Luxman MB3045 biasing guide/help

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DanBun

New Member
Hello all and thanks in advance

I recently purchased a beautiful pair of Luxman MB3045 amps. They are running well but given the age and the previous history being unknown I would like to check the bias etc on them.

I have done a bit of a search online and thought I would come across a service manual, guide or similar on how to perform it but no such luck.

If anybody has the info or can put me in the right direction it would be greatly appreciated. I am a qualified electrician and have worked on voltages up to 330kv and have a healthy fear so I am aware of the danger.

Thanks439728254_770254151751033_6778552697983472003_n.jpg
 
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ah the MB 3045. Looks like they have been converted to run a KT88 in place of the impossible to find original tubes. I've heard 2 different sets of these amps and they are just a nice piece of gear. Not bad to work on either.

You're after this set of docs

Elektrotanya for electronics experts

Electronics forum for electronics experts to repair defective appliance.
elektrotanya.com

Its 0.5 volts across the test points whether its in triode or pentode (they call it Ultralinear, but its not ultralinear) mode. The manual has pics of where the test points and the adjustment pots live. Have to take the bottom off to do this.
 
@gadget73 Thank you so much for that. Exactly what I couldn't find. Easy peasy.

Might post tomorrow after I have done it

Yep already had the conversion done thankfully.

Thank you
 
DanBun said:
..... but given the age and the previous history being unknown
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Good looking set of monoblocks!
Always good to check bias. Anything else to do? I like to check the fuse, bias, tubes, general visual inspection, etc.
These are getting close to 50 yrs old? If no history or signs of maintenance, the electrolytic capacitors may be showing their age. I am not familiar with LUX or their caps, but you may be due for a refresh, especially in the power supply.
Looks like there are a couple of threads here on these amps. Let's get these off the carpet, sort out the power cord, & celebrate your new amps!
 
coupling caps at least, and its worth cleaning the bias and balance pots if they haven't been done. Not sure what the OE Luxman coupling caps were but I don't think they were anything all that amazing. The two different sets I've had my hands in both had them replaced already, but one had some heat damage on the board from failed couplers. I think both sets were still running original power supply caps though. If we're being fair these were built in the 1970s though so thats a bit long in the tooth.

you can probably find some pics on here somewhere of @Blue Shadow 's MB3045 and the board repairs he did where it got cooked.
 
gadget73 said:
coupling caps at least... I think both sets were still running original power supply caps though...
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A bit O/T...
I am curious why you may prioritize coupling vs PS caps. Are you thinking about sound quality vs vintage equipment care?
I always thought PS caps were a primary focus for vintage care vs smaller coupling caps. Yes, they may all dry out, but get the big caps 1st!?!
 
more concerned about nuked output tubes. If the coupling caps go leaky the tubes will roast. Also they are cheaper and easier to replace. I don't get into that endless debate about sound quality and what caps have the magic mojo sound inside. I just hate spending $100 on a vacuum tube because a $3 capacitor went leaky.

Its admittedly limited experience but given that the two sets I've worked on still had original power supply caps that were working properly and at least one I know had some damage from bad coupling caps it would lead me in that direction first. Not saying don't consider changing the filter caps, they are a solid half century old now. At a minimum it would be worth testing them to see what sort of functional shape they are in.
 
So had some luck and a bit of an issue.

I used this process to do the biasing https://www.audioasylum.com/messages/tubes/101820/biasing-the-luxman-mb3045

Should add that the valves used are 6550 v6,v5, 6gc7 v4,v3, 12au7 v2 and 12ax7 in v1. Also a few caps have already been replaced by a good tech but he doesn't like valve amps and didn't want to do anymore than that

I was able to set the bias and DC offset fine on mono No.1 with it's installed valves.

Mono No.2 I could set the bias to 0.5v but the DC offset would stay at 0.03v no matter how much I turned the pot. It would affect the bias voltage when turned. There was no issue with the valves and would play music.

So I thought I would swap the 6gc7 valves in v4,v3 from mono no.1 and same issue of 0.03v on DC pot.
I swapped in the 6550 valves from mono No.1 and same again.
I then swapped in v2,v1 so all valves from mono No.1 were now in mono No.2. After about 4 minutes 6550 v6 started to red plate. I powered down let it cool and powered up. I thought I would check the bias voltage and DC offset voltage after only a minute of powering on. Bias was good around 0.5v and DC offset was initially 0.4v but rising. I continued to measure until it rose to 1v across DC offset. After a few more minutes it began to red plate. I did this same process twice with same result of v6 beginning to red plate.

I am kinda at a loss of what issue may be. I assume that with the valves that biased fine from mono No.1 that they should be ok. The No.1 valves also allowed mono No.2 to read a voltage other than 0.03v on the DC offset. Maybe there is another issue in the amp?

Will try to get all valves tested to see what the go is with them.

Any help greatly appreciated

439352858_949900340106139_1668921314175126920_n.jpg439914854_1101176937764747_3930931683372112944_n.jpg
 
mrwood200 said:
Good looking set of monoblocks!
Always good to check bias. Anything else to do? I like to check the fuse, bias, tubes, general visual inspection, etc.
These are getting close to 50 yrs old? If no history or signs of maintenance, the electrolytic capacitors may be showing their age. I am not familiar with LUX or their caps, but you may be due for a refresh, especially in the power supply.
Looks like there are a couple of threads here on these amps. Let's get these off the carpet, sort out the power cord, & celebrate your new amps!
Click to expand...
Thank you very much.

Yeah we found an issue with a cap leaking so replaced all in the area (blue ones) by a proper technician who doesn't like valve amps so left the biasing to me. He gave them a once over too. Also replaced the pot for volume control as one was dead.

