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Biasing KT88 question/issue on Luxman

gadget73 said:
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this amp is triode mode, not UL, and probably runs in AB2 mode but for voltage and current purposes at idle its pretty similar. The Ia+g2(0) line is idle current plate + screen with zero signal, 50ma per-tube in the 460 plate volts column.
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The schematic of this amp shows tetrode connection, so not triode mode.

These are the frequency response and distortion graphs from the manual of the Luxman MB88 Ultimate. These show 60 to 70 Watt maximum output power.

Luxman MB88 Ultimate graphs.jpg

Based on these graphs, the measured plate and screen voltages, and datasheets for the KT88, I think this amplifier runs in AB1, so without control grid current.

Running this amp at 50 mA per KT88 at idle seems a bit low to me. I would think 60 mA per KT88 at idle (which results in 27.78 Watt plate dissipation) would be more in line with the datasheets for the KT88.

Edit: The manual I found is in Chinese. I couldn't find info on bias in it. But if Luxman is still in business, you could ask them.
 
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err, yeah, pentode mode. Looking at the wrong thing, too many posts. Either way the voltages are about the same, so idle current should be comparable.

Surprising the manual would be in Chinese for a Japanese company.
 
Mr Boochie said:
I think many current Luxman products are partially or wholly made in China. :idea:

https://onahighernote.square.site/p...-vs-mq-88u-made-in-china-by-subcontractor-/11
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The CL36u (pre) MB88u and LX38u were some of the last designed one of Luxman's famous designers (before Alpine)

from Luxman's History:
The MB-88 limited edition amp (run of 200) was produced. This is one of the last pieces of work Chief Engineer Mr Susumu Uehara, one of Luxman's longest-standing engineers.
 
EL504 said:
Probably my wrong. It likely is in Japanese.

I found the manual here (its size is 5.38 mB, so too large to upload to the forum):

Luxman MB88 Ultimate Manual
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That is Japanese.

The Photo Translator app, available in Google Play. It translates Katakana and Hiragana pretty well. Kanji is okay. You take photo of screen with the app and it translates it, in a few seconds. Many languages.
 
After reading through this thread, I started looking at KT-88 prices.
I'm glad I don't have to buy KT-88's right now for my M-125 mono blocks!
Then I looked at prices for EH 6CA7's since thats what I have in my ST-70.
I payed around $25 each a couple years ago for a matched quad.
The prices I see now are shocking. :yikes:
 
Lavane said:
After reading through this thread, I started looking at KT-88 prices.
I'm glad I don't have to buy KT-88's right now for my M-125 mono blocks!
Then I looked at prices for EH 6CA7's since thats what I have in my ST-70.
I payed around $25 each a couple years ago for a matched quad.
The prices I see now are shocking. :yikes:
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Russian-Ukraine war is affecting tube prices. :idea:
 
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maxhifi said:
All of the current ones I've come into contact with are as Japanese as is presently possible. In fact, the new Luxman amplifiers are top notch equipment, I'd rank them in my top 5 new production tube amps.
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Yeah, I really wasn't implying quality issues with Made in China parts or sub-sections. It's the company's prerogative to maintain QA onshore and offshore.

After all, Apple has been manufacturing products in China for decades.
 
Mr Boochie said:
Yeah, I really wasn't implying quality issues with Made in China parts or sub-sections. It's the company's prerogative to maintain QA onshore and offshore.

After all, Apple has been manufacturing products in China for decades.
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Yes, totally agree, but "Made in Japan" is a big selling point for the Luxman gear. And poking around inside it, you can see why.
 
maxhifi said:
Yes, totally agree, but "Made in Japan" is a big selling point for the Luxman gear. And poking around inside it, you can see why.
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Yes, I agree. That's a big selling point for Japanese cars. Cars made from parts fully made and assembled in Japan, can be a dividend. :thumbsup:
 
Mr Boochie said:
You know, the schematic in that Japanese manual has the KT-88 in what looks like triode mode. :idea:
Click to expand...
The screen grids are connected through 100 Ohm resistors to the centertap of the output transformer, so to B+. That is tetrode connection, not triode mode.

Perhaps the fact that the beam forming plates are not visible in the symbols for the KT88's causes the confusion?
 
EL504 said:
The screen grids are connected through 100 Ohm resistors to the centertap of the output transformer, so to B+. That is tetrode connection, not triode mode.

Perhaps the fact that the beam forming plates are not visible in the symbols for the KT88's causes the confusion?
Click to expand...
Really, one of the more "creative" schematics I've seen. ;)
 
Posting an update.

I heard from Luxman Japan and the correct bias is 25mA for the MB88 Ultimates. They were set correctly.

I ended up replacing all the electrolytic and film capacitors, the 10 & 100 ohm resistors off the kt88's, one tube, trim pots & kt88 sockets.

Both are stable and sound fantastic now.20230904_133631.jpg

The original UK Gold Lion KT88's sure are something special!!!

Thank you all for your help!
 
There is no KT-88 like the original British Gold Lions. I even keep the original boxes and those cardboard inserts. Keep the amp's bias adjusted and they will last.

:thumbsup:
 
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