Power Scaled Express build complete

[Matt T.]

Here's a few really poor quality pix of my latest homebrew. PSX is supposed to stand for Power Scaled Xpress but I've changed it enough such that it's 'Express-based' at best. I'll make some clips soon...it's a mean mofo.

The power scaling seems to work like it's supposed to but this amp really hits the PI hard and it seems that the PI and preamp are responsible for much of the distortion/tone...I can get a good tone just by using the post-PI MV. But when you add the power tube distortion the feel changes as much as anything...chewier. The amp is very quiet...especially considering the amount of gain it has (about the same as a goosed non-MV Marshall).

I used the following to make this amp:
Ceriatone Express kit (sans transformers)
Heyboer Transformers
London Power Power Scaling kit
London Power Bias Mod kit
Stone Age Custom Cabinets
Amplates Custom Faceplates
Scumback 30W H75s in the over-sized 2x12
I made the power supply, bias supply, and preamp filter cap board myself (G10 with eyelets) and used the Ceriatone preamp board (G10 with turrets)

The chassis shots were before I got the faceplates (obv).

[LarryLarry]

That looks awesome! Please post some clips! What is the wattage on this do you think?

[Matt T.]

Thanks Larry. It's supposed to be in the 35W ballpark...with the power scaling at full voltage the plate voltage is only 395vdc and the OT primary Z is 5K1...allegedly just like the originals (I think some used 6K6 primary Z but not sure).

The preamp is a bit more like a Komet but still different.

[LarryLarry]

Can you make a comparsion to a 50 watt MV Marshall JCM800 volume and gain wise?

[Matt T.]

Do you mean a JCM with 390 plate volts or one with 490 plate volts?

I'd say it's close in both volume and gain...maybe a little less volume and low end...similar amounts of gain.

[LarryLarry]

My '85 4010 running EL34s is about 440v on the plates.

[Froumy]

Nice Job! Your amp looks great. I've read that you had some issues, and that you picked up an Osciliscope(I think I may need one myself....) `I'm curious as to how close it is to a Trainwreck(Minus the MV). I'm torn between building a plexi or a Trainwreck for my next project. I'm DYING to hear a clip.... If you search YOUTUBE you'll find a couple of great Trainwreck clips.
http://www.youtube.com/results?searc...&search=Search

How close did you wind up to the original? I'd probably incorporate a MV for real world application as well, so I've watched your posts closely. What was your final solution? Nice Amp. I'd love to hear some clips.......

[Matt T.]

Froumy,
Other than power scaling, which includes a post-PI MV, the mods are pretty basic. It's an 'Express' circuit except for the following changes:

Changed the cathode on the first gain stage to 2K7/.68uF
Changed the cathode on the second gain stage to 4K7/.68uF
Changed the 150K resistor to 270K
Added a 270K resistor to ground after the .1uF coupling cap after the third stage, then added a series .022uF cap after that (this .022uF cap is your 'new' PI input cap)
Changed the 10K PI tail resistor to 22K
Changed the 91K PI plate resistor to 82K

My post-PI MV is not like the one called out in the Power Scaling instructions...I used dual 1M pot and 2 each .68uF caps...each pot of the MV is connected after the coupling caps with the .68uF caps on the wipers (which then connect to the junction of the 1K5 grid stoppers and 220K bias splitters)...ground lugs to ground. The taper is great.

I also yanked out the Presence control and put a 4K7 resistor in it's place...then installed a dual-pot post-PI tone control...it's just a dual 1M pot wired as two separate variable resistors to ground with .0022uF caps in series. You can see from the gut shot photo that the two .0022uF caps just tap off of the dual-pot MV.

That's it. The preamp mods are sort of in the same direction as a Komet...so you might try that too. Oh yea...none of those preamp mods are in the photo above...only the MV and post-PI tone control had been added at that time.

[bob p]

yes, its a real beauty. an express is on my "to do" list. i'd like to hear some clips when you have chance to work on them.

btw, who makes those cool orange 'lytic caps?

[Froumy]

Thanks for posting the mods. It will give me some options with when I get around to making one. (hopefully within a month.) The only Trainwreck I've ever heard are those clips on youtube, and I love it. I'm guessing that you have a little more gain. How does it compare? Looks like there are a few of us interested in hearing some clips. If you get around to it, great! If you don't, I'll at least have some mods to play with. Thanks again. That's a fine looking amp.

[Matt T.]

bob,
Those are Ceriatone caps (from the kit)...I assume they're just IC or something with a Ceriatone sleeve. One thing to note is that there are three 22uF caps, one 33uF cap, and two 47uF caps but they are all the exact same size...and both of the 47uF caps were quite low...closer to 40uF, the 33uF was only 30uF...but all three 22uF were slightly high (23.5uF to 25uF). The layout calls for 47uF//33uF (80uF net) for the plates and 47uF for the screens but I did it 47uF//47uF on the plates and the 33uF for the screens (which netted 82uF on the plates and 30uF on the screens).

Froumy,
I don't really think I've got more gain...but it seems structured better. It seemed that the Express circuit hit the PI too hard. I changed the structure a little such that the third stage distorts right after the PI and the second stage distorts just after that. The second stage doesn't distort that much though.