Dual Channel Amp: Primer?

[Decko]

Hi,

I would like to build a two channel amp:

channel 1: Rocket
channel 2: Express

I will keep the express output stage.

I will want to use a footswitch on the floor or a switch on the faceplate.

Besides building two separate amps, what is the best way to combine the preamp stages?

Is there a primer out there to help direct me? I don't want to re-invent the wheel.

Thanks in advance.

Regards,
Danny

[Decko]

Hi All,

I found an interesting link that answered my question:

http://ax84.com/bbs/dm.php?thread=421012


Cheers
Decko

[Reeltarded]

Are you planning to switch circuits or use an extra tube?

These amps will be problematic when used as channels for several reasons. The Express distortion profile vs the Rocket's. you are going to end up with a psuedo-Marshall on the Express channel trying to bleed off volume to match the Rocket.

If you just want both sounds in the same box you can add a tube and some relays. Lots of relays.

[JoeCon]

What about building an Express and a Rocket and switch amps ala Eric Johnson?

A Lot less relays and problems...

[Decko]

Are you planning to switch circuits or use an extra tube?

These amps will be problematic when used as channels for several reasons. The Express distortion profile vs the Rocket's. you are going to end up with a psuedo-Marshall on the Express channel trying to bleed off volume to match the Rocket.

If you just want both sounds in the same box you can add a tube and some relays. Lots of relays.

I was planning to switch circuits. In short, I want a super clean channel and a super overdriven channel that I can toggle between using a footswitch.

Easier transition for the less skillfull.

[NickC]

As I understand it, the distortion in the Express is largely attributable to the output stage. Thus, I don't see how one could get Rocket AND Express tone/distortion characteristics from the same amp???

[Reeltarded]

Perfectly good example of what I was saying then.

You need a Rocket with that other triode as an added stage before the PI and maybe that switches in a simple diode circuit with a total of one added switch and a knob. All kinds of ways to skin an invisible cat.

So when you flip the switch it can add the mixed stage, turn on the trimmed diodes, defeat the trimmer on the clean volume, swap the bright.. and so on..

[M Fowler]

Might have to do like Dana Hall did build a Rocket and Liverpool (Pegasus) in one that way you can have four EL84 power section and keep it a true TW tone.

[Decko]

Hmmm. I see.

Maybe I can do something at the power section.

But then I might be talking about a whole new separate amp.

Over drive as you were saying is coming from the power tubes.

Would it be silly to add two more tubes and allocate those to the clean channel? Of course I would have to bump the power of the OT and the PT.

Separate volumes to each channel so that I can adjust the overdrive on each channel?

Sound crazy? Channel switching the power tubes?

Currently I use two 6sn7 GTBs in parallel for the power section in a push pull circuit. Each tube has two triodes. I could separate them out. It will change the impedance at the speaker but we can adjust for that.

Just thinking out loud...

Decko

[M Fowler]

You build the tone board with both Rocket circuit and Express circuit without the phase inverter section. Put the phase inverter on it's own board using Express values and power section of the Express. Uses 300-0-300 300mA Express PT and Express 6k6 OT. Put the treble cut pot of the Rocket at the end near the MV pot.

Use a switching circuit to go from Rocket circuit to Express circuit.

[Decko]

You build the tone board with both Rocket circuit and Express circuit without the phase inverter section. Put the phase inverter on it's own board using Express values and power section of the Express.

For the combined preamp board: both Rocket and Express would share portions of the same circuit? Or would it be better to have two completely separate board and then mount them one above the other? Each board would have it own tone stack. They would have to share B+ voltages and preamp tubes. Please bear with my inexperience with dual channel architecture.

The phase inverter is straight forward.

Uses 300-0-300 300mA Express PT and Express 6k6 OT. Put the treble cut pot of the Rocket at the end near the MV pot. /quote]

Why would we need a master volume?

I am wondering how to create a good balance of volume between rocket channel and express channel. But less gain post-phase inverter will affect the overdriving of the power tubes.

I want to be able to set the express channel full tilt then click over to the rocket channel being cleaner or more of a rhythm tone.

So we are talking a post phase inverter MV?

[quote="M Fowler"Use a switching circuit to go from Rocket circuit to Express circuit.

I see.

Thanks for your input. I maybe over thinking this but fun nonetheless!

Regards,
Decko

[Decko]

Might have to do like Dana Hall did build a Rocket and Liverpool (Pegasus) in one that way you can have four EL84 power section and keep it a true TW tone.

I would love to see the schematic. Is it posted?

Decko

[Decko]

Perfectly good example of what I was saying then.

You need a Rocket with that other triode as an added stage before the PI and maybe that switches in a simple diode circuit with a total of one added switch and a knob. All kinds of ways to skin an invisible cat.

So when you flip the switch it can add the mixed stage, turn on the trimmed diodes, defeat the trimmer on the clean volume, swap the bright.. and so on..

Draw me a schematic

[Reeltarded]

I'll do better than that. I'll finish building it and then I'll show ya!

[UR12]

Just My two cents since I have been there and done that. The amp you want to build is not as easy as it seems. The Rocket and the Express have two preamps that when the signal hits the PI is 180 degrees out of phase. So combining them before the preamp would require a mixer stage or at least another inverter stage before the PI. Or you could insert each pream on oposite grids of the PI like a 18watt marshall. Only problem there is that the Express uses a Presence control whis is fed off ta negative feedback circuit. Problem here is that if you set your switching up so that only one preamp at a time is supplying the PI when you switch to the Rocket channel you will now have positive feedback which wouldn'd be good. The Rocket also has a Hi Cut control after the PI. You could leave this in for both channels but I didn'tersonally care for the HI cut on an Express Pream. There are other problems too like different resistor values on the PI tail for both the rcket and the Express so you will need to either switch those or settle for a happy median.

Here is a link to my project from a few years ago

http://ampgarage.com/forum/viewtopic.ph ... ht=pegasus