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MB3045 KT88 Bias Levels

24.177.8.38
Posted on June 18, 2009 at 08:00:23
geraldm121
Audiophile

Posts: 748
Location: Massachusetts
Joined: January 9, 2007
 Earlier this week I posted to the Amp/Preamp Asylum about Changing my Luxman MB3045 from 8045 outputs to KT88's. Thanks to all that responded. The change over was not a problem as the KT 88's are in and running along with 6CG7's replacing the NEC6240 drivers. The Luxman modification bulletin suggests biasing at .5v. My issue is that the amp when Biased to .5v(after the modification) sounds rather bright and sibilant which usually indicates to me it is under biased. The 8045's suggested bias was .75v and while I realize the KT88's cannot take this level how far can I go with the bias level without running in to issues? Thanks in advance for your help. Oh, and Mike Samra I was going to call you later what time is good for you?

Jerry

 

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KT-88 bias in MB-3045, posted on June 18, 2009 at 11:43:39
elektron
Audiophile

Posts: 952
Location: midwest
Joined: February 18, 2004
 Jerry, hi. I have been working on the Luxman MB-3045 since buying a new pair circa 1982. Yes, 50 mA in the Lux update service documents. Did you see the changes in the feedback RC network? Are your amps using triode tied KT-88 or are you running them with specific screen B+? Don't give up on them. MB-3045 are among the best push-pull amps I have ever heard. In the aftermath of the 3045 US introduction and the 8045G fiasco there were lots of mods. Futterman used 6LF6. Underground Sound used EL34. GSI had mods too. Good 6LF6 are almost extinct. Tim deP wrote me 20+ years ago during my quest for reliable 3045 information and said the amps were offered in JP with a choice of tubes including KT-88/6550. I have used original TungSol with good effect. I have winged =C= in them at the moment. If you want to chat let me know. Glad to do it.

 

RE: KT-88 bias in MB-3045, posted on June 18, 2009 at 13:45:17
geraldm121
Audiophile

Posts: 748
Location: Massachusetts
Joined: January 9, 2007
 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

 

RE: KT-88 bias in MB-3045, posted on June 18, 2009 at 20:10:59
elektron
Audiophile

Posts: 952
Location: midwest
Joined: February 18, 2004
 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

 

RE: KT-88 bias in MB-3045, posted on June 19, 2009 at 00:26:58
geraldm121
Audiophile

Posts: 748
Location: Massachusetts
Joined: January 9, 2007
 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

 

RE: KT-88 bias in MB-3045, posted on June 19, 2009 at 04:57:31
elektron
Audiophile

Posts: 952
Location: midwest
Joined: February 18, 2004
 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.

 

RE: KT-88 bias in MB-3045, posted on June 18, 2009 at 15:20:07
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 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.

 

RE: MB3045 KT88 Bias Levels, posted on June 18, 2009 at 10:29:59
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Jerry
If you have the genelexs,you can go 55ma..Its too bad the units are so so cramped that we cant put any cap upgrades in without building an aux chasis..
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: MB3045 KT88 Bias Levels, posted on June 18, 2009 at 10:07:57
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Oh after six..I have go to the dentist and let her pluck on my teeth lol.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Bias Levels, posted on June 18, 2009 at 08:58:05
Chris O
 Instead of using a guideline of 0.5v or 0.75v as bias measurements, you can understand the true tube dissipation pretty easily.

What you are measuring is the voltage drop across a fixed resistor. If that resistor is 10 ohms and the voltage drop is 0.5 volts you can use Ohms law (Volts/Resistance=Current) to calculate the current passing through the resistor. In this case 0.5v/10ohms = 0.05 amps, or 50mA.

Just for the sake of simplicity, let’s assume that the current is from the tube’s plate. If the tube has 400 volts on the plate and 50 mA of current, then the tube is passing 400 * 0.05 = 20 watts. A KT88 is rated for 42 plate watts. You can go up to 80% of max plate watts without too much trouble. I like 70% on average. 70% plate watts would be 42 * 0.7 or 29.4 watts.

With 400 volts and 29.4 watts, your current is 29.4/400 or 73.5ma. Using your 10 ohm resistor and Ohms law (reconfigured as V = R * C) then V = 10 * 0.0735 or 0.735 volts across your 10 ohm resistor.

Note: I do not know the value of your current test resistor, if it is for one tube or both tubes in a push pull pair, or the volts on the plate of your output tube. Measure these first. If you learn to calculate what you want, you will not be haunted with uncertainty.

 

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