Luxman MB88 ULTIMATE

[jqmce]

Hello AK members.
I purchased a pair of Luxman MB88-Ultimate amplifiers and they have some issues.
I’m looking for a service manual and full schematic.

I did find this post but none of the schematics are complete and or correct.

Thread 'Biasing KT88 question/issue on Luxman'

Aug 19, 2023

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?

• Mark 1234
• Replies: 81
• Forum: Tube Audio

Thanks

 

[Dswankey]

Hello AK members.
I purchased a pair of Luxman MB88-Ultimate amplifiers

I did find this post but none of the schematics are complete and or correct.

What's not correct? it looks to me that it's the OEM schem from Luxman

See attached pdf

 

[jqmce]

Thanks Dswankey,
The
Schematic has no expected voltages given.
There are numerous components not shown on schematic.
For example pin 5 on KT88 have resistors connected but this is not shown on the schematic. (I did check other examples online and they all have this resistor in place)


Am I missing something really silly here?

Thanks

 

[jqmce]

Correction for above.
There is 10 Ohm resistors between pins 8 and
-ve rail that is not shown in schematic.

 

[Brice]

The 10R resistors on pin 8 (Cathode) are current sensing device to measure the current in the tubes. This is pretty much a standard. All good.

On pin 5 it's a grid stopper resistor, so nothing extraordinary here.

 

[jqmce]

Thanks Brice,
I found the issues. On both amps the 100ohm resistor (screen grid pin 4) for the KT88 tubes were blown. Perhaps someone in the past played musical tubes with a bad one? Who knows.

Second issue was the 110v RIFA protection caps swollen / cracked, measuring bad. Hate these things.

Third issue was bias was set really poorly. Was all over the place, but dialed in just fine.

These sound so different to the Luxman MB3045. It’s wild.

Thanks to all for the help.

Cheers

 

[gadget73]

Bad tube would do it, have run into that before on other amps. Last was actually an MB3045, replaced the resistor and the current ran off the rails. Tube was bad.

very different circuit on this, looks actually quite a bit like a Heath W6. Williamson front end plus a cathode follower stage to drive the output tubes into grid current a bit.

 

[jqmce]

Thanks Gadget73.
Yeah totally different methodology and the sound is bonkers different too… even though the graphs look fairly similar.

I have requested a service manual from Luxman but I’m not holding my breath. There is very little info out there on this amp. There are numerous components that are not on the (lovely but artistic) schematics for this unit.
Hopefully someone, somewhere has the info.

Fingers crossed.
Thanks all.

 

[jqmce]

Does it make sense that the bias across the 10ohm resistor be set at 25ma on the cathode?
This seems low to me.
I read the previous post (link above) again and again and I can’t figure why Luxman told Mark1234 that it should be set at 25ma in pentode mode?

I’m I missing something silly here? All my previous kt88 amps were set at 50 to 60ma.

Thanks in advance for your thoughts.

 

[gadget73]

If thats what the spec is, then thats where I'd set it. His post says he got that info from Luxman, so I'd hope they know their own gear. Not always the best assumption but so is any other bias point without seeing it in writing in the original documentation.