TMB + Negative Feedback Loop = Soiled Underwear.

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

Post by **zaphod_phil** » Tue 07/13/04 4:20 pm

I've also had good results reducing the EL84 cathode bypass cap to 68uF and even 47uF to get the low end a bit tighter. With NFB installed, you may not need to do that now...

jersey_aaron · Post by **jersey_aaron** » Mon 07/26/04 9:59 pm

Just wanted to add that I've got a 25uf in there now instead of a 1uf, and its really like having the presence up on 9-10. There is lots of sparkle and the clean tones you get when you roll your guitar volume down are very fender/ampeg shimmery sounding. Like you can hear your hands slide up and down the neck. What exactly is the proper method for adding a presence pot, so I can just dial it in at will?

Post by **Plexi** » Tue 07/27/04 9:59 am

JA its very simple..and since you have the feedback already installed.
There are different ways people wire it.You can go to the fender amp field guide,and look at the 5f6A-bassman layout.. Marshall added a 4.7k resistor to ground off the right lug. The .1[film cap] cap is what will add alot of the highs and hiss..

Richie

Post by **zaphod_phil** » Tue 07/27/04 10:37 am

The 56a Bassman uses a 27k feedback resistor from the OT, going to a 5k pot at the bottom of the PI tail resistor, with a 0.1uF bypass cap between the wiper and ground. The Marshall JTM45 has the same, while the Marshall 1987 Plexi has the same but with a 47k feedback resistor. The frequencies bypassed by the cap don't get reduced by the NFB circuit.

Post by **zaphod_phil** » Wed 07/28/04 7:28 am

jersey_aaron wrote:Here ya go, fellas:
The JA/Ritchie Negative Feedback loop mod aka The "Dookie" Mod.

I looked at the mod layout that was posted, and I don't see any reason for moving both channels to the same PI input, with the 220k summing resistors. The NFB will work perfectly fine with each channel going to it's own side of the PI as before, and that makes the NFB mod even simpler to do with any TMB 18W, as you no longer need the 220k resistors.

KHStudio · Post by **KHStudio** » Wed 07/28/04 6:37 pm

zaphod_phil wrote:
jersey_aaron wrote:Here ya go, fellas:
The JA/Ritchie Negative Feedback loop mod aka The "Dookie" Mod.
I looked at the mod layout that was posted, and I don't see any reason for moving both channels to the same PI input, with the 220k summing resistors. The NFB will work perfectly fine with each channel going to it's own side of the PI as before, and that makes the NFB mod even simpler to do with any TMB 18W, as you no longer need the 220k resistors.

Phil,
I tried the NFB and felt it changed the sound of the normal channel too much (Yes, it was switched off!). It lost volume & gain for one. So what your saying is very interesting.
Have you tried it & exactly which side would the feedback go into?

Thanks

Can someone make an updated drawing with these options & maybe the option for the presence control? (& where to put it)

KHStudio · Post by **KHStudio** » Thu 07/29/04 6:22 am

Post by **zaphod_phil** » Thu 07/29/04 8:08 am

No I haven't ried this. I was just speculating again. Anyway, here's the link for the "Dookie Mod" - http://www.18watt.com/modules.php?name= ... oad&id=506
Just don't fully apply it. Leave the Normal channel's volume pot connected to the 0.01uF cap on the 2nd input of the PI, just as it was before. Connect the TMB channels "MV" pot directly to the 0.022uF cap on the 1st input of the PI, also just as it was before. So no 220k resistors and all that's been added to your TMB amp now is the two resistors (820 ohm & 100 ohm) and the 0.1uF cap. That's it!

jersey_aaron · Post by **jersey_aaron** » Thu 07/29/04 9:07 am

Doesn't work. The Normal Channel is dead unless you disconnect the NFB wire from the .01 cap and plug it straight into the tail resistor. Then I'm pretty sure the NFB loop isn't working at all. The normal channel sounds pretty sweet this way though, maybe cause the tail resistor is being bypassed by my 25uf cap?

Post by **zaphod_phil** » Thu 07/29/04 9:49 am

jersey_aaron wrote:... you disconnect the NFB wire from the .01 cap...

Not quite sure what you're saying there, JA....

jersey_aaron · Post by **jersey_aaron** » Thu 07/29/04 9:59 am

It doesn't work, thats what I'm saying. The NFB loop cannot be on the same side of the PI as the normal channel.

Post by **zaphod_phil** » Thu 07/29/04 10:12 am

OK, I got what you're saying now - I think we'd both misunderstood each other there.

No, of course not. The NFB connection *must* go into the PI tail resistor to apply equally to both sides of the PI. However, once you've done that, there's no reason why the Normal channel can't be left where it was, connected to the "B" input of the PI.

Whether that actually imprves the Normal channel's tone is a different question...

jersey_aaron · Post by **jersey_aaron** » Thu 07/29/04 11:36 am

Ok, I see, so the NFB loop will work without using one side of the PI? This is what I heard when I did that -

The Normal channel was extremely sweet and clean (with the 25uf bypassing the PI tail)

The TMB channel was a tad bit on the harsh side, kind of sounded like the NFB wasnt working.

