Trainwreck Pre and Marshall Power Amp

[zendragon63]

Hello to all. Been lurking around here for several months absorbing ideas and advice from you tech savvy folks—I for one really appreciate that you guys readily share your R&D and inspire rest of us to build great amps or make the doggy sounding ones usable. And I just love the Trainwreck sound. I hope that my first post here can be considered a contribution even though it is 95% complete; I think I may need a little help with the last 5%. I have had a mediocre sounding Marshall 2204 on the shelf for eons—I cleaned up the wiring, replaced the PCB with a PTP board from Metro and wired it as a Wreck preamp into the Marshall power amp. I also wired V2 as parallel (why not use a perfectly good triode section) l but did not change any of the component values. There have a few posts that refer to trying this but have not found any with results. The tubes are Mullard AX7LP->Mullard AX7->GTAT7->W-C EL34. Attached is the obligatory gut shot.

I was surprised that I did not have any start-up oscillation and man, oh man did this amp sound different. Rich in many harmonics that the Marshall never had and even though it will allow you to feedback nearly anyplace on the neck at surprisingly lower volumes, this thing gets way LOUD. I opted not to wire in the PPIMV for the time being. The chassis is noticeably more microphonic at higher gain settings though I do not have to run above 3 to get a huge, full bodied sound. My question is: because I ran V2 in parallel, the plate internal resistance effectively half what a single section is so would this change the .002 uf/150K tuning?

I really appreciate anything you guys can offer. I had a blast creatively assembling this thing. Again comments and feedback (even order harmonics only!) welcome.

[Darkbluemurder]

That is cool! I was looking to do something similar to my JTM 45 RI once I get rid of the buzz in this amp.

I like the Metro boards. I assembled a master volume board for my JMP 1959 whose PCB was ripped due to my early experiments. As a result the amp became somewhat flaky. The Metro board cured that.

Did you use Mallory as signal caps? I used Sozo but I probably would use Mallory for a Wreck to tame highs.

Do you have a schematic?

[zendragon63]

Thanks for the feedback darkbluemurder! I know what you mean about the PCB--it doesn't take long to trash one of those. Sounds like you experiment quite a bit. Attached is a schematic I drew up when I finished it just so I could remember what I did--a little crude by modern standards but what the heck it show the values and such. The measured voltages were added in case there is any interest. Used mostly the Mallory and one Sozo--what I had available. I understand that the orange drop are bit brighter than either of the others and did not have any to compare against but the sound is not noticably shrill or piercing. Still curious as to how to correctly calculate the loading/gain for the parallel V2 so any help or insight would be very helpful. Then maybe onward to tackle a 18 watt Liverpool clone! Thanks again.

[Darkbluemurder]

If you want the parallel triode to have the same (or similar) characteristics as the original Express schem, try changing the 100k plate resistor to 47k and the 10k cathode resistor to 5k. Of course you could just add a 100k / 10k resistor to the existing ones. You just need to halve the values when putting the triode sections in parallel.

Yes I did quite experiment a bit but less these days since I have less time. I did a lot of experiments with medium to high gain preamps by comparing various schematics and trying them out in the 100 W Marshall. I did also experiment with my Fenders a lot - tried Dumble circuits in them.

[zendragon63]

Got it. I can draw an loose assumption that it should also lower the gain on V2 a scosh; could be a good thing. I will post a sound clip when this is all fininshed. Again, I appreciate the input.