Hooray! Got the tubes this afternoon, but unfortunately, I'm under the weather, and running a fever so was only able to get them installed, bias the amp and play for about 15 minutes.
* First of all, even though I am a self proclaimed do-it-yourself-er, this is a technical process involving a lot of current/amps/voltage. I describe the process as being fairly easy, but I WAS nervous. Make no mistake, any time you reach into the back of an amp under power, you are AT RISK of injury or death, and I am not responsible for you if you decide to try this. If you are not completely comfortable with the process DO NOT DO IT. Your life is worth much more than the $100 or so a tech would charge to do it for you.
A couple of observations:
- Should have checked the bias setting for the old tubes (just out of curiosity)
- I wasted a bunch of time trying to remove the amp from the chassis, unsuccessfully. In hindsight, this is not necessary.
- Wish I'd bought a few more of these JJ's tubes so I would have more than one matched backup set
Once I decided to try adjusting the bias with the amp still in the chassis, it was literally a 10 minute job. The bias contacts and the bias pot are all on the underside of the amp next to the tube sockets, and, for the most part, easily reachable once the two tolex back panels are removed. The bias pot is a little difficult to get to due to the speaker magnet, but a small phillips-head solved that, even though it kept getting stuck on the magnet.
Below was my process:
I layed the amp over on it's face and unplugged everything. Once I had the tolex back panels removed (and after giving up on removing the amp from the chassis) I used a latex glove to grip and pull the old tubes, and insert the new tubes, as I didn't want to get finger oils on the glass... not sure this is necessary, but...
Once the three new tubes were in (1 ECC83S in the phase inverter position, 2 EL84's in the power positions) I made sure they were fully seated a couple of times, and then plugged the speaker in, and a guitar and turned on the power. I also made sure it was set to Class A/B and the volume was about halfway up on each of the gain stages (Drive, Channel Volume and Master Volume).
V3 lights up first (I guess because it's first in line from the power supplies), then V2 and V1.
I sat there for a few minutes while it warmed up in standby, and used the opportunity to power up my multimeter and set it to DC Volts in the mV scale. Then I sat there for a few more minutes with a flashlight eyeballing the bias contact points, thinking about the old game "Operation" wondering if I could get the spare rib, broken heart and brain freeze out without getting "buzzed"!
After deciding there was really nothing underneath the chassis to be concerned about (not a good assumption as there is a massive power supply right next to V3). I inserted the black probe in the hole labeled "ground" and the red probe in the hole labeled "bias" for V2..... Nothing. No reading... must have missed the contact points... after several more attempts, and getting a little frustrated, I wiggled the black probe just right, and found an indention in the bias contact. Found the same indention with the red probe, and viola... 16.7mV... wait, it's supposed to be 25mV +/- 3. So I moved the red probe over to the V3 bias contact, and made the same measurement... 18.2mV...
Now you see why I was wondering what the old tubes were biased at... if they were that cold, it's great for tube life, but not for tone, and could explain why my highs sounded brittle, the lows were muddy and breakup was a little harsh, with the old tubes.
I pulled the probes and inserted the phillips-head into the bias adjustment pot and gave it a VERY slight turn to the right... seriously, maybe only an 1/8 of a turn at most... re-checked the bias on V2... 34.2mV... whoops, too much... readjusted the bias and checked again... 24.8mV on V2... okay, that's good, checked V3 and it was 26.2mV... pulled the probes, wondering what I'd done wrong (ha ha), and then rechecked it again... same reading.
Powered everything down, pulled the power cord and guitar cable, re-attached the two tolex pieces, sat the amp upright, and moved it back to it's usual resting place. Reconnected everything and powered it back up with a guitar plugged into the front input (I don't usually use the amp this way, it was just a test to make sure I had sound). I did! Played for a few minutes on "American Clean" and "Brit Crunch". Sounds good, but since I don't usually play the amp like that I couldn't compare to the way the amp sounded before.
Plugged into my POD HD, and reconnected the L6 Link and played through some familiar patches. Here are a few observations after 15 mins of playing:
On cleans (in Topology I, Pentode and Class A/B mode, but with various amps):
- Low end is definitely tighter with a more defined pick attack and better thump and compression
- High end is cleaner, and meatier with better extension - definitely more of that "jangle" EL84s are known for
- Mids have more body
- Sustain is clearly longer than before and there is more clean headroom... harder to drive the amp into breakup through aggressive picking, at least at moderate to moderately loud volume levels
On overdriven tones (same settings, but mainly with the PHD Motorway):
- More "air" in split coil settings (a recent mod to the bridge pickup of my Charvel Pro Mod)
- Fat, bluesy tones through the SH1-59 neck pickup, with plenty of pick attack and more sustain than before
- Better low-end definition from the bridge humbucker, while also being meatier with a subtle, but crunchy, staccato pick attack (This is the closest I've gotten to the sound of Warren DiMartini's guitar sound on the opening riff of "Lay it down". The low "D" was clear and thumped like I always thought it should).
- Through my Strat with the N3 Noiseless pickups, in position 2 (without the S1 switch engaged) the cleans had the body I always wanted from this setting, while still retaining that high-end sparkle Strats are known for.
Overall, I am VERY pleased with the JJ's over the stock EH's and Chinese 12AX7. Especially since they biased to within 1.4mV of each other - that's pretty good matching!
Again, I do not recommend this if you have never done it before or aren't completely comfortable with partially taking your amp apart, and replacing parts and reaching your hands in... If you try this, you do so AT YOUR OWN RISK!