gain control for trainwreck express.

[phsyconoodler]

I'm wanting to add a dumble style gain control to an express circuit.
  I'm thinking it should work by simply replacing the 150k voltage divider before V2 pin 2(control grid) with a pot wired as a voltage divider.Normally there is a 150k resistor to ground where I've got the pot grafted in.

 Take a look at my paint diagram and any thoughts would be welcomed!

Edit: apparently the voltage divider and cap form a filter,so doing my pot would alter the frequency as well.Hmm...this is getting interesting.
 What about a gain control off the plate of V2?Before it goes to the PI?
Or maybe just a PPIMV.I don't really want that,I want a gain control.Wah!

[SILVERGUN]

Works great just like your thinking...I did it recently and was very hapy with the ability to dial the gain into that third stage,,,,in fact I liked it soo much that I added another cascaded stage and put another pot in that grid leak spot and am now able to dial in all the OD you could ever want....in fact as I increase the gain, the low end fills out very nicely............dial-up-distortion
My intention was to just never use an OD pedal with this amp......I won't have to.

I would just suggest that you put a low value (10K?) resistor in series before the pot so you wont be able to completely ground out the signal by turning the pot all the way down to 0........I would also go as high as a 500KL pot there if you want more gain out of the amp (mine is 1M, but starts to get messy up above 500K, and doesn't show much more of a range of adjustment above that point)

Thank you jojokeo, for that mod....it helped turn my ho-hum T-wreck into a monster

[phsyconoodler]

Ok,what exactly did you end up doing?Sounds like the ticket!!!I rambled quite a bit,so point me in the right direction.I owe you!!

[jojokeo]

SG initially went w/ the schem that had a low value grid leak resistor (56k?) going into V2a which loaded down the signal too much like on some Fenders. Just a way to minimize too much gain w/out using voltage dividers and extra parts like Leo always hated to do, lol. Upping to 150k - 500k greatly increases gain here. He uses a pot instead of a set resistor value like what you want to do so it can be dialed around. But then he changed the biasing of V2a into more of a conventional stage and followed up on my advice of using the unused stage V2b w/ a 33k or 39k (unbypassed, very cold bias and soft clipper) which "treats" the high signal nicely at that point.

[phsyconoodler]

So you parallel V2A into V2B or cascade it like V2 in a Marshall?
i'm not quite seeing it in my mind's eye.

[jojokeo]

V2a is cascaded into V2b w/ V2b being biased very cold into a soft clipping type of stage.

[SILVERGUN]

I still have the pots hanging out of the amp, and haven't finalized anything,,,,BUT my favorite settings were 800K going into V2a and then 180K going into the additional stage V2b

But, this yielded a TON of gain,,,,maybe not what some would want, and definitely not conventional

I wound up putting a 12DW7 in V1 position to calm things down, and bam, perfection,,,for me

Here's the whole picture: http://www.el34world.com/Forum/index.php?topic=14575.0

[phsyconoodler]

Very interesting! I have built trainwrecks before but never really liked the addition of a master volume,so I want to make this gain control work.
 The voltage divider I used was 150k before V2A and it has TONS of gain.But it will alter the frequency going into that stage by inserting a pot there as a voltage divider,which takes away bottom end.
  Maybe it is best if I just add that master pot before the PI?
I certainly don't need MORE gain by adding the extra triode!I just want to be able to dial some out when I want a less over the top sound.Turning the guitar down accomplishes exactly what's needed so I guess I should think about that area.A simple volume control at a strategic spot would do the job.I had my sights set on V2A input grid for that but the frequency change is not something I may like.
  Guees I'm going to have to breadboard this one and have some fun like you guys are!
Thanks for the responses! You guys are the best!

What about this:

[sluckey]

What about this:

That pot is really gonna wreck the grid bias on that LTP PI. I don't have a clue what it will do to the sound.

