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Luxman MB-3045 Red Plate on a KT88

RichP4003

RichP4003

New Member
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 popping/staticy noises as it would warm up.
These amps I converted from the 8045G to KT88 in Triode a few years back and they have been pretty solid. Also replaced all the coupling caps at that time.

Thanks everyone!
 
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Swap the tube with one of the other KT88's and watch it closely on power up and see if it starts to red plate in another socket.

Usully if the tube does this in any socket, it's the tube. If any tube does it in just one socket, it's usually a coupling capacitor and/or socket connections.
 
After mine were converted, using some old tubes for testing, sounding fine, one day I got red plating, too. Turned out to be the tube.
 
If one of the tubes is worn out it is probably time to get a new quad. You can get by just replacing the one but the amp would sound better if you had a fresh set.
 
Thanks guys..

So far, I swapped around the tubes, and the issue appears to follow the socket/position, not the tube. I used a laser thermometer to monitor the plate temps and I could see a rapid comparative increase on that tube position vs the other ones (and in comparison to the other mono block)

I then got under the amp, and measured the voltages. On the bias, it was well over 1 volt! (should be 0.5 after the KT88 conversion) even with the pot turned all the way down, I am still seeing over 0.5v. I also noticed when first fiddling with the bias adjustment, slowly turning it down from where I started, that it would slowly fluctuate up and down, maybe 2 second? intervals, and I could hear the buzz of the power transformer doing the same with this fluctuation.

Im hoping I dont need to replace the quad if this tube sustained damage.. they aren't very old (maybe a year and a half?) they are gold lions.

So... not sure whats next? Replace the bias and DC balance pots?
 
Flying a little blind but I went poking around during lunch and did realize that one of the resistors is not the right spec, Ive attached a picture (they should both be 560) Of course, this amp has always been like this, but maybe its something that would make a problem develop over time?
 

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Scott is right,, unless that resistors' color band is burned, it certainly looks like 560R... is that about what it reads?
Brown is 1 Orange is 3, zeros...
 
The one next to it reads about 560 (as to the other 2 in the other mono block)... the one with the brown band reads 650 (which makes no sense to me)

ive attached a schematic (the last one are the changes to convert to KT88
 

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If the 560R overheated it would read high... unless its supposed to be 560K? What does the other channel/amp have in that position?
 
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knockbill said:
The one with Brown band should read 560R,,, its reading out of range as it got hot from overheating...
I can't see it on the schematic,,, where does it say 56K? 560R seems more reasonable for a K cap...
Click to expand...

Either side of the balance pot in the bias supply for the output stage, R301/302. In the pic, the one on the right is clearly 56k, green blue orange.

jeff
 
2 things about the green/blue/brown resistor:

-It should be green/blue/orange according to the wiring schematic and the other amp.
-I have a pic of the underside of the amp, it was the same color as it is now (so this wasn't from a recent event where it overheated)
-Its reading approximately 650R (which doesn't match its coloring)
-On this amp, that resistor is tied to the tube socket that is NOT giving the run away temperatures/where the red-plating occurred.

I really appreciate everyones help with this, and I hope that by having all this posted here its a good resource for someone, someday if it happens to them.
 
I'd be looking at leaky coupling caps. Could also be problems with V4. This thing is direct coupled, so C105/C106 or issues with V4 will affect output tube bias. Also, problems with balance pot VR2 or the resistors tied to it could be an issue. Same with bad connections on any of the tube sockets in the area.

I don't see any 560 ohm resistors on the schematic. I do see 56K resistors in the cathode circuit of V4 though. Stripe-wise, it looks like blue, green orange on one and blue, green brown on the other. Hot orange turns into brown though. Being a direct coupled amp, those resistors will affect output tube bias.
 
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Gadget73, I think we have a winner!!!!!!

Due to the beauty of mono blocks, I took one of the 6240Gs out of the good amp and put it in the other amp in the V4 spot. Within a minute the voltages were already relatively stable and easy to bias. 10 minutes later they are holding and responding appropriately.

Of course, now I need to make a judgement call on replacing the KT88 and its 3 friends...
 
Blue Shadow said:
He knows his stuff. He got mine running within a minute or two post conversion to available tubes.
Click to expand...

It is such a relief to know what the issue is. Have a pair of KT88s on order, and a complete set of 4 6CG7s to replace all the 6240Gs (which as a fellow MB3045 owner, I know you can understand the sadness of straying further and further away from the original tubes on this amplifier. But better it running well than not running at all!)
 
I'd have loved to hear one of these with original triodes. It sounds nice with the 6550 or KT88, but the original triode is probably something. Of course I've not heard a set with the output tubes triode-wired either. It may be a better match in triode mode.
 
gadget73 said:
I'd have loved to hear one of these with original triodes. It sounds nice with the 6550 or KT88, but the original triode is probably something. Of course I've not heard a set with the output tubes triode-wired either. It may be a better match in triode mode.
Click to expand...

I did the conversion myself from original 8045G triodes to the KT88s in triode configuration, so I have heard both. I can't say I lost anything in the conversion, apart from a maybe little power. Of course, the reality is the number of hours on the original tubes was completely unknown, and there isn't a way to even test them to see where they may have stood. (I do still have them just for posterity). There may have been a psychological component knowing the 8045Gs ran at the edge of their life and were rare.. but to me, the benefits outweigh the cost (cost being trying to find those tubes, and having to be selective for how much one runs the amps)

I also heard another set of these amps with the original tubes not that long ago (I went to someones house to audition some speakers) I can't say they seemed different to me than my triode KT88. Its an incremental difference at best, to my ears anyway.

Im not sure how hard it would be to go from a KT88 ultra linear to KT88 triode, although part of my choice in going triode was the conversion was only a few resistor add/swaps to convert, the ultra linear conversion is a bit more of a tear-up.
 
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