It took a li'l while for it to grow on me because I have "a few" amps lyin' around but the JSX IS A CONTENDER no doubt about it. I installed a 12At7 in the PI spot and the rest is bone stock (EL34s). She's biased up at 43 Mvdc and when the Master Volume is over 6 this amp sings. I just picked up a JSX cab...and am lovin' this thing. It is a finicky li'l amp with lots of "outside the box" thinkin' feature-wise and I fully understand why this amp is hated by some....it ain't so "plug in and play" right off the bat and at bedroom/bedtime volumes it can sound like ass if the MV is set low. But the more I discover about this amp the more I love it....the tonal range of this thing is astonishing. I wish I was set up to record clips as I've dialed in some great clean, low, and mid gain tones that rival many boutique clips I've heard....and this li'l video posted here does show only some of what this amp is capable of.
1) Know that it's a modern high gain amplifier with modern compression on the attack. You can do some things to warm it up including being careful with tube selection and biasing the amp (see next point for biasing). I would recommend Sovtek 12AX7WC preamp tubes for warmth without sacrificing much (if at all) gain and articulation, while adding a somewhat more vintage attack character. For output tubes, I'd probably use EH EL34 for a bit more classic midrange character but I've found SEDs/Svetlanas sound nice in it too.
2) Bias it properly and know how to bias it. The bias points Peavey provides are for measuring the bias voltage on the grid pins of the output tubes, so it does not directly correspond to current draw. Use a bias meter and check the current, and you'll be surprised.
3) (If it's the combo version) Set the impedance correctly for the combo; it's shipped with two 8 ohm speakers in series which makes it a 16 ohm load.
4) (If it's the combo version) Check the connections on the speakers. I've seen a couple combos already with loose female quick disconnects, which had to be crimped tighter.
5) For a modification, try adding a choke in place of the big power resistor (to regulate the screens and preamp B+). The typical (as in the 5150) 400 ohm / 10W resistor can be replaced with a choke as long as the voltage rating is sufficient (500V), and the exact resistance and inductance aren't critical. Don't look to replace the resistor with a choke that drops 400 ohms thinking it's critical to get the same resistance value because using something with lower resistance (150 ohms for instance) will only raise the screens and preamp B+ slightly. I'm betting Peavey used a 400 ohm resistor so that it would sound more like a choke (as opposed to using a lower valued resistor). Anyway back to the point: replacing this resistor with a choke will make the amp more rich sounding and keep it more focused at higher volumes (you will use the "fat" switches and have no problems at higher volumes anymore, for instance).
6) Oh yeah, I nearly forgot...you might want to replace the 750 ohm / 5W screen resistors with 1000 ohm / 5W (and use somewhat better ones). Peavey seems to have problems with those 750 ohm cement Dale resistors from time to time with certain EL34 in these amps (and also the Triple XXX, evidently). Getting the power tube board out will be fun, and you'll love the way it's mounted. (Don't shoot the messenger...)
I like the JSX, for a modern high gain amp. And the combo sounds really good too, which surprised me since I figured an open-backed combo with those Peavey (made by Eminence) speakers wouldn't impress me.
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