"
And now for something completely different."
OK, so I tried to ramp up the difficulty level a bit lol. Don't try to reproduce the experiment I'm about to show you at home, unless you
really know what you're doing -I hardly am!
So, I'd been looking for 7-pin socket adapters -versatile adapters, not one-shot adapters that make a tube type into an other and nothing else- for a while, and finally bumped into a website that sold quality NOS adapters for cheap. Shipping across the world was cheap too, so I bought three pairs of adapters. They finally got here today.
Now, I'd had -and still have- a number of ideas in my head of new tube types to try on our LD amps, most of which would require some thorough pin reworking, but there was this one experiment I wanted to try first; ironically it was the most challenging one. Using one of those ubiquitous double triodes that everybody loves on my LD amp! It's a good thing I had an old mil spec Mullard M8162 lying around (it came with a box of very random tubes months ago, back in the EF91 days). So, I started working on adapting that single 12AT7 tube on my two-separate-pentode LD!
Here's a picture of the adapter/tester/savers I got. Brand name is Vector, and they seem to have been a reference back in the tube days.
The tube I adapted: Mullard M8162 / CV4024, mil spec 12AT7, Mitcham-made; barely legible when I took the picture 8 months ago, and even less now that I manhandled it into submission...
Just your basic 12V, center-tappable, high-mu double triode. I just realized that this a $30 tube on ebay, actually, so maybe not that basic.
I'll cut to the chase and just illustrate what I did with a picture (how's my "reach out to the gods" setup Mikelap lol?), just so you can see just how unpractical this whole setup is -and how crazy an attempt to adapt a tube it is!
Basically, I chopped up some old computer cables and stripped them on one end. The other end I used with its metal hole to squeeze onto the tube pins as DIY air socket holes (R) (with great difficulty and little fate that the contacts will last long). The stripped end of the wires I attached to the socket adapters' tester tabs in a non-destructive way (the adapters can be unscrewed and opened -which I had been hoping for since it was what I needed- so I squeezed the bare wiring underneath the tester tabs and screwed the adapters closed again).
Made each of the nine wires (9 pin double triode) go to the appropriate pins on the tube, and socket holes on the amp; using the heater center tap to power the tube (so pins 4+5 to one of the heater pins on one of the sockets, and pin 9 to the other heater pin on the same socket), and rerouting each triode pins to the appropriate pins on each socket. The pieces of paper below the tube aren't to make it pretty; they're there because I was very scared that the wires would touch each other, and they would have otherwise!
Interestingly, it only took me two attempts to make this mod work (I was expecting it to fail; I didn't even know if the tube was good). First time I had messed up some wires on the right side, so the heaters -fortunately- didn't power on, and I was spared the awful PPSHhhh noise, just got silence.
On my second attempt -a bit more thorough than the hasty first one- everything worked perfectly!! Filaments lighting up, music in both channels: success! I did get a bit of noise in one channel for half a second, but I know that's because the tube is barely attached to the wires and
kind of hanging from nowhere, so I wasn't surprised. It worked flawlessly for 15 minutes after that.
What is even better is that it sounded great, which is not an easy feat considering that everything was wrong with this mod: wrong number of tubes, wrong tube type, wrong number of grids, extra 50 years old adapters, 15cm of crappy wiring without shielding, dubious tube... But still, it sounded as good as my current fav' 6DT6 tubes, so top-tier. Very natural and musical sound, detailed and realistic but still toe-tapping; I guess double triodes are cool too huh?
It does kind of look like the amp is holding up a tube as an offering to the gods though lol. I'd imagined it would have looked even more rigged, so this isn't even that bad for a quick test.
Like I said earlier, the adapters unscrew, and the whole top part comes off with the pins and tester tabs, so you can squeeze stuff in there and not destroy an adapter by soldering wires for every new pinout you want to try.
Anyway, interesting experiment. Unpractical for sure, but my goal was just to prove that it worked, not to leave it there. Not only did it work but it sounded great! So, I'll be investigating this a bit longer. If a 12V center tapped double triode can be made to work on this amp, you can sure bet a 6V double triode like the 6DJ8 would too! And these are supposed to be very very nice tubes.
Notice how such a setup would use less power than our typical tubes: a single 0.3A heater instead of often two in our pentodes. Then again, the "prettier" solution using double triodes here would probably be to just use one per channel with a clean 9 to 7 pin adapter, and only use one triode out of the two in each tube. More expensive and a bit silly, but it would be the only way to not have a tube floating in the sky on top of your amp between sockets...
I had to open up my amp a few days ago and took the opportunity to take -non-destructive- pictures as best I could. You can't see much but it gives you a pretty good idea of what is in a MK IV SE, seen from the front. Pics can be enlarged if you actually want to see something.
Edit: See that wooden thing on the picture above? That's my solution for changing jumpers easily. Glued a match stick to an elongated jumper so that the wooden part pokes out by maybe 2 mm outside the amp, meaning that I can grab and change the jumpers by hand.
