Marshall AVT help

[Jay]

I have a Marshall AVT 50 head, and over at the Marshall Forum
a fellow claims to have figured out some mods to fix a few of the complaints
with the amp. He has since not been around to answer questions.
Here is what he says about it.


"I came up with some great modifications for my AVT50 amp:
1. The obvious mod: add a second speaker jack.
2. Installed a clean channel midrange control.
3. Increased preamp tube gain by around 25% (switched).
4. Installed a 6 position mid shift switch on the Overdrive channel with the stock midrange frequency, and 5 others [a great mod in any amp].
5. I reinforced some key components in the amp with higher wattage and voltage components (cheap insurance).

The fatter tones from the mid shift switch along with the extra tube drive really really brought the amp to life. The amp sounds 100% way better, and I didn't even drive the tube nearly as hot as in other tube amps. The clean channel tube sound really rocks on full gain, and rolls off at lower gains. Very impressive improvements."


The main thing I would like to do is #3, more gain would be nice. Any ideas? And the mid control on the OD channel is crap. Can
I just change out c79? Will I also have to change other values to compensate? Even adding the 6 way switch would be fine if I could figure where to put it.
(schematic link below) OD tonestack is on the lower right of the schem.



http://www.indyguitarist.com/schematics ... _60_02.pdf

As always any help is appreciated.

Jason

[jupagblkxten]

easiest way to get more gain out of that tube is to add a cathode bypass cap in parallel w/ the 1k resistor off the v1a cathode. start w/ a .68 or 1 uf cap and tailor it to taste.

[Jay]

Thanks for that tip. I setteled on a 22uf. At least for now, I might change it later on but that seems to give it a pretty good
boost. I replaced the channel footswitch jack in the back for a stereo jack an have a 2 button footswitch so the gain boost is
switchable.

The gain boost actually cleans the sound up quite a bit. I still changed out c79 (15n) for a 10n. That seems to help give
the mid pot a little better adjustment. Its not quite a muddy sounding as before, but like I said the gain boost helps there as well.
Does that sound like a good change or should I also swap out c78 to compensate for the c79 change?
I could use a little more bass so maybe I should raise c78.

I'm just guessing on some of this stuff so if anything I'm doing is wrong or could be dont better please let me know.


Thanks
Jason