problemes while building an Express from a Jet Ciy JCA20H

[Jmc12]

HI,
A friend of mine gave me a jet city JCA20H Head.
I Wanted to build a light version of the Express, cos I don't need 50 Watters anymore, I got plenty of them.
The jet city with 2xEL84 is easy to modify.
So I used the PCB, got rid of the unwanted components, and made a Francesca circuit with it. I kept the final Power amp witch is similar to the Trainwreck. I just rewired the presence, And used 0.1 ùF before the PI.
It's working fine, but it's overdriving very Quick. I have read that with 100mV sinus signal on input, with the volume at noon, I should have a clean PI, and a square signal on the output. My PI is breaking up at 9 o'clock on the volume...

on the scope with volume at noon :
100mv clean sinus on input
4 v clean sinus out of the second stage
28 v clean sinus out of the third stage, and input of the PI.
100 v out of the inverter output, already square on the top
80 V on the non inverter output, square both sides.
and 30 V square on the 8 ohm output.
my voltage chart is different than the forum's one.
for B3=304 V, I have 210 v on the anode pin1 and 200 v on anode pin6
20 v and 22 v on the grids, and 34 v on the cathodes.

I like the sound, but I should be able to clean up with the guitar pot, but passed 2 on the Lespaul it's getting dirty
Maybe somebody has an idea....
I checked everything 20 times.....
Thank you all
JM

[martin manning]

EL84's require less signal level to over-drive as compared to EL34. I suggest you keep the preamp the same as in the Express, and install a KF Type 2 MV to attenuate the signal coming out of the PI. With that you should be able to find a setting that produces the guitar volume knob effect you are looking for.

[ChopSauce]

On a related matter - for that seems to be a common issue for tinkerers like me, who copy/paste portions of circuits to make their own "sauce" :

- is there any source of information that would you recommend to learn some more about the theoretical approach, which would help with the default "trial & error" tuning method...

[Jmc12]

I tried to add the KF mod with the 820 ohms grid resistor before the 2nd stage. It's helping a little.
I Don't want to install a MV, cos it's not the spirit of the Express. I 'm gonna try a PI with less gain, like a 12AT7 or a 12AY7.
Anyways, the amp is working fine, and it's doing what we expect from an express, dynamic with the right hand, and guitar volume sensibility.
Now that I'm ok with the result of the project, I'm gonna get rid of the jet city PCB and replace the whole thing with a turret board with a more logical layout.

[Jmc12]

Talking about tinkerers.., Ken Bran, who was just a tech for Jim Marshall, Just Copied and Pasted the leo fender bassman, He just changed a few things in there and it became the JTM45... We don't need to redo what is already working. Ken Fisher and Howard Dumble were skilled technicians too, and they just made their own sauce based on their knowledge sure, but inspired by Fender and Marshall, and a lot of tries, and the most important, big ears.

[martin manning]

I Don't want to install a MV, cos it's not the spirit of the Express.

The essence of the Express is slamming the PI with a large signal and overdriving the power stage. I'm suggesting that you maintain the original circuit up to the PI, and then attenuate the signal with a PPIMV so that it over-drives the EL84's by an amount that is comparable to the situation with EL34's. That should get you as close to the spirit of the Express as possible.

[Reeltarded]

STOP!!

R7 on the Liverpool scheme/layout... that resistor. Lower value smaller (and still really large) signal down to about 47k so do that with a pot or a trimmer until you find the line then swap in nearest larger resistor value.. tahdah.

[Jmc12]

Do you mean the 150 k grid leak resistor on the third stage ? I'm talking about the Express schematics.

[wpaulvogel]

Do you mean the 150 k grid leak resistor on the third stage ? I'm talking about the Express schematics.

Yes, and increase the 2nF coupling cap to maintain the frequency response of the formed high pass filter 2nF/150k. Use 10nF/47k. The filter will allow a little more low frequency or use 4.7nF/47k but it will thin bass a little.

[bepone]

too much gain for easy drive requirements of EL84..volume after PI should be reduced 10x, otherwise poor overdriven EL84 will return in very raspy , unpleasant sound..... ECC82 can be nice shot. or leave all how it is, just form attenuator before PI, add resistor R9a=10k, 100k in series with PI also has function of reducing blocking distortion, 2 problems fixed in one shot

vo.png

[Jmc12]

Thank you guys for your possible solutions.
I did not try everything.
The 12AU7 in place of the PI is not good, we lose the spirit of the Trainwreck, the sound is not enough overdriven, even with the volume on 10.
I tryed the divider before the PI, and ended up with a 68K and a 39 K. I begin to clip at 9 oclock, and got a full overdriven sound at 14 oclock.
This guy is really nasty, a bad boy, love it, on my 1970 100w 4x12 Marshall, I have a sound in the spirit of Mr Eddie on the first Van Halen.
So here are some Pix of the CITYWRECK....