Multi-preamp 5W SE amp

[OrganicEffects]

Hello,

This started out as a Champ with some bells and whistles, but then I came across Sluckey's Dual-Lite and I had to have it. Rather than make two amps, I wanted to try and roll the whole thing into the Champ build. The Champ controls are much more robust than the Vox or the Marshall type preamps, which is fine. I believe the circuit will pull some 50mA from HV taps, and the PT can give 70, so that should be ok.

I added in 220k summing resistors as that's what I've seen on AB763 schematics. Will this effect the gain of the preamp hitting the 6V6? Looks like a 220k / 220k voltage divider, so I guess it would cut the voltage gain down by half? Is there a way around this?

I've been killing trees trying to figure out a switching schem that allows you to put any two preamps in series, as there's a thread on the Dual Lite where Sluckey says the marshall into the vox circuits doesn't sound bad. I can't seem to come up with a way that doesn't involve a half dozen+ switches and gets convoluted quickly, so I was thinking.. can it be modular?

Keep the schematic how it is, but rather than input switching, make each preamp on a jack. Then add switching jacks just before the summing 220k resistors to allow for a 1/4" cable to feed the output of one preamp back into the front of the other. This might be asking for hum problems.

Thanks for your comments.

[shooter]

Quick notes, since my brain is in outta work mode 

1.   your “3rd” pre will be outta phase with the other 2 so I wouldn’t mix that
It also has nfb, not sure if that would be a problem

2.   I would give up on “in series”, parallel, maybe. 

Can't recall the Amp, but it has 3 channels, called out by color, blue is one.  that would be a good start for how too, although if I recall it might be a complex amp

[shooter]

Over the winter I did a 4ch pre on the BB, tweaked some, made lots of scrap paper, and settled on the best of the 4.  here's the schematics for reference, they are probably not "build approved".  the architecture is basically plexi-ish from in to TS out.

[sluckey]

... as there's a thread on the Dual Lite where Sluckey says the marshall into the vox circuits doesn't sound bad.

Can you post a link to that thread? I don't recall ever saying that. And I have never cascaded the channels on my Dual Lite.

[OrganicEffects]

... as there's a thread on the Dual Lite where Sluckey says the marshall into the vox circuits doesn't sound bad.

Can you post a link to that thread? I don't recall ever saying that. And I have never cascaded the channels on my Dual Lite.

Ok

http://el34world.com/Forum/index.php?topic=18330.0

Just for you Franco...

I played around this morning with cascading the preamps. I first connected the EF86 preamp output into the 12AX7 preamp. Didn't care for that.

Then I connected the 12AX7 output into the EF86. Much better. Plenty of nice distortion, but trying to roll back for a nice clean or slightly overdriven sound was not easy. Really didn't sound good to me with anything except full out distortion. Think American Woman.

I didn't try to optimize any of the gain other than twisting knobs. The 12AX7 into the EF86 has possibilities for anyone that wants to experiment. I'm gonna just stay with my original circuit. I already have a few buzz saw pedals and I can always turn my little PeeWee into a Herzog!   

[OrganicEffects]

Thanks for the schematics Shooter! I’ll have a look at them and also try to find the colored preamp amp you mention!

[sluckey]

Thanks for the reminder. Seems my memory is fading. Wonder what else is gone?   

[shooter]

Wonder what else is gone?

It's Achieved in the ppl you've helped along the way

[Bantam]

Maybe Custom audio Electronics, or damnnit I just had it and forgot.... think guy from pearl jam played it...I'll go search n git back.   one I can't remember particularly had colored channel s

[OrganicEffects]

1.   your “3rd” pre will be outta phase with the other 2 so I wouldn’t mix that
It also has nfb, not sure if that would be a problem

2.   I would give up on “in series”, parallel, maybe. 

Can't recall the Amp, but it has 3 channels, called out by color, blue is one.  that would be a good start for how too, although if I recall it might be a complex amp

Perhaps I should put the selector switch at the output of the coupling caps of each preamp rather than mixing them with resistors?

Any ideas on how divvy up the guitar input to each preamp without another switch? I could do a 3 position rotary switch to switch the in’s and out’s of each preamp, which would also disconnect any of the NFB coming through, but I’m worried about putting the inputs and outputs so close together on the same switch. even with shielded cable, they still get pretty darn close.

