Underwear.

myker · Post by **myker** » Wed 07/07/04 10:21 pm

damn! my britches are saturated!!! ALL HAIL THE DOOKIE MOD!!! thanks richie and aaron!!
that really keeps the tone together and provides bassy balls when cranked

i got to go change my pants so i can soil another pair!!
mike..

dboul · Post by **dboul** » Thu 07/08/04 1:26 am

jersey_aaron wrote:The "Dookie" Mod.

Absolutely Too F'in funny!!! Thanks for the laugh and the fun sounding mod. I'll be trying it this weekend!

dboul · Post by **dboul** » Thu 07/08/04 1:30 am

Crap! Pardon the pun . . .

I forgot to ask about doing this in a non-Master 18. What would it look like?

Post by **zaphod_phil** » Thu 07/08/04 3:05 am

Pretty much the same, as the output stage and PI are the same in both regular and TMB 18watters.

Those feedback resistor values are however, very different from Trainwreck ones. TW amps use only a small amount of NFB compared with typical Marshalls or Fenders. Typically the NFB reistor from the OT's 8 ohm tap (not 16 ohm), to the PI tail is around 47k to 100k, feeding into the top of the usual 5k presence pot.

Post by **Plexi** » Thu 07/08/04 7:01 am

Yes it might be the same for either as Phil said,but probably help the non master even more,since you don't have the MV to control the level gain sound as much. Yes the values may be a bit different,when you use the 47k or 100k you don't have as much use of the volume control for clean,it comes on quicker and dirtys up faster than with the lower ones.
Dan, if you use the switch you'll see very easy how it effects the channels.
If you forgot to switch out the feedback,and plug into the normal channel,it will loose alot of the poop..lol
Or if you have the feedback kicked in,flip the switch and you'll see what a boost it adds to the TMB channel,not haveing the feedback.

Also as for the values,you can use alot of different ones,to make the amp act different,or tailor/tweak it to what your after. And since this is more of a preset presence control,the lower values will make it be more like if you did have a presence control,set at only maybe 2.And not real bright as if it were on 10. The other thing that is important in making the presence control is the .1 cap,thats what adds hiss and alot of the high end,and you can change that,since you don't have a control,you can try maybe a .0033 across the feedback resistor,it will take out the hiss,and the high end,but still with it hooked up as feedback,it will help control the huge boost of the TMB..
I believe some caps are also used series with the feedback resistor. There might even be a way to use both,with a dual pot.
All kinds of ideas you could add,resonace control,or even a cut control,even though its used in a different spot.I always think of those as a master tone control..

I used different values in my TW feedback circuit.But whatever works for you,is ok.. You can try many things to see what they do.

Richie

Post by **Plexi** » Thu 07/08/04 7:11 am

"""Those feedback resistor values are however, very different from Trainwreck ones. """
The values i posted in an early drawing were for more of a common Marshall circuit. Since thats what the TMB is. I never posted the values for the TW that i know of. Since no one is useing it but me..
Some use and try different taps of the OT. But i like the 8 ohm tap.
Richie

Post by **zaphod_phil** » Thu 07/08/04 8:49 am

You said you we're using the NFB to get the TW sound and feel with your 36W. So are you using different values in that amp then? Also, I'm curious, which Marshall amp use a 820 ohms? Most of the Marshall schems I've seen use anywhere between 22k to 100k as the NFB resistor. Obviously the higher the value, the less NFB actually taking place...

Post by **Plexi** » Thu 07/08/04 3:20 pm

Phil: If you look way back to when i posted the feedback circut..for the TMB
i drew a little picture..with "see note" that drawing shows a 47k resistor.
Belive me i know what a marshall feedback circut uses or used,some amps do vary. And yes i am useing different values in this 36 watter.
The fender is most know for the 820 ohm resistor,but fender amps didn't use a presence control as much. Some of there amps used 820 ohm..other amps used many different values. 22k 27k 47k 100k and some i probably left out.
I posted the different values you could use,and what they do or how they effect the amp.Look back for the"adding feedback to the TMB" drawing.

