I recently scored a quad of Chinese KT88s marked "Audio Research" off of E-pay. I'm kinda short in the beefy 6L6-type department (just some Sovtek 6550s and some "super" 6BG6GAs), so it was nice to get some inexpensive tubes to play with. The innards look fairly reasonable, though one looks a little crooked on its base. Are these firecrackers with glass envelopes or reasonably decent tubes?
Wait, do you mean they're Chinese-made for Audio Research or Chinese knock-offs of Audio Research tubes?
Back in the mid 90's I spent a lot of time in Nutley New Jersey at JFF Research Lab. This was a small testing facility that tested vacuum tubes of all types, big and small-DC to microwave, for the industry. Back then Audio Research was buying Chinese tubes for their production amplifiers because there was little else available in quantity. The Eastern European sources were in distress. While there I had the dubious honor of watching John test Chinese tubes for AR and others.
Testing was done with custom built (by John) setups that would simulate any actual operating conditions spelled out by the customer. By dubious honor I mean that I witnessed a failure rate of as much as 25 to 40% per batch sent in. Not only KT88s, but 845s and 211s as well. The Chicoms were great at copying the look, but woefully deficient in matching the operational quality. This is one reason why I've never liked Chinese tubes. So yea OK, they've improved a lot since then, but the bad taste never fully went away.
I have no Idea at all what your KT88s are like or what time period they're from. And I couldn't tell even if I saw them. They might be OK, they might not. You'll just have to try them and see. I do know that "for a few dollars more" they will put your (or anyones) name on them. I do have two 25 packs of very early 6550s that I bought cheap from a dealer I was friendly with. But I've always been afraid to use them.
They say "Audio Research" on them, and they are labeled "made in China". They were tested and matched by the E-pay seller. The seller has another quad up at this moment, so you can check out the pictures on "that auction site" if you're curious. Just search for "Audio Researdh KT88"
Hmmmm? Very curious! Tested on USM-118 and Tek 570 curve tracer. Lots of negatives and all about tubes. I wonder it that's an old acquaintance of mine Tom Bruckner. I suspect it is since he operates out of that area.
Vacuum Tubes and Electron Tubes for Industry
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HollowState has it right. I scored a pair of the Audio Research tested and selected tubes on Ebay at least 10, probably 15 years ago. They worked and are still alive. They have Audio Research in white letters screened on the side along with "made in China"
Stan at ESRC had purchased a lot of the same tubes untested directly from China. He had such a bad experience with them that he decided not to carry them any more and sold me the remaining tubes cheap. They do not say Audio Research, but do say made in China. They are otherwise identical. I blew up many, but I still have 20 or so, I may get them out of the warehouse on July 4th.
The untested versions do blow up...a lot. 25 to 40% failure rate is about what I get. One of these tubes was one of only two tubes that I have seen blow up so badly that the glass broke! The other was a Russian 6P19 that I wired really wrong.
The construction quality of the tubes is randomly bad, or worse. The alignment is such that most of the plate current occurs in one spot leading to a bright red spot at dissipations of 15 or 20 watts. The good ones will light up one side of the plate at 25 watts. I decided that these tubes are no good for any class A applications with continuous dissipation near 25 watts or more. I was building P-P guitar amps in the 35 to 40 watt range with 375 volts on the plate and the tubes that lived for 15 minutes with the amp head sitting on top of the speaker seemed to last OK. None would work in an amp with 450+ plate volts. Those amps got Sovtek 6550WA's.
The Audio Research pair are decent and well matched, but nowhere near capable of eating 40 watts continuously. I use them for testing experiments before plugging in the good stuff.
One of the untested tubes is shown here:
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This suuports my suspicion that these are "banging around tubes" (hopefully not too much of that "bang") and not much more. They may be of a later vintage than George's, as the "Audio Research" is in black. Maybe there was even some time for the manufacturer to get his act in gear. I stole a picture so folks can see what they look like. I guess I'll have to buy some EH KT88s for more serious work.
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Chinese KT88s have an up and down history. Early specimens definitely were "firecrackers". Then, the maker got his act together and built some pretty good tubes. Good things came to an end. The most recent stuff is trash. Important things, like multiple getters and good Q/C, are missing in the latest iteration.
I scored an octet of EH KT88s for about the same price per tube as the iffy Chinese versions shown here. I should be set for a while.
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Keep your fingers crossed on that one, I've lost two so far in low stress series pass regulator service and in just a couple of years of use. I've also lost a Tungsol 6550 reissue in the same sort of service under even lower stress conditions. (230V plate to cathode 45mA).
By the same token, people sing the praises of the EH KT88 as being one of the best sounding, as well as some Penta Labs units made in China. We'll see what happens when I actually apply voltage. These are "rainy day speculation" tubes and we're out of the rainy season here in Cali, so that may not happen for a while.
The EH KT88's are the best tubes for rock music in an SSE. Some users have seen early failures, but my SSE is still on the original pair despite being biased at a power transformer cooking 100mA on 450 volts. The amp is 4 years old and saw daily use for about 2 years. The Hammond sourced Allied 6K7VG power transformer is rated for 150 mA and I am sucking about 220 mA from it. It gets too hot to touch and smells funny after being on all day. The amp hasn't missed a beat since blowing the IXYS diodes on day 2.
The New Sensor sourced Tung Sol 6550's seem to show loss of getter after a few months. Several users have seen this.
Maybe the New Sensor stuff is batch related. Some Sovtek 5AR4's refuse to die, and some blow on initial power up.
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