Ignorant Question #638 - PI Question

[solderstain]

(reposting this after it got lost overnight)…

Thanks to everyone for the discussion in the Cathode Resistor thread.

New question on a different project.

I have an amp that’s configured like a TW Express (electrically). The physical layout isn’t even close, but this is an engineering project. Once I get done twiddling with it, I’ll build something with a more accurate layout.

I decided I wanted to do something with the unused preamp triode in the Express circuit. I chose to try a Marshall 18w ‘Normal’ channel. I mocked it up in this mule, disconnecting the Express preamp completely. I ran the Marshall 18w channel into the Express PI (through a .022uf cap) and the regular Express output stage.

It sounds pretty damn cool, though it needs a bit of tweaking.

FINALLY! MY QUESTION. I want to tag it up to the other side of the Express PI, much like the Trem channel of an AC30 and Marshall 18w. The reason I want to do that is to avoid the necessary mix resistors that would be required if I ran the 18w channel into the high side of the PI. I can’t just copy the hookup of the AC30/Marshall exactly since both of those amps don’t have NFB/Presence.

I think I can do the hookup by eliminating the 0.1uf cap between the PI opposite grid and the NFB/Presence line, and then just hook the .022uf coupling cap for the 18w channel directly to the second grid.

But part of my little pea-brain is whispering that it can’t be that easy.

Is it?

[Andy Le Blanc]

Look thru the bad cat, matchless, and top hat bits at schematic heaven.
its been done, but these didn't use feed back.

[solderstain]

Look thru the bad cat, matchless, and top hat bits at schematic heaven.
its been done, but these didn't use feed back.

Thanks. As you said, those have no NFB/Presence... I guess I'll just have to heat up the soldering iron to try my theory myself and see what happens...

[collinsamps]

If I followed the verbiage correctly yes it's that easy. I do something similar when adding reverb between the tone stacks & PI's on JTM and 5F6A builds., or eliminating the mix resistors on single channel ab763 types. If you want to draw it up I can verify it, but in my head it sounds right.

[solderstain]

If I followed the verbiage correctly yes it's that easy. I do something similar when adding reverb between the tone stacks & PI's on JTM and 5F6A builds., or eliminating the mix resistors on single channel ab763 types. If you want to draw it up I can verify it, but in my head it sounds right.

Kool. Thanks for posting that. Between all the twidget-heads on this site, I figured SOMEONE had to have done this by now...

[Richie]

Look up the Trex .. Mark Huss dew up a schem for it.. Its an 18 watt channel and wreck channel. It doesn't use a presence control,but has a feedback switch. The 18 watt has to have the feedback disconected to sound right. Or it will suffer in thegain department. The Wreck channel needs the feedback. You could add a presence control,and use a pull or click switch to switch the feedback in and out. I think Mark uses a footswitch circuit on his.. But you should be able to get some ideas of how to do the channels from the Trex..
A couple of the links are to 18 watt.com ii think you have to be a member to listen to them. And remember this is an Trex,and uses 2x EL84s So the power section is a bit different values than a wreck or bassman would be..

http://mhuss.com/18watt/schematics/


http://www.18watt.com/modules.php?name= ... trex+clips

http://www.18watt.com/modules.php?name= ... 71&start=0


http://mhuss.com/Clips/18wSteel.mp3
http://mhuss.com/Clips/18wTie.mp3
http://mhuss.com/Clips/18wTom.mp3
http://mhuss.com/Clips/18wZZ.mp3

[solderstain]

T"he 18 watt has to have the feedback disconected to sound right."

Thanks, but I'm not worried about that - I'm not trying to make it sound like a true 18w. I just liked the idea of trying that style preamp configuration with a standard Express PI and output. Just to see what it sounded like. It's a neat sound - not like an 18w, and I don't even want it to. I'm not going to bother with switching NFB in and out and all of that. The only thing I'll be switching is the input from one channel to the other.

The 18w preamp actually saturates pretty quickly, but it's naturally less loud, having only one gain stage before the PI. It really sings at a more tolerable volume level. It's a little bit mid-heavy, so I'm going to mess with it a bit and see if I can't dial that back a bit.

Now that I'm basically done with my 2xEL84 project for the moment (is anything ever really DONE???? ), I'll try the lash-up on this other little abomination.

[Merlinb]

(reposting this after it got lost overnight)…
I think I can do the hookup by eliminating the 0.1uf cap between the PI opposite grid and the NFB/Presence line, and then just hook the .022uf coupling cap for the 18w channel directly to the second grid.

But part of my little pea-brain is whispering that it can’t be that easy.

Is it?

If you do that you will have feedback being injected only to the PI tail. This will subtract from one PI output, but add to the other! In other words, you don't have NFB any more, but a combination of NFB and PFB.
However, so little feedback signal will make it to the PI cathodes that your feedback will be redundant- you may as well get rid of it completely for all the good it will do.

If you still want feedback, then you will have to remove it from the tail and inject it into one of the PI grids again, via a blocking cap. NOt very elegant, but hey.

[solderstain]

Good - more discussion.

...If you do that you will have feedback being injected only to the PI tail. This will subtract from one PI output, but add to the other...

This had already crossed my mind, but I thought it was still worth a try. I'm now having more second thoughts about this. The only way I would have tried to add this second channed is if I could do so without effecting the original Express sound and feel. I'm getting a strong feeling that it's actually not possible to do it without screwing with the Express side.

That other channel is a neat sound, but maybe it just doesn't belong in bed with an Express.