Richie

KHStudio · Post by **KHStudio** » Fri 07/09/04 5:59 am

I've been following this post and would love to try this out but I'm a little confussed. It seems all you guys use different values for one reason or another. I'm not to sure where to start.
I usually ask this a lot, but what would be the most standard(middle of the road) way of adding NFB (definitly SWITCHABLE)?
And as long as I'm going to put it in I'd like to add the Presence control... how would I do it & what components would have to be changed or removed?

& is it really worth it?

Richie, I looked at your ruff drawing with the "see note" and didn't see any "notes"?

1 more question on Richies drawing, why does it tap off the speaker jack instead of the OT taps?

jersey_aaron · Post by **jersey_aaron** » Fri 07/09/04 7:04 am

Just use the Jersey Aaron drawing and the values listed there. They are the values that seem to work well with the TMB (I tried them all).

Post by **Plexi** » Fri 07/09/04 7:14 am

You ask good questions..

"""And as long as I'm going to put it in I'd like to add the Presence control... how would I do it & what components would have to be changed or removed? """

You'd have to find a place to put the control,and if you use the control you could use the standard marshall values for the feedback,and maybe play around with the value of the feedback resistor till you get what your after.
You could solder in a couple alligator clips,then it makes it easy to try different resistor values,then when you find what you like,remove the clips ,and solder in the resistor. If you use a presence control,you could get a pot with a switch,and use it all on one.
So if you wanted to use the TMB the control/switch would be on..
And for the Normal channel..roll off the presence control,and click it off,and the feedback not used. Since your parts would be mostly on the pot,the switch would mostly just be used as a ground,or to lift it from ground.

"""& is it really worth it? """

That is a YES.. when this first came up,i never thought of using it,because the TMB was just an option for people not using the trem circut. And to have the normal channel work like it should,it doesn't have a feedback circuit. The switch should have been added a long time ago..lol

""Richie, I looked at your ruff drawing with the "see note" and didn't see any "notes"? """

Well the notes were on the post that has the drawing..not on the drawing itself.i think i put what the value changes might do,or how it sounded.

1 more question on Richies drawing, why does it tap off the speaker jack instead of the OT taps?

My drawings suck..lol I just figured people would think of the tap being on the jack. Some use switches,so you could put it on the tap that is on the selector switch,some of us just use a jack for each tap..Hope that explains it..pretty much the same thing..

Richie

Post by **Plexi** » Fri 07/09/04 7:16 am

JA posted as i was writing..lol

As JA said..

""Just use the Jersey Aaron drawing and the values listed there. They are the values that seem to work well with the TMB (I tried them all).""

Richie

myker · Post by **myker** » Fri 07/09/04 10:27 am

I have tried it on both channels,it really has improved the tmb channel. I actually found the tmb channel to be unusable before, i thought it sounded flabby and farty and lacking focus. But now the tmb channel is rocking.
on the normal channel it seems to take away a bit of that shimmery richness in the lower volumes. have any of you found this to be true on your amplifiers? and maybe i just need to tweak the values a bit or take it off the normal channel? im using the stock jersey aaron dukie mod values.
aaron and richie, are you guys using the dookie mod on the normal channel too?
thanks
mike..

Post by **Plexi** » Fri 07/09/04 3:25 pm

""are you guys using the dookie mod on the normal channel too? ""

No, if you use the feedback circut on the normal channel,it will take away a heck of a lot of the volume /gain sound.

Thats what the switch is for,to put it back to usee the normal channel.

Richie

KHStudio · Post by **KHStudio** » Fri 07/09/04 4:38 pm

PLEASE HELP!!!!!!!!!!!