Haha yep only on the floor to test and yep power cable wasn't like that for long not that it is anything special, wasn't putting tension on amp either
 
gadget73 said:
coupling caps at least, and its worth cleaning the bias and balance pots if they haven't been done. Not sure what the OE Luxman coupling caps were but I don't think they were anything all that amazing. The two different sets I've had my hands in both had them replaced already, but one had some heat damage on the board from failed couplers. I think both sets were still running original power supply caps though. If we're being fair these were built in the 1970s though so thats a bit long in the tooth.

you can probably find some pics on here somewhere of @Blue Shadow 's MB3045 and the board repairs he did where it got cooked.
Click to expand...
Thank you

I do hope the bigger PSU caps are fine...
 
gadget73 said:
more concerned about nuked output tubes. If the coupling caps go leaky the tubes will roast. Also they are cheaper and easier to replace. I don't get into that endless debate about sound quality and what caps have the magic mojo sound inside. I just hate spending $100 on a vacuum tube because a $3 capacitor went leaky.

Its admittedly limited experience but given that the two sets I've worked on still had original power supply caps that were working properly and at least one I know had some damage from bad coupling caps it would lead me in that direction first. Not saying don't consider changing the filter caps, they are a solid half century old now. At a minimum it would be worth testing them to see what sort of functional shape they are in.
Click to expand...
Thank you. Given the above biasing issue on one I guess there may be an issue but both played music fantastically for quite a few hours. I'd think it wouldn't if these caps were dead?

Cheers
 
There is no offset adjustment on a tube amp, so not quite sure what you mean about that. Amp with output transformers really can't have any offset. This should have a DC bias and a DC balance pot. The bias you use to set the voltage across one set of test points to a level. The balance is used to make both test points the same. It will affect the bias on the first set of test points so there is a bit of back and forth to get it all happy but at the end of the day both should run at 0.5 volts. If you're 0.003 volts different thats not going to matter. I think these are 10 ohm resistors so 3 millivolts across it would be 30 microamps of difference. Nothing in the grand scheme of things, those resistors are probably mismatched enough just because of production tolerance that even if they showed exactly the same voltage the tubes are that far out of balance.

I might consider replacing those grey caps on the other board. Those are the coupling caps and if they go leaky bad things happen. Thats the usual fail mode, DC voltage leaks through the capacitor and it messes up the voltages on the tube. They look to be some sort of film capacitor which typically aren't problematic but I don't know these caps specifically.
 
gadget73 said:
There is no offset adjustment on a tube amp, so not quite sure what you mean about that. Amp with output transformers really can't have any offset. This should have a DC bias and a DC balance pot. The bias you use to set the voltage across one set of test points to a level. The balance is used to make both test points the same. It will affect the bias on the first set of test points so there is a bit of back and forth to get it all happy but at the end of the day both should run at 0.5 volts. If you're 0.003 volts different thats not going to matter. I think these are 10 ohm resistors so 3 millivolts across it would be 30 microamps of difference. Nothing in the grand scheme of things, those resistors are probably mismatched enough just because of production tolerance that even if they showed exactly the same voltage the tubes are that far out of balance.

I might consider replacing those grey caps on the other board. Those are the coupling caps and if they go leaky bad things happen. Thats the usual fail mode, DC voltage leaks through the capacitor and it messes up the voltages on the tube. They look to be some sort of film capacitor which typically aren't problematic but I don't know these caps specifically.
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Thank you again. Yes I mean DC balance when I say offset. Across TP9 and TP10 in picture attached.

So yeah I got mono No.1 with both set to 0.5v no problem.
Mono No.2 0.03v on DC balance with first set of valves and rising over 1v and red plating with valves from mono No.1

The coupling caps look ok but yeah probably good idea to replace thank you.

I did read through some of the thread you mentioned last night but more reading to do

Thank youScreenshot 2024-05-03 120249.png
 
so 1 volt across one test point and nothing across the other? yeah something is very wrong there. Either a coupling cap is leaking and driving one tube much too positive, you have something causing one tube to not conduct at all, or some combination of that.

I really wish there was a better quality scan of the schematic for these, but I've never seen one that was good and legible.
 
Have you checked those test point resistors with the power off? Looks like one of them may have been hot. I had a pair of these I rebuilt, almost 20 years ago, that had bad board capacitors (that you have replaced) and also had both test point resistors burnt badly into the board. The amps worked fine for a decade after the capacitors and resistors were replaced. I sold them so they may still be working.
 

Luxman MB-3045 Red Plate on a KT88

Hi, As I had the hifi on today I walked past the amps and noticed one of the KT88s was red plating. Is there an order of things to check to see what the root cause is? Im assuming this tube is probably fried as a result? To note: For the past few weeks, occasionally the amp would make light...
audiokarma.org
 
The damage I had one of my amps that Thain mentions was a burned board under the two 68kΩ resistors which are between the two large gray caps on the long board. I believe one of those two caps died and cause the voltage drop to make the resistor drop more than 2W. The resistors were mounted close to the board. It appears on your amp, these two resistors are mounted with long legs lifting them from the board. I know this board is face down in use but even that ¼-½" space could help keep the board in good shape. Check to see if it is getting discolored from heat.
 
Congratulations on your Luxman monoblocks. I have a pair being serviced and recapped atm. My tech guy told me that apparently the new 6550 tubes are not suitable for these amplifier, he explained the technical reasons behind it, but I cannot recall how he put it. Apparently the 6550 winged C are OK though.
 
Willmax said:
My tech guy told me that apparently the new 6550 tubes are not suitable for these amplifier,
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Gonna need more info on that as mine use 6550s and seem to work just fine. They have been modified to use the KT88/6550.
 
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