jj tube upgrade kit for marshall jvm 410h, 410c, 210h, 210c amps el34/ecc83s jj electronic audio tubes direct has over 3000 successful transactions on other websites with a 100% feedback rating. imported from slovakia, these brand new tubes have been fully tested on the industry's most reliable, voltage controlled testing equipment and exceed all manufacturer's specifications. they are guaranteed from the manufacturer to be new and free from any defects in materials or craftsmanship and are backed by our 30 day warranty. matching: we consider power tubes to match when the plate current(lp) and the transconductance(gm) vary less than 5%. we burn-in all power tubes for maximum stability and accuracy during testing. test results are printed on the box.