Post by **zaphod_phil** » Thu 07/29/04 12:29 pm

If the NFB was connected to the PI tail resistor, like shown in the dookie mod layout picture, and each channel was being fed into it's own side of the PI, then NFB was being applied equally to both channels. However, the 220k(or even 470k) resistor may help roll off a few highs to sweeten the sound up...

KHStudio · Post by **KHStudio** » Fri 07/30/04 9:47 am

OK, Now I'm confussed>

I've asked several quenstions on this topic that never got answered... some were, (I thought) very important. I tried it with the switch, as I explained earlier & it didn't seem to work. I could not hear the switch affecting EITHER CHANNEL / ON OR OFF. Keep in mind i couldn't crank it up past about 3-5 on each channel.

Anyho, my biggest complaint is I firmly believe (and can hear) that the 220 mixer resistors (or the mod in general) hurt the normal channel: Volume drop, fullness, drive. etc...

So, I've been following you guys posts & doing some research myself to find a way to NOT BOTHER THE NORMAL CHANNEL, and still get the NFB to work.

I think it was plexi that introduced a new concept on hooking things up in another thread recently, I'm not sure if that would do it... Has anyone tried it. (Plexi's pluging into the TMB TUBE thing?)

Also, If any of you guys figure out the best way to add the presence control please let us/me know. With minimal effort it could be a cool added effect. I did see JA ask about this a few post ago.
JA, how are you making out with that?

Once again, Thank you for your help.

Post by **Plexi** » Fri 07/30/04 10:06 am

""I tried it with the switch, as I explained earlier & it didn't seem to work""

If it didn't work then you have something hooked up wrong,or wrong value part. The 36 watter that i took the normal channel and moved it to go into the TMB tube,i posted some sound files of that amp..they are under "Trex" in the Zounds...so you can hear how it sounds,and the feedback circut is used in and out of the clips..

Tell me what amp you have built TMB/normal channel ? And i can try and help you figure out the wireing.
The TMB should use a feedback circut,where the normal channel shouldn't,and if the feedback is used with the normal channel,it will cut alot of the sound or volume.

Richie

KHStudio · Post by **KHStudio** » Fri 07/30/04 10:29 am

Plexi wrote:""I tried it with the switch, as I explained earlier & it didn't seem to work""

If it didn't work then you have something hooked up wrong,or wrong value part. The 36 watter that i took the normal channel and moved it to go into the TMB tube,i posted some sound files of that amp..they are under "Trex" in the Zounds...so you can hear how it sounds,and the feedback circut is used in and out of the clips..

Tell me what amp you have built TMB/normal channel ? And i can try and help you figure out the wireing.
The TMB should use a feedback circut,where the normal channel shouldn't,and if the feedback is used with the normal channel,it will cut alot of the sound or volume.

Richie

Richie
I built the TMB/normal channel.
I explained in an earlier post how I hooked up the switch & asked if it was a problem... It just might be.

Maybe thats why it sounded better when I unhooked it all.
==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??????????

My other questions were:
(check out #4 too)

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.

==============

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)

Thanks

Post by **Plexi** » Fri 07/30/04 3:00 pm

The 100ohm to ground, thats probably only used if you use a small feedback resistor. 100ohm to ground,is almost like having the feedback grounded. The effect or voltage of the feedback will change with the resistor you use to ground. Try A 1.5k for the resistor going to ground and a 27k feedback resistor,see how that works. What you should hear,if you turn the volume up on the TMB,with feedback switched in to start..when you flip the feedback out..it just grounds the PI tail and lifts the feedback connection. JA seemed to have some problems trying other values. I haven't had any problems trying many different values for the feedback. But you have to keep in mind the resistor off the tail to ground will vary the feedback too. You can bypass it with a .1uf cap,it will add some high end presence.

If you are useing the 220k make sure they are 220k not 220ohm..Or just use one that is on the TMB channel. then the normal channel will be as it is stock. these just help to keep the channels from bleeding together. Some people remove them in fenders.

If you try the newer way of putting the normal channel into the TMB tube,then you won't need the 220ks.

As for some other questions..no it won't hurt the OT,when you lift the feedback it just disconnects it..I see what you are saying,but to disconnect the feedback you have to lift it somewhere,so one end or the other will have something kinda hanging connected to nothing,so to speak.. I chose to lift it closer to the OT connection. But you could probably do what your talking about. But it won't hurt a thing as it is.

If you use the PI with feedback,the values are .1uf and .022

Hope i'm not confusing you more..its really very simple. I probably just make it sound like its hard to do..lol

Richie

Post by **Plexi** » Fri 07/30/04 3:01 pm

"""when you flip the feedback out..it just grounds the PI tail and lifts the feedback connection"""
I should have said..you should hear a boost when you do this..

Richie

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TMB + Negative Feedback Loop = Soiled Underwear.