Biasing KT88 question/issue on Luxman

Mark 1234

Mark 1234

Addicted Member
Just bought two Luxman MB88 Ultimate monoblocks. I cannot find a manual or biasing info. Below is all I can find. I measured the Cathode (pin 8) to ground voltage on the 4 KT88's to see what they are set at. They are all about .260mV.

I am trying to figure out why tube 1 on each amp has that value fluctuating up and down from about .190 to .320? Could this be a bad bias variable resistor or tube socket?
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Does the bias current fluctuate after 20 minutes warm-up, when no music is playing, with the input disconnected or shorted?
 
maxhifi said:
Does the bias current fluctuate after 20 minutes warm-up, when no music is playing, with the input disconnected or shorted?
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I was measuring bias voltage after 20 minutes with an 8 ohm resistor across the speaker plugs and amp not connected to anything. No music.
 
Mark 1234 said:
I was measuring bias voltage after 20 minutes with an 8 ohm resistor across the speaker plugs and amp not connected to anything. No music.
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1. When you swap the two output tubes, does the unstable bias remain on V1, or does it move to V2?

2. If it stays on V1, try and swap the second and third 12AU7 tubes. in this amplifier, the bias is set through the 12AU7, so a bad 12AU7 could also affect the stability of the bias.

3. Make sure pin 5 on the output tube sockets is tight. This can be done with a tiny flat blade screwdriver, after the amplifier has had a chance to cool down, and bleed off all voltage.
 
I did find out the output Alps pot on the rear was really bad. Ordered new ones. I hooked it up to see if I got sound and touching that dial created awful static and loudness fluctuations.
 
Thank you. I also ordered new kt88 sockets. Tubes seemed very loose. I did test the 12au7's and all were well above weak but after changing the Alps VR and the sockets I will see if things are better and if not try swapping tubes. Thank you again.
 
You may want to try tightening the sockets, and cleaning the pot before resorting to replacement. A squirt of contact cleaner, and working the pot back and forth, and then re-tensioning of the sockets will probably be all it takes.
 
I thought about detoxing the pot, may try that, but don't see any way to make the socket hold the KT88 pins tighter.

Those pots have screws to take one side off so should be able to clean easily.
 
Usually you can carefully get a pick or small screwdriver between the insulator and the metal contact to bend it a bit tighter. Exactly how depends on the contact style
 
The schematic you posted shows the cathodes of the KT88's connected to each other and to earth. The voltage between the cathodes of the KT88's and earth would therefore be 0 Volts.

So I guess this schematic doesn't represent your amplifiers.

I think that in the posted schematic, the bias is set with the 18K adjustable pot, while the balance between the KT88's is set with the 10K adjustable pot. To measure the bias of each KT88 you would need to measure the voltage between each grid of the KT88's and ground. To know if the bias is within reasonable limits, you need to know the plate voltage of the KT88's.
 
Unless it's been modified for cathode resistors, which is common. I don't trust the grid voltage method. It relies on tubes that accurately match the datasheet values and that's asking a lot sometimes.
 
I think the drawing is correct. I was measuring across the larger 10 ohm resistor. Pin 8 to ground. Would the plate voltage be across the two blue wires from the power transformer?20230821_195805.jpg
 
adnick said:
Mark, here’s an informational video on biasing, it may not answer your question, but it’s worth watching….

Andy
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Thank you but I don't see a link.

I was thinking it would bias like my other Luxman amp where I measure across pin 8 and pin 1 and adjust to .4-.5mV but it looks like this is different?
 
I downloaded what looks like a more complete schematic of your amp. Looks like you have two pots. One for hum/balance (VR2) and one for bias (VR.5K). :idea:

luxman_mb_88_schematic.jpg
 
Mark 1234 said:
I think the drawing is correct. I was measuring across the larger 10 ohm resistor. Pin 8 to ground. Would the plate voltage be across the two blue wires from the power transformer?View attachment 2965020
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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).
 
Mr Boochie said:
I downloaded what looks like a more complete schematic of your amp. Looks like you have two pots. One for hum/balance (VR2) and one for bias (VR.5K). :idea:

luxman_mb_88_schematic.jpg
Click to expand...
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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