TMB + Negative Feedback Loop = Soiled Underwear.

[Budso]

Hi Richie,

You're right. I was mistaken about the 2ohm tap. And yes, there's probably a thousand ways to design and implement a NFB.

Maybe the intent of my post wasn't clear. I was merely adding to what Phil had to say when Wornadapter was questioning how much NFB signal was actually acting upon the PI. My point was that Jersey Aaron's Dookie mod works well partly because he is using the 16ohm tap rather than the 2ohm tap. I was obviously wrong about the 2ohm, but the point is still valid that he's getting a whole lot more NFB from the 16ohm tap, rather than the 4ohm tap seen on (most) Fenders.

I also want to thank you for your design efforts that made the TMB possible. I've learned a helleva lot from you, Phil and others here. Now I'm thinking of another question but I think I'll start another thread.

Bud

[mcv]

"It might make more sense to look at wornadaptor's drawing. My idea then is to run a wire from between the tail resistor and smaller feedback resistor over to the grounding lug on the input jack. I guess it depends on the type of input jack you're using. I had in mind the three prong swithcraft jack. It includes a center switchable grounding lug that you would attach the wire to. On the Marshall type jacks, if you were facing the input jack from the front, you would attach the wire to the left rear lug. Then ensure that a wire runs from this lug to ground"

i did this, no switch though, so i have NFB always on the TMB chanel and never on the normal chanel, even when jumpered, i´m i correct?
thanks budso!

[Budso]

When you asked about the channel jumper still working I thought of course it would. And it does, but I'm afraid that with the channels jumpered, both channels will now see the NFB unless you have the switch installed. There may be a way to jumper both channels and keep the NFB only on the TMB channel, but can't think of any. I obviously didn't pick up on this part of your question. I hope I didn't lead you down the wrong path. Sorry if I did. Maybe Richie or Phil can weigh in on this one with more guidance.

[zaphod_phil]

There may be a way to jumper both channels and keep the NFB only on the TMB channel, but can't think of any...

As both channels are sharing the same PI and power stage I don't think there's any way you can actually do that....

[Plexi]

No, but if you think about it, your question is just the opposite of how it is set up.
The TMB should use feedback,and the normal channel needs to not have feedback,to be more stock like. When you have the switch..And you flip it,you can make both channels have what they are supposed to use..or switch it,and make them not have what they are supposed to use [in feedback] With this setup,you can get alot of options. You could probaly get more added sounds with maybe flipping in the bypass cap,or useing one in parallel or series with the feedback resistor.

Richie

[KHStudio]

I'm getting ready to work on my amp right now & wanted to know:

What values are you guys are prefering the most?
& if its possible, what guitar/speaker config are you using - this may help give a better idea WHY you are choosing certain values over others.

Also, is it still a no go with the presence?


Thanks

[jersey_aaron]

Try the .1uf first, and then keep going towards 25uf if you want more sparkling highs. Presence is still a no-go.

[Jef]

could a rotary "presence" switch perhaps be an option? Which gives you
1. no NFB
2. 0.1 cap --> up to 6. 25uF cap?
Kinda like the Matchless DC30 tone selector which let you select between 6 different coupling caps?

Just a thought

Jef

[BrianH]

Man, tonight I finished converting my 18w clone into a TMB with NFB loop. I'm dying to try it out but it's too late! Also definitely need to double check everything before flipping it on and I've already cracked open a beer (Yards India Pale Ale), so that ain't happening. I ran the ground bus under the board for neatness, and only ran it up in a small loop that I'll solder to a metal standoff when I've checked all my wiring for any errors.

[Plexi]

Brian..VERY neat work! Are you useing the new method Phil was explaining? Also looking at your board..the tone slope resistor,i can't see the 3rd band very well, is that 47k? it looks like a 470k..just checking,make sure its 47k. Keep us posted how it works..

I still see no reason a presence pot wouldn't work if someone wanted to use one.. I'm going to have my amp on the bench over the weekend,i may try some things to see whats what.

Richie

[BrianH]

Holy cow, thank you, Plexi! I DID use a 470k instead of a 47k. Thank you for pointing that out, I'd have missed it! I'm glad I have a couple 47k's lying around... they're old carbon comp but unused and hopefully are ok. Again, thank you.

I followed Jersey Aaron's last picture of the NFB loop, but I used a .1/100V instead of a 25uf to start.

[Plexi]

Brian, good thing you posted pics of your nice looking amp,or i'd never have seen the resistor..lol and your tone slope would be into mud..lol
You'd have thought it was something else causing it to sound like that..
We all make those kinda mistakes every now and then..

Richie