[SILVERGUN]

My favorite move to tame the front end gain was putting the 12DW7 in V1 because the dis-similar triodes worked perfectly there...
It left a 12AX7 in V1a but let me slip a 12AU7 in V1b without having to make any physical modifications, which made the preamp volume control much more useable and acted more like a gain control......you might want to try that in combination with the grid leak pot going into V2a for total control over gain.....I would put a 250K(or 500K) pot there just in case the 12AU7 drops the gain too much....
Also, what I've found is that cranking those grid leak pots really fills out the bass nicely.

I adjusted your layout pic to reflect what I think you were suggesting with the Master vol. (pre PI),,,,,,,,but IMO the method I suggested above would yield more desireable results in gain control, playability and feel of the amp

[phsyconoodler]

Thanks guys.Yeah I don't know what I was thinking about with the grid leak to one side of the PI.Duh!

 I bet a type 4 master might be a great way to do it after all.

[SILVERGUN]

That's the only spot I didn't try to put one, so I have no help for you.....but, good luck, I hope it works out for you...let us know

Don't be afraid to try your initial idea, a pot would definitely make a good gain control in place of that 150K grid leak resistor before V2a...in fact a simple way to try it without beating up your board would be to parallel (clip in) a 1M pot across that 150K resistor, and that would give you a decent range to try

[phsyconoodler]

I've been following your other thread and it sounds like you've created a real monster over there!
 I'm going to get one done up soon and try some mods on it.
The circuit is brilliant to work with.The purists will freak a bit,but in the end if it sounds good it is good.

And you know exactly what I'm talking about don't you? That feeling you get when you play in a band setting onstage with a killer rythym section behind you and the tone you are getting thast seems like it's flowing out of you fingers into the air....that kind of tone.It's an eerie feeling,better than sex.Well...that depends.

[jojokeo]

phsyco,
That last schem drawing is off. You have one vol/gain going into the other the way it is. I think you want it between V1b & V2a correct? This will help control your overall signal to that last stage before it hits the pi. In fact you may not need the original volume pot after the tone stack at all and rather have this new positioned one only. The signal load by the stack plus it's lower settings could act is a pseudo gain/vol control by itself? You then would have the same controls on the faceplate and the same basic circuit except the position relocation of the volume control in the circuit. Little changes like this can make a lot difference as you know. It'd be easy to build & try and you could always drill another hole if you feel the need to have the additional control later? Since you've built a few of these already, I'm really curious what your results would be with this single change?

[SILVERGUN]

That feeling you get when you play in a band setting onstage with a killer rythym section behind you and the tone you are getting thast seems like it's flowing out of you fingers into the air....that kind of tone.

Exactly,,,, that's what I'm trying to achieve...and I don't care how I get there.....if that's really what you want and your not married to the schematic,,,,re-read my earlier post and take it to heart, that 12DW7 in that V1 position is a winner for lead playing....it softens up the feel of the front end, just enough, and then add the gain back in with that grid leak pot before V2 (it's worth trying)

The circuit is brilliant to work with.The purists will freak a bit,but in the end if it sounds good it is good.

F the purists, they will be sitting home complaining about not having the right NOS tubes, or speakers, or OT, while we're out melting faces
Or, it'll be the guy in the back of the room with his jaw on the floor, saying, "my Express CLONE doesn't sound like THAT"

Everything I said in my thread is true (it really got me that excited), and those little changes that jojo is talking about make for BIG improvements.

