Matching the current is critical to sonics here. If you could find matched tubes, great. But over time, even matched tubes will age and most probably, they won't be matched anymore. Therefore it's critical to have some adjustment on the tube currents.
There you have it! By varying the pot, we could adjust until both tubes are conducting the same current. Well, as close as you can get. Without too much effort, I managed to get one channel within 1mA while the other channel was within 5mA. If you have trouble getting the tubes to "behave", sub the 15 ohm with 20 ohm.
After all the improvement, complicating a simple amp, I brought it over to YH's place. YH uses Harbeth 82dB, 6 ohm speakers. Room size is 16' x 24'. To say the Simple EL84 faces a daunting task... is an understatement. Well, what can I say? This 10W little bugger drives the Harbeths very well! Except for some complex passages and musical passages where it clipped, the Simple EL84 is a winner. We played a Consonance (Opera Audio) M99 integrated in this same system a week ago. Let me tell you this: the Simple EL84 CREAMED the M99! There's simply more life in this little bugger. Bass control is simply excellent... for a tube amp.
The EL84 naturally has a sweet sound so the usage of neutral ECC99 and Auricaps help it excel. Sweet front end tubes and caps might give you diabetes! But if you like it, then what do I know? Experiment and tell me what you like!
Hey, if you are longing for a tube amp, but can't afford one. Here's your ticket.
If the Simple EL84 can drive the 82dB Harbeths with aplomb, it should be able to drive most commercial made speakers. 88dB? No problem! Unless your listening room is many many times bigger than YH's.
After the Simple EL84 survived the Harbeth test, brought it to test with other speakers as well. Tried it with 85dB Musical Technology speakers, and 91dB Monitor Audio bookshelves. Again, the Simple EL84 proved itself equal (and even surpassing) some of the much more expensive gears over there. I luv this baby!
Let's face it, comparing to single ended amps, reveals a not so easy-going midrange and treble but if you favour better bass control and dynamics, you know which I prefer. : ) Single ended amps need very efficient speakers, which are mostly HUGE and COFFIN-LIKE in appearance. Here's another alternative for you, courtesy of DIY Paradise.
Options? Lots of! You could build all NOS line-up. 5687 driving 8BQ5s. You could build all new tubes line-up. JJ ECC99 driving JJ EL84. Or you could mix and match. There's certainly more but this will have to do for now.
Before I wrap up, here's what you need to build the Simple EL84.
Continuing the journey... Variation of the Same Theme
Output trans, 8kohm primary impedance. 10W is fine. (New Hammond stock will have this little critter.)
Power trans. 240-0-240V @400mA, 4-0-4V @6A (for 8BQ5), 3.15-0-3.15V @3A (for 5687 or ECC99).
The 23kohm plate load resistor consumes ~2W. Use 5W or higher.
The cathode 390ohm, you could use a 1/2W.
Ditto with 220kohm grid leak. 1/8W is fine as well.
500ohm pot can be 1/2W or more. Ditto with 1.2ohm current sense and 15ohm current adjust.
The LM317 on YHLMCCS (highly recommended) is best fitted with TO-220 heatsink. Run it at 70mA if you think 80mA is too hot. (It is, to a certain extent.)
Lastly, TOOBS! NOS 8BQ5 @RM20 each for a limited time. JJ EL84 matched quad @RM140. JJ ECC99 @RM45. NOS 5687 @RM40. See price list.