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Tubes, KT88s Vs. Mothra

Discussion in 'Amps and Cabs [BG]' started by corinpills, Feb 15, 2003.

  1. corinpills


    Nov 19, 2000
    Boston, MA
    OK, tube freaks, where are you all? I have to replace two of the KT88s in my Trace Elliot V6. Of course, Ideally, I'd replace all 6 of them at once, but as 4 of them are OK, would it be a major hugey to just replace the two bad ones for now? Financial considerations, you understand. There doesn't seem to be any sort of bias adjustment on the amp.

    Also, are there huge differences in the different brands of KT88s? I have found some websites where people actually argue about the Electro Harmonix ones VS. Czech Republoc and Chinese ones, but they seem to be audiophile stereo freaks rather than bass amp freaks. Will the brand make a discernable difference in the tone? There definitely seems to be a pretty wide range of prices.

    opinions, please. I yield to your expertise.
  2. I have only used 6550's but have heard that the Sovteks and Electro-Harmonix (same factory) are really good and are not too expensive. The Tesla's are good as well. Not so sure about Chinese KT88's but it seems that Chinese KT88's have a better reputation than Chinese 6550's. If money is no object try and find some new or good used Genelex KT88's those were the best ones ever made or KR Enterprise is a Czech company that hand makes their own KT88's. Don't know anyone who have tried those ones.
  3. I've heard (and read) pretty good things about the Chinese tubes, but EL / Sovtek / Ruby are easy to find and relatively inexpensive. Much less than Genelex. Unfortunately the bias pots for the V6 are on the board itself if my memory serves me. You'll probably have to haul the chassis out of the case to adjust it.
  4. Shuguang Chinese KT88's have a good reputation.
  5. amper


    Dec 4, 2002
    Ah, and now we come to the major problem of KT88 amps, and the V-Types in particular. Yes, there is no external bias adjustment on the V-Type and V6 heads (there is on the V8), which means that ideally you need to replace your tubes with matched quartets or sextets, and matched quartets or sextets of KT88's are not only hard to come by, but expensive to boot (about $75/tube, last time I looked).

    Fortunately, I only have V-Type's, and I found a matched quartets of VHT KT88's on eBay for spares for about $50/set. I'd still get a V6, though, if I came across another one (kills me that I passed on what was probably the last NOS V6 in the world--it was only $800 new!).

    Since the V-Types (and most high-power tube amps) are Class AB, the tubes have to be matched to reproduce the waveform accurately. If they aren't matched, but still within spec, they'll work, but your tone may or may not be what you expect. But, as long as they aren't wildly out of spec, you won't cause any damage.

    As far as brand, I've heard that Svetlana makes the best current production KT88's, but as I haven't had any failures yet, I haven't needed to buy any, so I can't really speak from personal experience. There are still NOS types out there, but expect to pay a princely sum for them, as KT88's are more common for audiophile gear than for MI applications.

    Groove Tubes is reliable, and is probably the best source for side-matched dual pentode (ie, 12AX7) preamp tubes for phase inverters for Class AB amps (GT Special Applications Group). Whatshisface over at <http://www.guitaramplifierblueprinting.com/> has a great article on this subject. GT also has KT88's.

    As far as differences in brand. Yes, they are slightly different, but remember that MI apps are not as broad spectrum as reproduction equipment, so try not to buy or read too much into the NOS/brand difference arguments.

    Try giving Gibson a call. They may still have replacement parts in stock. I was able to get some parts for one of my V-Types last year.