Pearl tube coolers are great for making power tubes last longer but when I tried them on my preamp tubes I didn't notice much of an improvement so I left them off.
There are other areas that will make more of an improvement like replacing the quad op-amp in the output stage of the C-19. The Brown Dog RC4136 Upgrade Adapter will let you use a standard quad op-amp like the AD823. The 8 pin op-amps are TLO-72 which is a dual op-amp.The Burr Brown 2604 is a drop-in replacement for the TL072 but I used to use the Analog Devices AD-712.
I am new to the site and I must say this is pretty hard core. Never heard of a C-19 and I am amazed you guys are modding them. This looks almost dead, but hopefully you'll update on your progress. Am I correct that Anders added the IC sockets for the TLO-72? I saw something on another site that scared me if you upgraded all of those: "The big problem, as I understand can also be the unexpected current draw - especially with TL072 upgrades. These draw less 3ma or so but up an "upgrade" of say an OPA2604 is 12ma. That adds up pretty fast to your power supply giving up it's soul. It could also sound worse or nasty things." I was looking at the C-19 schematic online and call me crazy, but it looks like it uses seperate 6DJ8 for MM and MC? This thing is half solid state, but you might be able to yank the phono tube not in use and perhaps some ICs around it. Looks like the unit runs the 6DJ8 at very low voltage and they all run off the same transformer taps, so this might be something that would take the load off of the power supply.
If you get the Burson I'd love to hear what you think of it as i have solid state crossover for which this might take the upgrade to another level. Hopefully Anders has directed you to the key ICs to replace in your situation as I see a lot of them. If you go Burson crazy the expense would be pretty high. Tube rolling and IC rolling all in one, yikes.
You might get a lot of mileage upgrading the bridge rectifiers in the unit. Those in design often claim they can use more mundane diodes to accomplish what hexfred's or stealth diodes do, but I have my doubts having experienced diode upgrades which are pretty cheap.
You might be surprised what you might find in the blackgate FK, C series, and PK at low voltages. I recently upgraded a solid state amp with these and was quite pleased with the result. High voltage black gates are insanely priced these days.
