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    Unbiased opinion about KT 66

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    Tube Nube

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    Unbiased opinion about KT 66

    Post by Tube Nube on Wed Sep 02, 2009 6:15 pm

    I was undecided whether to send this as a PM, or to post it. Thinking it might be helpful to others, I decided I'd post.

    Here's the deal. I swapped in some Gold Lion repro. kt 66 tubes into the VTA ST70, as recommended by Bob and Junky Jan.

    The amps been singing along nicely for months with many hours of play. Probably more than 50 hours already.

    I pushed the tubes down gently in their sockets (power off), so they all looked... well, straighter. When I turned the amp on, the front left KT 66 was glowing brighter than all the others -- way brighter, and I was hearing popping from the left speaker. I checked the bias, and it was way out.

    VOM read about .44 on both right side tubes, but on the left, I had something like 3.6 and .22.

    I was able to re-bias, and the noise (and brightness) settled down.

    Next day, same problem, but this time the front left potentiometer had no impact on the bias voltage at all.

    I assume I've got a problem with the tube, so tonights plan is to move the tubes around and see if the problem follows.

    If so, that'll tend to confirm it's the tube. If not...I guess I've got a problem inside, and better open it up and snoop around.

    Alternately, I could go back to KT 88s I've got in the closet.

    Suggestions and advice are eagerly welcome, and I hope this isn't a waste of bulletin board space.

    Thanks.

    Bugs

    Posts : 91
    Join date : 2009-04-17

    Re: Unbiased opinion about KT 66

    Post by Bugs on Thu Sep 03, 2009 10:01 am

    I've been using Genalex KT66s in my ST70 for a few months now. I've had single problem right after I purchased the tubes where one of the tubes started to glow and would not bias. I contacted the distributor and he replaced the tube and it has run fine since. As Bob recommended, I've been keeping them biase at .45 V.

    I've been happy enough with the tubes to buy a second quad for another ST70 I just rebuilt with a VTA board and it has been running great for a few weeks.

    My input power transformer on my original ST70 picked up a pretty bad hum, but I believe that is a seperate story.

    Not much help here, but that is my expereince with the tubes.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Unbiased opinion about KT 66

    Post by Bob Latino on Thu Sep 03, 2009 11:29 am

    Tube Nube wrote:I was undecided whether to send this as a PM, or to post it. Thinking it might be helpful to others, I decided I'd post.

    Here's the deal. I swapped in some Gold Lion repro. kt 66 tubes into the VTA ST70, as recommended by Bob and Junky Jan.

    The amps been singing along nicely for months with many hours of play. Probably more than 50 hours already.

    I pushed the tubes down gently in their sockets (power off), so they all looked... well, straighter. When I turned the amp on, the front left KT 66 was glowing brighter than all the others -- way brighter, and I was hearing popping from the left speaker. I checked the bias, and it was way out.

    VOM read about .44 on both right side tubes, but on the left, I had something like 3.6 and .22.

    I was able to re-bias, and the noise (and brightness) settled down.

    Next day, same problem, but this time the front left potentiometer had no impact on the bias voltage at all.

    I assume I've got a problem with the tube, so tonights plan is to move the tubes around and see if the problem follows.

    If so, that'll tend to confirm it's the tube. If not...I guess I've got a problem inside, and better open it up and snoop around.

    Alternately, I could go back to KT 88s I've got in the closet.

    Suggestions and advice are eagerly welcome, and I hope this isn't a waste of bulletin board space.

    Thanks.

    Hi,

    It's probably the tube. You will be doing the right thing by placing the tube in another socket and see if the problem follows the tube or is with the socket. If the problem turns out to be the socket check ...

    1. The coupling capacitor on the driver board for that tube. You should read an "open" (no continuity) across that cap.
    2. The 1000 ohm resistor between pins 5 and 6
    3. The 270,000 ohm resistor on the edge of the driver board
    4. The 10 ohm bias resistor for that tube

    I have had the Genalex KT66's in my own ST-70 for almost a year with no problems at all. They sound great and bias easily. The use of a KT88 in an ST-70 should only be done on an ST-70 with an upgraded transformer. For those that are reading this > an original PA-060 ST-70 power transformer has a stack lamination of 1 1/2 to 1 5/8 inches while the upgraded replacement ST-70 power transformers are usually 2 to 2 1/2 inches. The use of a KT88 in an amp with an upgraded power transformer will not give you any more power in the ST-70 but the KT88's (or a 6550 tube) will last longer than EL34, KT66, KT77 tubes because the KT88 and 6550 tubes are designed to look at higher voltages. In an ST-70 they are being operated at maybe 60 to 80 volts LOWER than what they were designed to look at.

    Bob

    Tube Nube

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    Re: Unbiased opinion about KT 66

    Post by Tube Nube on Thu Sep 03, 2009 2:16 pm

    Hi Bob and Bugs,

    Thanks for the replies here.

    Last night I shifted the front tube to the rear. Same side of the amp though.

    The tubes were all easily re-biased, though I thought I heard a bit of noise (intermittent pop) while I played music.

    I gently tapped the tops of the tubes with my finger, with no music playing, and did hear some so-called "microphonic" noise, but only on that one problematic tube. The others were silent. Even when I tapped them a little harder.

    It's looking more and more like the one tube.

    Perhaps I'll now switch it over to the right side. Play it a while longer, and see what happens.

    Or, do I risk blowing a speaker driver?