OK, I did this mod last night. Seems alright but I'm not sure

1st off, I couldn't crank it up too loud, it was late. 50% both volumes (TMB). I couldn't hear very drastic changes but I did hear (with the switch off) that my normal channel wasn't as open or sweet as it was before. I did however hear better headroom & Bass control you guys talked about earlier concerning the TMB but not too drastic.
So I have a few questions:

1. Is it noticeable at low volumes?

2. Does the 220k's affect the sound at all, especially the normal channel?

3. I used JA's values & 16 ohm tap with RICHIES switch layout.
I followed Ritchieâ€™s advice in swapping the sides and had no problems with howling or squealing.

3a. Instead of hooking the .1uf/100r straight to ground I hooked it up to the other side of the switch that is going to ground anyway to make it easier.
????????? IS THIS OK??????????

3b. JA recommended the channels join together at the .022uf.
When you swap the sides like Richie said the channels join together at the .01uf.
????????How does this affect things?????????

3c. With the switch off & the channels linked this way, how is the channel input linking affected?

4. I noticed the feedback resistor (on the switch layout) is permanently connected to the speaker ohm tap & the cap/res. - wouldnâ€™t it be better to have it disconnect using the unused poles on the other side of the switch?
4b. Does this constant hookup hurt anything at allâ€¦ like the OTâ€™s performance? (just curious)

Iâ€™m really learning a lot here as I go along. I really appreciate everyoneâ€™s help; it has helped me build a GREAT amp! Probably my favorite.
The only problem is I donâ€™t want to ruin what I like the best - The absolutely killer, smooth, sweat normal channel.

Thanks, Kevin

KHStudio · Post by **KHStudio** » Tue 07/13/04 3:30 am

Would it be possible to add a more complex switch that could put it back to the original design or is that too much? (will probably pop like hell

)

Also, what would the values be & where would you put the pot for the presence control.

Thanks again

Post by **zaphod_phil** » Tue 07/13/04 6:20 am

myker wrote:I actually found the tmb channel to be unusable before, i thought it sounded flabby and farty and lacking focus.

If that was the case, what values of cathode bypass caps were you using in your TMB channel preamp? They shouldn't have been any higher than 1uF or 2uF. I say this because most other TMB builders have found the amp very useable even without the NFB mod. The NFB then just ices the cake...

KHStudio · Post by **KHStudio** » Tue 07/13/04 2:36 pm

Is there anyone who can help me with my last 2 post?

jersey_aaron · Post by **jersey_aaron** » Tue 07/13/04 2:53 pm

[quote="KHStudio"]PLEASE HELP!!!!!!!!!!!

1. Is it noticeable at low volumes?

The benefits of the NFB loop occur mostly when the amp is cranked. Lower volumes will = cleaner tone.

2. Does the 220k's affect the sound at all, especially the normal channel?

dunno

3a. Instead of hooking the .1uf/100r straight to ground I hooked it up to the other side of the switch that is going to ground anyway to make it easier.
????????? IS THIS OK??????????

dunno

3b. JA recommended the channels join together at the .022uf.
When you swap the sides like Richie said the channels join together at the .01uf.
????????How does this affect things?????????

both channels are fed from one side of the phase splitter, the other side feeds the nfb (or so I understand).

3c. With the switch off & the channels linked this way, how is the channel input linking affected?

dunno

4. I noticed the feedback resistor (on the switch layout) is permanently connected to the speaker ohm tap & the cap/res. - wouldnâ€™t it be better to have it disconnect using the unused poles on the other side of the switch?
4b. Does this constant hookup hurt anything at allâ€¦ like the OTâ€™s performance? (just curious)

dunno

myker · Post by **myker** » Tue 07/13/04 3:03 pm

yeah phil, i used the 1uf caps. but the tmb side of the amp never sounded anywhere near as good as the regular side to me, i pretty much ignored it until i installed the nfb. my 18 watter is really bassy. i want to get it to sound like richies clips in zounds. i think it might be my 1960a marshall cabinet to tell you the truth. that cab sucks.
mike..

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TMB + Negative Feedback Loop = Soiled Underwear.