I had always felt my DT could sound better. The clean highs were a little thin, the midrange lacked body and punch and the lows were a little spongy. So I swapped out the tubes on my DT25 last night with a set of matched JJ EL84s and a balanced ECC83S (a direct replacement for the 12AX7) and performed a re-bias on the new tubes.
(This is a very dangerous operation, and should not be done by anyone who lacks the knowledge and experience of re-tubing and re-biasing an amp. If you choose to do this, you do so at your own risk).
This is just an initial review as I have bronchitis and was running a 102* fever as I was putting the amp back together. Not feeling good at all. Anyhoo, below are my initial thoughts after about 15 minutes at moderate volume (just loud enough to get the tubes warm).
First thing I noticed is that both tubes glowed with the same intensity. With the stock tubes, V3 glowed significantly hotter than V2, which probably indicates their bias voltages were significantly different.
Sounds on some of my clean patches (in Topology I, Pentode and Class A/B mode, but with various amps - PHD, Blackface, AC30):
- 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 and an AC30 thrown in):
- 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, tight 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 - I definitely recommend the change, and recommend matched sets since there is only one bias adjustment pot. I also recommend the balanced-triode ECC83S for the phase inverter position (in place of the 12AX7).
Again, I do not recommend performing this kind of work if you have never done it before or aren't completely comfortable with partially taking your amp apart, replacing important parts and putting your hands into an amp that is pulling current... If you have any reservations at all, it's best to pay an amp tech the $100 bucks instead of risking your life. If you do try this, you do so AT YOUR OWN RISK!