Possible to build 2203 preamp only type thing?

[Eric Klinger]

Greeting Everyone.

It's been a while since I have posted mainly because after building several amps, I got into some different types (Fender Bassman etc)...

I currently have a 2203 half built in my workshop... been sitting there for 6 years, undone (needs trannys and tubes). Lately I have had a big urge to somehow separate the preamp and power amp sections and sort of box up just the preamp and pre-tubes to run into power amps or Effects return of other amps.

Is this possible in anyones experience or knowledge??

Thanks much.
Eric

[RockinRocket]

You could make a JCM preamp for sure. The tricky part is finding a transformer and getting the volts right.
I don't really know but am assuming using a JCM 50 watt transformer isn't the right way to go.

[Eric Klinger]

Indeed... the most confusing part - the power supply... I assume some stripped down version of the power section still needs to be present.

I have built several amps, but my "knowledge" in the subject is more on the safety than anything else.

[Krinkle]

It is possible, I've done it with a 1959/plexi preamp (it's torn apart now). Here's what I found, sorry it might get long but I think this is what you are looking for;

It originally started as a copy of a CAE SE 3+, then I just broke down and bought one (and later sold it, go figure). So I was left with an enclosure with a toroidal transformer some filter caps and a front and rear panel with pots, jacks, etc. I decided to make a 1959 preamp using a turret board and eyelets like my Metroamp build. I have a VHT 2502 power amp so the plan was to run the preamp into that and then make every preamp version of Marshall, JTM 45, etc, plus other brands, you know kitchen sink type stuff.

I built the preamp up and played around with the power supply voltage dropping resistors to try to get close to what I had in my MA. I think I got fairly close from what I remember but it was a lot of screwing around with resistor values. I ran it into my VHT, I can't remember what I used after the treble pot output to reduce the signal voltage down to line level, maybe a pot, but it'll make sense once I explain where I ended up. It did not sound good, I would use the very nontechnical term "lame". It was lacking distortion, no sizzle, very meh. So I thought that I must have made a mistake.

I examined my solder joints, component values (only to find that I had the 0.022 and 0.002 caps interchanged on my MA build, so I stopped to fix that!). In the end I realized that the problem was that the preamp signal voltage hitting the phase inverter in the 1959, my MA, was a LARGE part of the sound of a plexi. So I got a Zero Loss FX loop, connected it to the treble output of the preamp build, ran it into the return on my MA, and there it was, pretty much identical to my MA alone. I'd read all the opinions about power amp distortion and I think that a lot of what people are describing there is the huge signal, 90VAC or more, slamming the input of the phase inverter.

So then I thought, well I'd still like to be able to run the homemade preamp into my VHT, but how do I get the phase inverter distortion added in there. I'd always wondered why designers had added gain stages to their Marshall style preamps (unsuccessfully matching that clear 1959 plexi tone in my opinion) and now I knew why, they all built them and needed more gain because the PI distortion was missing.

So for a few seconds I thought I was a genius (jk) and that I had figured out some holy grail type of thing. Sidebar - Along the way I'd been obsessing with early Billy Gibbons tone, as usual, that's why I got the MA, and I'd read that he used a Legend Super Lead 100 for Eliminator (not early one, I know but I was still interested, maybe it held some clues to the early tone if they had made mods or something). So I found a schematic online and took a look at it to see what the fuss was all about.

The SL100 is a tube preamp into a SS power amp. So I took a look at the schematic and noticed a very neat circuit, albeit one that took away my fame and fortune. They had designed a standalone 12AX7 PI stage, with feedback from the inverted output. In other words, the 12AX7 looked like a PI, the noninverted signal was the output, but the inverted input was fed back around to the input and acted like the feedback in a Marshall. Very clever.

I took that portion of the PI circuit fron the SL100 and adapted it to my preamp but I kind of stopped there. I built it up and it didn't sound exactly like my MA, so I wasn't sure if the VHT just wasn't a good match for 1959 preamp, that it just didn't sound like a Marshall output/power amp. I was still using the ZL FX Loop in the preamp so that meant I was double distorting the PI, first in the pseudo PI in the preamp, then blasting the PI in the MA through the Return of the ZL FX loop.

So the next step would be to build a Marshall 1959 standalone power amp, as I wasn't willing to disconnect the Loop in my MA and go external preamp straight into my MA power amp, and I left it at that.

[Eric Klinger]

Thank you for the info, and apologies I didn't reply sooner... I have been relentlessly jamming;.. go figure

Certainly an interesting story... I read it four times to absorb it... as I said I am not a circuit wiz, but I know enough to remain alive after have built up four or so amps...

I am still quite interested to move on with it somehow and will seek out this SL100 and see if I can't check out that PI section...

This was my worry - the way the PI interacts with the circuit - however I did intend to use it with amp effect returns, though at the moment I only have a Fender/EVH 5150III (50W) and a 40W Hot Rod Deluxe...