The following is from a guy who used to work for SUNN in the 60s......Regarding my 67 Coliseum PA head and 2 Pwr. tubes vs. 4 pwr. tube and 1 rect. tube vs. 2 rect. tubes... HERE IS HIS OFFICAL ANSWER TO THE QUESTION:
Specifically noted are the four type 6550 or KT88 (again british but same tube) power tubes and they are visible from the top down and marked as V6, V4, V5, V3....Ok so far? The entire audio power output circuit stretches from tube V2 a 6AN8 (shown as 2 tubes but really 1) to the right side of the page.
This is a 4 tube PUSH-PULL audio circuit. Note the ground connection between V4 and V5? This is a little line that jogs to the left to a ground symbol (triangular shaped 3 lines).
This is the center of the circuit and the 2 power tubes above, (V6 and V4) are a "mirror image" of the 2 power tubes below, (V5 and V3). This is an extremely balanced circuit and by that I mean that the top part conducts on the positive side of the cycle (sine wave) and the lower part conducts on the negative side of the cycle. Picture a sine wave in your minds (what's left of them 
) and that is like a sideways S with a horizontal line thru it. The top part of the wave (or S) is + and the bottom part is - (minus). Now look to the right of the 4 power tubes and you see a long vertical coily symbol. That is the primary input winding of your precious 120 Watt audio transformer! In the center of that you see a line to the left marked R to a dot marked A. That is the center tap or zero and is like the horizontal line thru the sine wave I mentioned. The top of the coil to this line conducts on the positive cycle and below this line conducts on the negative side of the cycle, just like the push pull amplifier section to it's left (mentioned earlier). This push-pull audio circuit is just like the 2 tube version (the 60watt Sunn) except to gain more power, the KT88's V6 and V4 and likewise on the bottom side, V5 and V3 are in parallel with each other. You guys still with me? OK....If you run this contraption with only 2 power tubes you are basically UNBALANCING the circuit causing undue stress on the unpowered components and ESPECIALLY the transformer winding. UNLESS, you happen to be running in 2 tube mode, either V6 and V3, or V4 and V5. Any other 2 tube combination is detrimental and here is the question: How do you know just which two KT88's you have pulled out? There is no number on the chassis saying which tube socket is V5, or the rest! If you were to be running let's say, just V6 and V4 your signal would only be conducting on the positive half of the sine wave and yeah, distortion up the ying-yang!!! If you have an amp you don't like and got lots of repair money then no problem 2 tubing it!...but I think you like your amp better than that!
NOW for the power supply, below all this on the lower part of the schematic is the POWER supply. You can see the 2 GZ34 rectifier tubes (5AR4 american) in parallel. You can see the 3 primary coils on the power xfmr to the right of the tubes, you can even see the lower coil section with the two leads marked GRN or green going to the six 6.3 volt tubes filaments (shown as an <) This is one of your green wires gettin HOT! By running only 1 rectifier tube you are severely unbalancing the load on the power supply causing excessive HEAT. Lastly, the Sunn modification schematic I included shows how they modified the power supply to use a plug in to the tube socket solid state replacement diode. Notice that they refer to it as a "plug-in diode unit OG-5R4, by Diodes Inc. or equivilent" and the note below "other octal (tube) socket not used, leave blank" If you don't want to mess with 2 spendy hard to get Brit GZ-34's or 2 easier to get U.S. 5AR4 tubes, I would suggest looking for one of these easy to find plug in solid state rectifiers. My own opinion is that you are doing more damage just running 1 GZ-34 than you would be running just 2 KT88's. Hence the heating up of the green filament wire. Still with me guys?..Hope this helps!
I have had a amp guy disagree with all of this too.....ANY COMMENTS/HELP 
I should have read this before I answered pickinatit!
For what it’s worth, here’s what I think; If you “have an amp guy” who disagrees with this dissertation you need to find a new “amp guy”. Your ex Sunn employee seems pretty much right on the mark to me. You should ask this guy to contribute to this forum if he doesn’t already.
The rectifier is not part of the signal path, so pretty much the only effect it has on sound is through “sag” (or lack of), which is basically the amp being starved for current in high demand conditions. As a bass player I personally have little use for sag, therefore I want my power supply as healthy as I can get it within reason. I’m hesitant to modify a vintage 40 year old amp (I have, but I hesitated first…
) meaning I don’t really want to hard wire a bridge rectifier in a classic amp that was built with tube rectifier(s), so I have some of these;
http://www.webervst.com/ccap.htmlthat I use in place of the tube diodes as sort of a compromise.
The rectifier issue has been address here on a few occasions, just do a search if you want more opinions on the tube vs. solid state devices.