I could do relays, but I don’t know any relays with three throws. Could use two SPDT on the input and another on the output controlled via a micro controller...

[Bantam]

Was the 3 channel pre with blue (along with green or yellow and probably red for high gain out) a Demeter ...Th3 or sumthin like that...nice sounding pre amp, so was CAE. Dave Mason played CAE when I went out with him years ago, through Crate 2x12 V50s w el34s, into the fx return.

[OrganicEffects]

Does the Demeter TGA-100 sound right? Schematic and elusive

[shooter]

Not sure, I'm a schematic guy, but NOT a musician.  Did find the Mesaboogie Nomad 45 is a 3 channel.

Personally I would mix, easier, proven, more "possibilities".
I would mix your top 2 ch and leave your 3rd as a single because of the reasons I mentioned above.

[OrganicEffects]

Not sure, I'm a schematic guy, but NOT a musician.  Did find the Mesaboogie Nomad 45 is a 3 channel.

Personally I would mix, easier, proven, more "possibilities".
I would mix your top 2 ch and leave your 3rd as a single because of the reasons I mentioned above.

Do you mean leave it as a single as in leave it out of the amp, or put it on a switch?

Found the Nomad 45 you're speaking of, the schematic makes it look like there's two input jacks for the two preamps, with the third preamp being a gain pot switch and a couple other things within the second channel. It appears there's another switch at the end of each preamp to switch which is going into the PI.

http://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_nomad45.pdf

I suppose what I could also do is have an input for each channel with a switch on the ring contact that sends a signal to a PICAXE, telling it to switching on the preamp corresponding to the plug, sending it to the 6V6.

[sluckey]

There is no problem mixing those three channels together ***IF*** you will only be using one channel at a time. But you will have phase cancellation issues if you try to run your guitar through the bottom channel and either of the top two channels simultaneously. Your schematic shows that only one channel can be selected at a time, so no issues. Using three 220K mixing resistors as you show is fine.

[sluckey]

One issue... As shown, when you select a channel with the input switch, the grids of the other tweo triodes are left floating. This creates an unknown bias state for those two tubes. The fix is to use three 1M resistors from grid to ground for all three tubes. Then remove the 1M from the switch.

[OrganicEffects]

Alright, here's a schematic and two wiring diagram. I don't know how you guys make wiring look so damn good. I couldn't even do all of the wiring in one sheet without looking like rainbow spaghetti. I suppose I could make the enclosure deeper and make a 'row' of the first preamp tubes and a second row with the power amp and transistors. 

The grounding scheme is a bastardization of a JTM45 I made and the layout from the Dual Lite. And some guesses. I wasn't sure where the load resistor and associated circuity should be grounded, so I took it to the power amp section ground. Also added an extra power filtering per a recommendation on another Champ thread by PRR.

The input switching may be a little convoluted, but it should ground the two preamp inputs that aren't being used. (EDIT: realize I drew up the wiring on this switch wrong, will edit the layout).

 The cap switching on the vox preamp is out of the Dual Lite, but I added the 5M1's in the schematic that aren't present in the layout.

I used shielded wires for the three inputs, the return of the volume after the tonestack, and the wires running to the preamp out/power amp in jacks as they run close to the heaters and output wires, and I thought that might be an issue.

Planning on using the Classictone transformers
www.classictone.net/40-18031.pdf
www.classictone.net/40-18019.pdf

Doing layouts and wiring is a whole different beast from building an amp. I suppose using 3 preamps made things more difficult than it had to be.

[sluckey]

The cap switching on the vox preamp is out of the Dual Lite, but I added the 5M1's in the schematic that aren't present in the layout.

If you use this switch you won't need those 5.1M resistors...

     https://www.tubesandmore.com/products/switch-rotary-2-poles-6-positions-shaft-mbb

[OrganicEffects]

The cap switching on the vox preamp is out of the Dual Lite, but I added the 5M1's in the schematic that aren't present in the layout.

If you use this switch you won't need those 5.1M resistors...

     https://www.tubesandmore.com/products/switch-rotary-2-poles-6-positions-shaft-mbb

Lovely, thanks!