Ok. the 12DW explains the PI imbalance, as you proved by trying the 12AX and finding it was balanced. 
Yes, you can plug 6L6's right in with this circuit.  Just have to be good enough to handle the high plate voltage.
Of the old tubes, not likely they are all bad, try them one at a time, probably the one or two that redplate are pulling down the bias for the others.

Triple check that both side heaters are lit.
Remove PI tube, power off.  Measure resistance from pin 8 (at the socket not at the board) to ground.
Now measure resistance from socket pin 6 to point D of power supply resistors.

  Sorry to nitpick, but standard "safe" wiring of fuseholder is for incoming power to go to back end of fuseholder first, then out the side.  This is so as soon as the fusecap is disconnected the hot can no longer touch anything.  With the hot on the side terminal like you have it, as the fuse is being pulled out it can contact the "hot" side terminal.
Of course we assume the unit will be unplugged before the fuse is removed, but mistakes happen.

Quote from: ilyaa on July 03, 2014, 04:49:28 PM
if i just pull v4 out and play without a tube in that socket, will the amp be mad?

Should be fine, preamp supply voltage will probably come up a little.

Quote from: ilyaa on July 03, 2014, 03:19:45 PM

QuoteWhere are you getting those "should be" voltages from?  Are you using a different schematic than the one from your first post?

its a classic A, not a classic B, i discovered

the new schematic is attached above (in my post that starts with NOTE:)

Ok, got the correct drawing now, sorry about that!
One thing very critical for power is screen supply.  Have you checked the voltages at pin 4 of both power tubes?

Quote from: ilyaa on July 02, 2014, 11:33:38 AM
on the power supply end of the 82K 1W resistor, the voltage looks high (should be 460 and its at about 490), but on the PI end, its low (should be 300 and its only 260). the collectors of the PI transistors are a bit low at 130-140V, instead of 150.

Where are you getting those "should be" voltages from?  Are you using a different schematic than the one from your first post?

Quote from: BrianS on June 29, 2014, 01:08:42 PM
I got to thinking a little bit and decided to disconnect the primary wires of the PT from the amp and measure that way.  Doing this, I get 29vac. 

I think you mean the PT secondary?  So when it's connected to the board it reads 3.4VAC, but when disconnected it reads 29VAC?
Is the fuse blowing?

  There are no parts missing.  The points you are looking at are where the reverb cable would go if it were mounted from the component side.
I have circled on your picture where the 2 cables would  connect, the pair on the left goes to tank input, on the right to tank output.
  It looks like you have them connected to the same points (but double check), just mounted from the solder side rather than component side. 

  I have had new caps like that with some kind of white powder on them, it's not leakage.  Generally you only see issues like that with electrolytics.

  I think where you said preamp in jack, you meant power amp in?
You may still have an issue with the power amp in jack, give it another shot of cleaner and work a cord in and out of the jack several times.
See if it makes a difference.  If not, try connecting preamp out jack to power amp in jack with a patch cord.  Then try playing through the regular input again.

  Stick with 1458's and see if it improves reliability.
Normally 4558 is ok to sub for 1458.  However, there are odd cases where they don't work well, and if I'm remembering correctly, the only place they seem to have issues is in some Marshall amps.  I can't remember which models or why it is a problem.  I think I have seen Enzo mention the same issue and he may remember more details.

  If the ripple is so low that a good meter will not accurately measure it, then it is not a concern. 
Yes, it is nice to be able to see it on a scope, but if there really is a problem, it should show up on the meter as AC volts.

  Yes, 100nF and 47nF.  They should also have a voltage marked on them.  Do not use them in a circuit that will exceed the voltage rating.

The value is in between the letters, so 473 and 104 for those.
The first 2 digits are significant, the 3rd digit is the multiplyer, giving the value in pico-farads.  So for the multiplyer, add that many zeros to the significant digits.
So in this case, 47000, and 100000 pico-farads.
To convert to nanofarads, move decimal place 3 digits to the left.  You get 47 and 100 nano.
To convert to microfarads, you move decimal place 6 digits to the left (from pico).
So .047 and .1 microfarads.

Quote from: Roly on May 20, 2014, 02:30:39 PM
Quite likely not;

Title: GFX120a
No: 07S252-XX
Sheet: 1 of 2
Date: 6/19/98

The basic point is that one has extra bypassing the other doesn't, and this change in DSP supply arrangement may be a clue they were having some problems in this area.

Yes, that is the same drawing I have.  It's a 470 ohm (R88), not 47.
It is used as a voltage dropper because they are tapping off the +40V supply.  The other circuit has it's own winding and rectifier for a separate supply and has a 10R resistor.
I'm not arguing that the .1uf bypass cap may help, I just wanted to point out that the supply circuits are quite different and the 470R is not just for filtering.

Agree to try the .1 bypass as close to the DSP chip as you can and see if it helps!