    Maybe I should just order a new tube. Or, should that be a matched pair?

    (In future, instead of ordering a matched quad, It'd probably be wise to order a matched 6-pack, in order to have perfect replacements).

    Tube Nube

    Posts : 603
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    Re: Unbiased opinion about KT 66

    Post by Tube Nube on Thu Sep 03, 2009 2:18 pm

    Oh, let me add, Bob, thanks for the specific instructions on just what to do, should I need to take the bottom off. That's helpful advice.

    Bob Latino
    Admin

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    Re: Unbiased opinion about KT 66

    Post by Bob Latino on Thu Sep 03, 2009 3:10 pm

    Hi Tube Nube,

    With the VTA driver board you don't absolutely need a "matched pair" but what I would do in your case is order a NEW matched pair - then put the new matched pair in that channel and then keep the one good tube of the older pair as a back up tube. In the VTA ST-70 the rectifier and the output tubes are the most likely to go bad. The 3 driver tubes can last 5000+ hours. When you buy the new matched pair keep the sales slip in a safe place. Most tube vendors will give you a 90 day warranty on the tubes.

    Bob

    GP49

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    Re: Unbiased opinion about KT 66

    Post by GP49 on Thu Sep 03, 2009 4:47 pm

    Tube Nube wrote:(In future, instead of ordering a matched quad, It'd probably be wise to order a matched 6-pack, in order to have perfect replacements).

    Once you've played the four originals for months or years, if one goes out and you install one of the brand-new ones, how well is that going to match?

    JunkyJan

    Posts : 102
    Join date : 2008-12-09
    Location : BC, Canada

    Re: Unbiased opinion about KT 66

    Post by JunkyJan on Mon Sep 14, 2009 1:38 am

    Hi Brenton...

    Sorry to hear about the hassles you had with these tubes. I bought a matched quad about a year ago, my son "borrowd" two out of the quad so I had to buy another two. Not one of the six has ever exhibited the symptoms you describe - and it has been running many, many hours already (so much so that the red bakelite bases are discolouring, becoming darker & darker). This must have been a bad tube that slipped through their quality control.

    If this will make you feel any better, the Quad II fanboys run these New Sensor - made Genalex KT-66 Gold Lions very successfully in the Quads (IIRC the Quad II monoblock amp was a self-biasing design specifically for the original Genalex KT66 tubes in mind) and they consider that these repro tubes are a "worthy" alternative to the staggeringly-high-priced original Genalex tubes.

    So, I also run a non-matched quad - and had no issues with the biasing, that has been rock-solid for months now.

    I hope by this time you have received a replacement... Any thoughts or observations regarding the sound? I ran these in a SET class A amp at one stage, and it was like night and day compared to the original tubes it came with.

    -- Junky

    Tube Nube

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    Join date : 2008-12-06
    Age : 53
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    Re: Unbiased opinion about KT 66

    Post by Tube Nube on Mon Sep 14, 2009 11:09 am

    Hey Jan,

    Nice to hear from you!

    I haven't got the new tubes yet. The last set came fast, but I forgot what postal option I chose then. I k now I chose a different, slower delivery method out of concern to avoid UPS, who charge a "brokerage" fee for everything. It adds between 1/3 to 1/2 the cost to the purchase price. There's quite a bit of gumbling on the web about it. As I understand it, there's even been a class action lawsuit or two.

    When I get the new ones running, maybe I'll do a comparison with my original kt88s for fun.

    I find it only a slight inconvenience to be ordering another tube so soon. It's part of the cost of having this type of gear. And it necessitates more involvement with, and a better understanding of the equipment than that boring old plug-and-play transistor stuff.

    ;-)

    I sure delight not only in the joyful sound of this amp, but also in the curiously delighted looks and comments by everyone who's seen it. "WHAT is THAT?!"

    JunkyJan

    Posts : 102
    Join date : 2008-12-09
    Location : BC, Canada

    Re: Unbiased opinion about KT 66

    Post by JunkyJan on Sun Sep 20, 2009 2:16 pm

    Hey Brent

    Good to see you're doing good too! I'm relatively quiet because of still being snowed under with things work-wise, else I would be far more "vocal".

    I'm hoping that you will be able to do a comparison between your current tubes and the KT66s... I have no idea what the sound quality difference will be like to you, but as I said, it was like night and day when I plugged it into a 6L6GC / 5881 / KT66 - based SET power amp.

    -- 'Jan

    Tube Nube

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    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: Unbiased opinion about KT 66

    Post by Tube Nube on Tue Sep 22, 2009 2:18 pm

    Hi Jan,

    Well, the two new KT 66's came yesterday and I was eager to hear them. I did a quick comparison, first the Commy Tron (Sov Tek)Kt 88's, with a Mingus recording, Fable for Faubus. Then, swapped in the 66's, and again with Faubus.

    I really do need to spend some more time going back and forth. One thing that struck me...or, less emphatically, perhaps it was more of a nudge than a strike... I think the kt88s were more brash.

    More than what I can "quantify", there's a sense that I just want to listen more to the amp with the Gold Lions than the Sov Teks. Maybe it's a fuller sound? Smoother? I can't really say what.

    Maybe Sunday I'll sit down and have a longer go at it.

    I really do like the look of the 66s also, with the red-brown base, and the larger size relative to the 88s.

    Can't imagine how immasculated the thing would look with EL 34s. Speaking of which...so, you're going for a pair of monoblocks!

    I look forward to hearing your update.

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