I've been silly... Why bother soldering a wire on very-hard-to-solder-on-because-oxidized tube pins or even wrap a wire around to link two pins semi-permanently when you can just... bridge two hole in the socket without soldering anything. Just by dropping a small |__| shaped wire into the holes (stranded copper wire btw, solid core wouldn't work here, it has to be able to ease around the pins) that will make perfect contact between the tube pins, wire, and socket holes over most of the length of the pins, and be flat enough that the tube rests perfectly in its socket barely higher than usual and perfectly vertical.
Voila, so easy a mod even a cave man could do it, and now you have the "schematics" too. That basically makes your amp compatible with all 7BK base tubes like the 6AU6. Nothing permanent, very good mechanical contact (stranded copper stuck between tube pin and socket hole wrapped around a long side of the pin, very hard to get a failed contact). Just chop up two pieces of wire and bring them down slowly in the holes with nose pliers or whatever. Wires need to fit nicely in the holes to avoid bad contact, or worse, contact with other pins. Just insert the tubes on top of that trying to make somewhat sure the wires don't like fly away from the tubes socket that very second, you never know. Only problem is that when you pull the tubes out, the wire has a 50% chance to come out too. Then again, you should probably re-straighten the stranded copper and reinsert the wires properly with every tube change, just in case; hardly very difficult. I used 26 AWG OFC copper wires btw (from a leftover Mogami 2893 quad cable).
Edit: Yeah, definitely take the mod wires out when you change tubes and re-twist the stranded copper back into a straight shape before reinserting the wires in the socket and then the new tubes. When I changed tubes the second time I tried this, the wires only popped out on one side and stayed in on the other, so I only re-twisted the one that popped out: bad idea, both tubes worked fine until I tapped the amp very lightly to check for microphonics and the tube on the wrong side went SHHHHH on me. Pulled tubes and wires back out, put everything back in properly, no problems. Then again, the pins were pretty dirty too -should have cleaned them first- which was probably not helping to make contact with the wire. At least it shows this "mod" won't make your amp explode or anything (you'll just get a floating cathode and lots of noise if the contact isn't perfect).
I tested that "mod" of sorts with "regular" tubes first, and it worked perfectly. 6AK5 jumper setting and do NOT do it with EF92/91 tubes (as pins 6 and 7 are inverted), but on any 7CM base tubes (6CB6, 6DK6...) it works by strapping grid 3 and cathode together (instead of using the EF92 jumper setting to strap grid 2, grid 3 and anode together), while grid 2 is still strapped to the anode. So, it can also be a good mod for those of you that hate changing jumpers with a passion. Note that for EF92/91 tubes, one could also imagine placing a small wire between pins 6 and 7 to strap grid 2, grid 3 and anode together by hand and stay in the 6AK5 jumper setting all the time -if you really hate jumpers that much. Strapping grid 3 to either anode (EF92 jumper) or to cathode (this "mod") might yield sonic differences -note that the shield is usually connected to grid 3 so connecting the shield to either anode or cathode might also make a difference in noise rejection- or might not, feel free to try while you wait for your 6AU6 tubes you know you'll end up ordering ;) .
Now that the amp is "compatible" with 7BK base tubes, and since I just got my 6AU6 in the mail, it's time to test some new tubes!
GE JAN 6AU6WC 1978 matched in box NOS NIB
Tektronix matched in box GE 8425A/6AU6A 1964 NOS NIB
The Tek matched 8425A are currently burning in for the workday. From just testing them after 15 minutes, I can say these are incredibly clean and musical, and natural and detailed with a black black background and possibly the best clarity I think I've ever gotten out of this amp. They have a very similar structure (shield, grids, top, getter flash) compared to the GE 6GX6/6GY6 I have, which is interesting. What is immediately apparent is how awesome the imaging is on these. The singer is right here, smack in the middle of the scene, on the stage or in the studio, singing with all the qualities and inflections of his/her voice, truly impressive! Only the bass could have a bit more punch and slam at this point, but that's probably my headphones more than the tubes. I would be very surprised these don't come out as A+ tubes after burn-in -as I had speculated they would. Try 'em, they're dirt cheap anyway, and their clarity and imaging might be even better than the beloved CV4015 -which is no easy feat.
Edited by Audiofanboy - 3/14/13 at 7:11am