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In Reply to: RE: KT-88 bias in MB-3045 posted by elektron on June 18, 2009 at 11:43:39
You sound like exactly the person I need to talk to. I too have owned these amps since the the late 70's.I did the basic (resistor changes and additions) tied triode modification per the 1st page of the LUX service bulletin as I wanted to stay as true as possible to the original configuration and hopefully the keep the sound I had been so happy with. The following pages of the bulletin seemed to relate only to a mod that reconfigured the amp into a pentode circuit. Kt 88's seemed the way to go as availability should be good for quite some time and I hate changing something I have been happy with. Also I have used Kt88's as triodes before successfully.
The issue as I mentioned above is that with the current Sovetek KT88s I am using the sound is rather shrill. It could just be the sound of the Soveteks or it could be that they are under biased. I wondered what the maximum safe bias would be for the Soveteks? Unfortunately I only have the Sovetek KT88's, some SED 6650's and a pretty well used set of (my all time favorite) GE 6550's at this house. I do have a set of the Genelex (Russian)re-issues and a couple sets of new Shuguang KT 88's at my vacation home.
So with all that said any suggestions or comments?
Regards, Jerry
Follow Ups:
Jerry, hi again. Caucasian Blackplate and Chris O have good advice. But, I think you are up to speed on bias setting. I did my 3045 triode - g2 tied to the plates with a 100 Ohm R as I recall. I think you are correct on the RC time constant network being for what Lux called 'Ultra Linear' although obviously a misnomer. My 3045 documents are at another location as I write this. The so-called 'UL' operation was merely putting some B+ on g2. Obviously there are no Ultra Linear windings. One of the reasons the stock amps sounded so good was the output tubes were being run on the threshold of thermal runaway. Jeez did they get hot. The NEC made for Lux 8045G output tubes appear to be internally triode connected horizontal output tubes. G2 is tied to the plate, beam plates connected to the cathode. 485 V is B+ to the finals. Funny, I never had my amps sound shrill. As you say probably the kind of KT-88 and bias. I would like to see exactly what Futterman did on them. The 6LF6 is probably closer to what Lux had NEC gimmick up than either 6550 or KT-88. On the other hand, if you are going to run same at 75 mA I'd be prepared for early failures much in the pattern of the original 8045G fiasco. Sorry it took so long to post a reply. Turned out to be a much busier day here than anticipated. Shoot me your phone number via AA e-mail and I'll try to phone over the weekend. Let me know when it would be convenient to call. I have toll free in CONUS. These amps can be a challenge with typical one inch deep chassis. Still, there are things that can be done and some that must be done. Ultimately unless Nichicon decides to make a double 220uf at 550 and another triple whatever at 550 WVDC re-do of PS electrolytics is impossible without outboarding
Electron,
It turns out that the Sovteks were the issue. When I got home tonight I pulled the KT88s and replaced them with a quad of Winged C 6550's (luckily when I looked they were original Svetlanas Winged C's not SEDs)and Biased them at 51ma. No more shrillness. The sound now had the same characteristics as when the 8045G's were installed. Funny you should mention the 220mfd Nichicons as I just noticed one of mine had started leaking any suggestions for a replacement source? The Final tube complement for each amp ended up as follows:
12AX7'S - Mullard Re-Issues
12AU7'S - Amperex UK
6240's - Zenith Branded (Probably Sylvania)6CG7's
KT88/6550-Winged C Svetlanas
Mike Samra and I have spoken about the lack of room in the chassis and to me maintaining the original look is important especially with an amp that looks as good as this one (It drove Mike crazy that I insisted he maintain the original look when he was working on my Mac 1500).
I would like to speak with you more about the Luxman and I also Have an original Futtermen H1a (the 6LF6's are as much unobtanium as the 8045Gs) but I am leaving on vacation tonight returning on the 29th can I reach you sometime the week of the 29th?
So thanks to all for the help. From Chris O & Caucasian Blackplate for the education and to you for the specifically Luxman Insights.
Regards, Jerry
Jerry, hi. Yes. That should be fine. No worries while on vacation. You can contact me when you get back home and get settled. Once upon a time I had a pair of H3aa (6LF6). Julius kept adapting the latest advance in tube building to his amps. The Amperex 6LF6 was the zenith. What a bottle that tube had. 40 W dissipation. Have fun on vaca. When you get back we can talk.
Chris O. posted all the advice you need to find the acceptable range of biasing for your KT88's, but if you have indeed triode wired them, there are a lot of other variables.
1) Was the feedback network altered along with this mod? (Hopefully the feedback resistor went way, way up in value, or was removed completely)
2) What kind of plate voltage are you using? Lowering the plate voltage a bit and running higher current would likely decrease the shrillness you speak of.
If the mod left you with tubes in pentode mode, it is likely just popping open the cover, looking at whether you have one or two cathode current sensing resistors, and biasing the tubes a little hotter.
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