Solid State Guitar Amp Forum | DIY Guitar Amplifiers

For years I had not been entirely satisfied with my tone and have done various mods, all of which led to nice improvements.

Finally, some fourteen years after getting that MP-1, I attacked the dreaded tube board. So far, I've been zapped twice.

I did a full filter cap job (tube board, main board).  I tweaked the cathode bypass on V1A from 33 uF to 0.68 uF (tantalum).   V1B from 33 uF to 10 uF.  Replaced all other electrolytics on the tube board. At that point, I was no longer hearing differences nor expecting to.

And then ... I replaced the four forty-year-old 1N4007 recitifier diodes on the tube board with UF4007 ultra fast recovery ones. I was not expecting anything; maybe to hear a difference in the level or profile of noise when gains are cranked. But, holy mother of god, that change was the money! The magic mojo suddenly descended into the preamp and declared residence. The diffuse fuzziness causing poor note definition toward the bass end was suddenly gone.

I have a mod whereby I can switch the OD1 drive circuit's filter between 1.6 kHz, 2.2 kHz (stock) and 3.4 kHz.   I mostly have been using 3.4 kHz. 1.6 kHz was way too tubby/flubby at high gains, but good for certain darker blues or classic rock type tones. After the diode replacement, I'm using 1.6 kHz all the time and enjoying it, instead of going "meh" after a minute. It is no longer flubby. It has the plump bass and rounded off top end, but the chugs have a clarity: like froth on top of a stout ale. Notes are clear. When I'm doing neoclassical, it faintly reminds me of old Symphony X, say.

Man, oh, man. Damned recto diodes in your tube circuit, who would have thought? Nobody ever seems to talk about those. It's always caps and tubes. If I had only clued into this over a decade ago ...

That 0.68uF bypass in V1A is great though; that's a keeper. The 10 uf versus original 33 uF on the second stage doesn't make much difference; the 6.8 nF coupler does all the bass cutting there; anything larger than 33 uF makes no difference; 10 uF just slightly cuts a tiny bit of bass. You do need one stage with a small valued bypass in a high gain amp or preamp. Everyone has that. Marshall, Soldano, Boogie, Engl, ... Ada did a weird thing there with all four bypasses in high gain mode being 33 uF, and the coupling capacitors not being very small either at 6.8 nF and that's why MP-1's are known for a kind of thick, woody tone that isn't that great at high gains. 0.68 uF on V1A and it's good. The cathode resistor is 1.1 k, whereas Marshall likes 2.7 against 0.68 uF, but that's not a big deal.