Biasing KT88 question/issue on Luxman

Since these are mono blocks in push pull configuration ( I believe) would you have to be able to bias each KT88 tube separately?
 
The 18K adjustable resistor in the schematic in your post #1 sets the 'basic' bias, but by turning the 10K adjustable resistor you can 'fine-tune' this, so as to let each KT88 pass the same current. Turning the 10K adjustable resistor will make the bias on one KT88 go up a bit, while the bias of the other KT88 goes down a bit (and vice versa, when turning the 10K adjustable pot in the other direction).
 
Thank you! Back to figuring out why bias on tube 1 is fluctuating. Will do some more tests after I replace or fix the holders.
 
EL504 said:
The two 10 Ohm resistors are not on your schematic so the schematic is not 100% correct.

The plate voltage is on pin 3. Also measure the screen grid voltage on pin 4.

The two blue wires are for the heater (pins 2 and 7).
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Maybe, that 10-ohm resistor was added later? :idea:
 
Mark 1234 said:
I did see that one but I think that is for the regular MB88. In this description is sounds like they changed things but not sure what.
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Yeah. No version number on the schematic. :(

All other ones on the Net, want you to buy the PDF. Kinda crazy, if you don't know it the schematic matches your particular model.

Do you have two VR pots for balance (5K) and bias (0.5K)?
 
Mark 1234 said:
Thank you! Back to figuring out why bias on tube 1 is fluctuating. Will do some more tests after I replace or fix the holders.
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might be worth checking the 100 ohm screen resistor. If thats dying for some reason (usually a tube failure) it will affect the screen voltage which will show up as dancing voltage across the cathode resistor.
 
gadget73 said:
might be worth checking the 100 ohm screen resistor. If thats dying for some reason (usually a tube failure) it will affect the screen voltage which will show up as dancing voltage across the cathode resistor.
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Are the screen resistors 100-ohms or 500-ohms? :idea: I don't like it when companies change circuits and not edit their schematics.

Thanks!

luxman_mb_88_schematic.jpg
 
Odd but I had a magnetized 300B tube (Maybe stored close to a magnet) that BIAS fluctuated often in warm up process.

After using a demagnetizer the problem was solved.

Note: Don't use a demagnetizer on a 6SN7 tube, I damaged some, on smaller or bigger tubes that may be fine.
 
gadget73 said:
might be worth checking the 100 ohm screen resistor. If thats dying for some reason (usually a tube failure) it will affect the screen voltage which will show up as dancing voltage across the cathode resistor.
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That is one of the first things I plan on checking when the KT88 is tight. Very loose now.
 
Mark 1234 said:
That is one of the first things I plan on checking when the KT88 is tight. Very loose now.
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I presume you mean the socket-to-tube contact is loose. These tubes run hot. Expansion and contraction, with heat cycle causes more loosening with age, than most tubes.

I usually reserve my good Amphenol Micanol sockets for 6550 and KT-88. Or amps that run their output sections in Class "A" topology.
 
Mr Boochie said:
Are the screen resistors 100-ohms or 500-ohms? :idea: I don't like it when companies change circuits and not edit their schematics.
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hard to say because these two schematics show a completely different output stage design, different enough that I'd actually make the case they shouldn't even be considered the same amplifier.

Whatever its got going on, if there is a screen resistor, it needs to be good.
 
My first drawing is pretty close. Pin 8 to ground 10 ohm, pin 4 to ground 100 ohm and pin 5 to ground 3.9k. Just not showing the Cathode to ground 10 ohm on schematic.
 
Like Mark wrote in his post #20, the schematic Mr Boochie posted is for the "Luxman MB88". The amplifiers of Mark are "Luxman MB88u" (in which the "u" stands for "ultimate"), which, like gadget73 already wrote, is quite different from the "Luxman MB88".

Note that the first picture Mark uploaded in his post #12 shows the type number of the power transformer to be S-2633, which corresponds with the schematic of the MB88u in post #1 (while the schematic of the MB88 shows a different type number for the power transformer). Very likely the amplifiers of Mark don't have 5AR4's in them.

The 100 Ohm resistors at pins 4 of the KT88's are connected to B+, so not to ground. The 3.9K resistors at pins 5 of the KT88's are grid stoppers and are connected to the cathodes of the preceding 12AU7, so also not to ground.

I think it is safe to assume that the 22K adjustable resistor has the same function as the 18K adjustable resistor in the schematic of the MB88u, so it sets the 'basic' bias voltage. To be sure, you can check this by looking at the values of the resistors around the 22K adjustable resistor (according to the schematic the two outer terminals of the 18K adjustable resistor connect only to 33K resistors, while the two outer terminals of the 10K adjustable resistor both connect to 68K and 2.2M resistors).
 
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Mark 1234 said:
I could be wrong, thought it was a buss bar in my picture above but it might just be a connection block.
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Yeah, I'm guessing that they go back to some tap off the power supply.

The way Luxman or whomever drew these schematics is a bit unconventional. Artistic, but unconventional.

veggie_tales_-_im_confused_sCH6AV5aobd7UqtdAkk1gt.gif
 
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