HAVE FUN......find that feeling

[jojokeo]

HAVE FUN......find that feeling

I hope to be there soon enough with you guys. I've got trannies, a chassis or two to pick from but I've drawn up 5 schematics so far and getting closer...in the meanwhile I've put together a new JTM 45-like pedal called Purple Haze, similar to the Box of Rock using mosfets. Very tubular she is...but I want the character of both, wrecked in the marsh. Don't have to be exact to each either for the same exact reasons. If she performs, she is a winner and she will get paid. SG, I'll need to borrow your welding mask 

[phsyconoodler]

 

f...the purists, they will be sitting home complaining about not having the right NOS tubes, or speakers, or OT, while we're out melting faces icon_biggrin

 Yes I agree! LOL! Melting faces is soooo what I love doing! Lately I've been melting them with my bastard Super Reverb with 1-12 Speaker.It's a Silverface that I blackfaced and use with a 1-12 cabinet fitted with an Eminence Gov'nor.I use my strat and an Ibanez Jet Driver pedal. Killer blues tones.

 The missmatch from amp to speaker is huge but it's not wimpering....yet. 2 ohm amp and 8 ohm speaker.
Frickin' awesome tone.

 The master I drew is right from the trainwreck pages,type 4 master.I've used them on other amps and they seem to work just fine.I can wind up the preamp and the master does the job.
  Some guys look at it and say 'two volume controls' and it looks like that,but it works.

 I may add the v2 divider on the back panel

[jojokeo]

back to back vol pots on purpose?

[sluckey]

The master I drew is right from the trainwreck pages,type 4 master.

Yeah but the amp you put it in already has that MV! If you read the text description about the type 4 (several pages earlier) rather than just look at the pic, you'll see that type 4 was meant to be placed between the preamp and PI, more specifically between the treble wiper and the PI input on amps such as the 1987 Plexi. But your amp already has that type 4 between the treble pot and the PI. And you just added another one. Would you call that a type 8? Or a type 4x2? Or maybe a type 4²?

You can't do anything with those two MV that you couldn't already do with the one.

[phsyconoodler]

I've had this mental block before and I think I finally see why.The difference is the marshall uses the cathode follower tone stack and the wreck uses a plate driven stack similar to a Fender,but a bit different still.
  The type 4 can't work on that amp as drawn.Somewhere the master needs to hook directly to the PI and it can't in this instance.
So,a voltage divider pot(gain control) before v2 and a PPIMV is where I'm heading.

 I seem to love to have two of everything I guess!

[Tone Junkie]

I wanna build another trainwreck and play to ,Damn I got 2 amps in my chassis holders now I gotta finish.
So much to do so little time. (LOL).
 Keep us posted I just finished my blues version of trainwreck gotta get pictures up. used 12ay7,s in first 2 slots and paralled the first one with the unused half, if I would have been smart I would have put a 250k pot between them like on the first tweed overdrive special. Still melts ya the way it sits.
 Bill

[sluckey]

Silvergun's pic will work well if you want to put a volume pot just prior to the PI. You must use the additional cap. That will allow you to crank up the  preamp for distortion and cut it back just before it hits the PI.

Or, you could replace that 150KΩ that connects to V2-2 with a pot. Remove the 150KΩ and the blue wire that connects to V2-2. Connect the .0022µF cap to the top side of a pot. Connect the other side of the pot to ground. And connect the wiper of the pot to V2-2 (maybe use shielded cable). This gives you a volume pot for V2.

Maybe even use both pots for the ultimate control.

[phsyconoodler]

Thanks Sluckey! You are awesome!
 and you too Silvergun!

 

[jojokeo]

no me digas

[phsyconoodler]

JoJO! You know I love ya bro!  Well not that way!

[phsyconoodler]

Hey! Now I'm drawing up a 'wreck with just one tone control,gain control,VVR and single ended.One 6V6.

 Should blow those new little marshall 1 watt amps into next week!

That Marshall 5 I handwired into the 'wreck is fantastic.The dude I did it for uses it as his main home amp now.He has quite a few other amps to choose from.

[smackoj]

I hope you will share the one tube 'wreck' you are creating. I like the simpler one tube builds because of time, cost and experience level.

Jack      D

[jojokeo]

JoJO! You know I love ya bro!  Well not that way!

ha I'm feelin the love brother all the way past the border. Here's a Pete Townsend windmill power A chord back at ya!