Express w 4gains

[surfsup]

Okay I've done a hand sketch to lay out the circuitboard and pots/switches for this build:

http://img707.imageshack.us/img707/8835/dbm39v11.gif

My question is, I have it currently set up with V1 using only one triode at the input, all the other stages are in the other tubes. I am thinking about not wasting the triode and doing a parallel triode tube for gain stage one. I assume:

1) I have to use a reduced resistor (1/2 value?) for the plate and cathode resistors. With two triodes drawing current instead of one, to keep the same voltage I need half the resistance?

2) Half the value of the gridstopper? Thinking about RC filters, the input RC filter is based on the tubes "miller effect" so two triodes is double the capacitance so I need half the R to get the same freq response?

3) I assume the Ck can stay the same? or does it have to change because I now have two plates offering more resistance (12ax7 ra=33k so now 66k) but half the Ra/Rk value (here 100k+2.7k at 1/2=50k), boing, my head exploded just then. so originally it was 33k+100k now its 50+66 so its nearly identical?

[M Fowler]

I just did this on a guy's amp I moved the rest of the board wiring over to the second preamp tube. Wired the first tube as parallel no additional components needed. See Matchless Spitefire or Lightning.

[surfsup]

Mark, do I need to change the cap value for the cathode bypass? What about the resistors? The Matchless Spitfire has a 220k/1.5k with a 25uF. The 4G express has a 100k/2k7 and 0.68uF. I think I'm going to go with my gut and halve the R values at least.

In case anyone wants to build the Express with 4gain stages, here is my layout I just did on a 10" board. I can't get it any more efficient than this. The 100k plate load on V3 is between the pins on the tube, hence just the B+ turret on the board:

[img691]http://chicagocadcam.com/ChrisHahn/Expr ... ress4G.jpg[/img]

[M Fowler]

I didn't change any values.

You did a good job on that board layout everything was good to me.

I added the jumper in red that was not filled in yet.

I did add a .001 cap with 33k grid resistor on the input recently. I don't think I actually needed it, but wanted less treble.

[M Fowler]

I don't have a cliff jack shape in my Visio so had to use Switchcraft to show the interrupt serial effects loop wiring. Easier with cliff jacks.

[Cliff Schecht]

In the Express the tonestack is driven directly from the V1 plate. Paralleling the triodes would decrease the effective plate resistance which I would think would change the response of the tonestack. Any thoughts on this guys?

[surfsup]

Is the iron for a Rocket and Express the same iron?

You did a good job on that board layout everything was good to me.

Thanks, means a lot coming from you. You know your sh-t! I think I've got the schemo tracing down. Main issue for me was tonestacks and I now have a rule of thumb, "signal to 3" "signal to 3" "signal to 3"...

to show the interrupt serial effects loop wiring.

I will study up on a serial loop tonight. Looks like you just put two jacks in series between the wiper and 100k. That's crazy simple so I wonder how this affects stuff.

I added the jumper in red that was not filled in yet.

whoops I had that dashed line on my pen/paper drawing. forgot to add it.

I did add a .001 cap with 33k grid resistor on the input recently. I don't think I actually needed it, but wanted less treble.

I assume you threw the cap on the input jack with the 1M. I will take your advice. I was thinking of changing the Ck value to shift the cutoff frequency a tad lower. Seems like the design is really cutting bass.

In the Express the tonestack is driven directly from the V1 plate

Cliff I'm still clueless but thanks because I just learned something that I didn't think about. However, this design with 4 gain stages has the tonestack off V3 not V1.

[img:632:568]http://chicagocadcam.com/ChrisHahn/Expr ... SoffV3.jpg[/img]

[tweedeluxe]

i've done an express with parallel triodes in the 1st stage. use 47kΩ for Ra and 750/820Ω for Rk.

This keeps same gain as standard express input, but drops output impedance closer to 20k. Change tone stack values to 47k slope, 500k treb/bass, .001/.047/.047 caps, 10k mid pot, and the tone stack will have the same response as standard Express.

Keep in mind that miller capacitance doubles so there might be some high-end rollof when using volume control on certain guitars which isn't the case with a standard express input (no grid stopper, tone stack loading down the 1st stage, reducing AC gain/miller capacitance).

the sound is a little hairier and "bigger" with parallel inputs, but clean to mean is worse.

[Zippy]

Cliff I'm still clueless but thanks because I just learned something that I didn't think about. However, this design with 4 gain stages has the tonestack off V3 not V1.

You've just left Express/Liverpool territory and entered Rocket Land...

[M Fowler]

Well yeah this is Marshall Territory now not Express or Liverpool.

[Zippy]

Hah, I was thinking about that when I wrote my previous post.

Thanks for setting that straight, Mark.

[surfsup]

Well its too late (maybe) to go back to 3 gain stage. Well I probably could but thinking I will press on because the eyelet board is built. Whipped this out in the last 30 mins. I'll decide later if I wanna parallel the stage though I'd appreciate a bit more explanation regarding "clean to mean is worse" (meaning there's less clean?):

[img1552]http://chicagocadcam.com/ChrisHahn/Express4G/board.jpg[/img]

[M Fowler]

Just a warning which you should have known already, the four gain stage amp is a shredder amp which works well for me because I always use two amps one clean with a little gain on tap and my second head has always been the higher gain amp but this one is killer for what I want in a high gain amp.

If you want one amp to do everything you need to have a low input jack for the 3 gain and high input jack for the higher gain circuit or foot switchable between them.

Hope this answers some questions.

Mark

[Zippy]

Well its too late (maybe) to go back to 3 gain

Not at all, Chris, but maybe you should pause long enough to decide whether you want an Express, Rocket w/octal output, or Marshall.

What sounds are you looking for from this project?

[M Fowler]

If you want to go three gain stage I could use that eyelet board let me know. It won't go to waste.

Mark