Biasing a Luxman MQ-88uSE - Voltmeter reading "overload" no matter what?

[Warm_tunes]

Hey there, looking for some help with a 3 week old Luxman MQ-88uSE tube amp.

About a week ago I had a massive crackling and popping sound in the left channel and swapped the V1 and V2 KT88s with the V3 and V4 KTs. Everything seemed to clear up, but about 3 days later, same issue now in the right channel and I could see arcing in the V3 tube (one that I had swapped), so I assume it was THAT tube that was causing the issue in the left channel.

I turned everything off and ordered a set of Gold Lion KT88s to see if that cleared up the issue.

Today I swapped the GLs with the Luxman JJs and after leaving them sit about an hour I used my voltmeter to bias the tubes.

V1 shows up 480mV just like it should.

V2 shows up "Overload," and no matter how I turn the pot it remains on OL.

V3 shows up "Overload," same as V2 - cannot dial it down below OL.

V4 shows about 480mV, but it is fluctuating all over the place, up to about 500mV and down to 440 or so.

Seems like something I should be concerned about... I've only ever biased my Cronus Magnum II which is very easy with the built-in meter.

Any ideas what my problem could be?

Thanks!

 

[gadget73]

wild guess but either the coupling caps are leaking very badly, or the cathode resistor has gone open. Can you change the range on your meter to a higher setting and see what it has to say?

 

[Warm_tunes]

wild guess but either the coupling caps are leaking very badly, or the cathode resistor has gone open. Can you change the range on your meter to a higher setting and see what it has to say?

Thanks, the 2000mV is the highest DC setting on the meter unfortunately.

The amp is only 4 weeks old, so this is a big disappointment. I do think there's a defect with the amp. Since I was having issues in the same valve sockets with the Luxman JJs and the Gold Lions I think there must be a big problem with the amp itself. I swapped positions on the Gold Lions, and the same V2 and V3 sockets measured overload (with the different tubes).

I am guessing this needs to go back to Luxman ASAP. Probably too late for a swap on a new amp so I'm guessing I'll be without the tube amp for the foreseeable future.

 

[gadget73]

if one of those tubes failed it may have popped the cathode resistor. Probably not a big deal to fix. If its under warranty, no worries.

didn't realize this was new. I generally assume anything with tubes is decades old and in questionable health. Thats how I get them anyway.

 

[Warm_tunes]

Wanted to follow up on this with a resolution...

Luxman USA was exceptional in dealing with this situation. Apparently there was one more MQ-88uSE left in their New York warehouse, which they sent overnight to me along with a return shipping label to send back my original MQ-88uSE. They didn't dick around with trying to figure out what the problem was or sending me parts to replace, just sent the new amp, no questions asked. This confirms my great affection for Luxman - previously it was the sound, aesthetics and heritage of the brand that made me a huge fan, but now the excellent customer service is the icing on the cake.

Anyway the replacement arrived last night... Oddly enough, the replacement did not ship with the styrofoam tube reinforcement in the cage, and the 4 Luxman-branded JJ KT88s required re-seating into the tube sockets once I unpacked the unit.

After warming the amp up for about 30 minutes I checked the bias on the tubes, which is supposed to be set at the factory at 480 mV. However these tubes all tested well over 500 mV. I brought them all down to between 470 and 485 mV (those pots are really sensitive). I listened to some music for another hour and re-tested... they had drifted back up to around 500, so I re-biased and checked again after another 30 minutes and they seemed to have stabilized.

I found this a little weird, in that the MQ-88uSE is run in for about 40 hours at Luxman and tested meticulously. I know they are fixed bias set at the factory, so I thought it was odd that they tested out of the box so high. I think maybe the fact that they shipped un-seated and unsecured in the sockets may have had something to do with it.

After a couple hours with the stock JJs, I figured I'd try out the Gold Lions again - initial bias was crazy, well over 900 mV upon installation. I cranked them down to spec and after about 30 minutes they had drifted again close to 700 mV. They seemed incredibly hot compared to the JJs.

So I swapped back to the stock JJs, which sounded better to me anyway (a bit more clarity and separation) and the bias on those tubes was all the way down to around 60 mV. I got those set back to 480ish and let the amp run for another 2 hours... tested again and the bias was the same, so all is good.

I won't be doing any further tube rolling with the MQ-88uSE... I'm happy with the JJs and I don't think the power tubes are going to have that much of an impact on the sound to be worth messing with any further. Being the "Special Edition," the MQ-88uSE also has premium ECC803 & ECC802 tubes that have been through Luxman's "high confidence" selection and testing process, so I don't think I'll be rolling any of the smaller tubes either. The Mazda Cifte's in my Backert Labs Rhumba Extreme seem to compliment the MQ-88uSE very well.

All in all a happy ending and kudos to Luxman for how they handled the situation. Looking forward to getting some quality time with this amp over the weekend.

 

[GordonW]

I suspect that your wall line voltage is a bit higher than what Luxman tested with.

The difference between 117 and 120v AC from the wall, could easily make the bias shift 5%.

Did all the Gold Lions test high, or just some of them? In any case, I would be wary about that PARTICULAR set of tubes. I haven't had issues with Gold Lion KT88 tubes being that out of whack, BTW- I would suspect a problem with this one set...

Regards,
Gordon.

 

[Warm_tunes]

I suspect that your wall line voltage is a bit higher than what Luxman tested with.

The difference between 117 and 120v AC from the wall, could easily make the bias shift 5%.

Did all the Gold Lions test high, or just some of them? In any case, I would be wary about that PARTICULAR set of tubes. I haven't had issues with Gold Lion KT88 tubes being that out of whack, BTW- I would suspect a problem with this one set...

Regards,
Gordon.

They all tested high, well between 900 and 1000 mv... I cranked them back down to 480 but they drifted up pretty fast. I may reach out to Viva Tubes on this.

I did suspect my local power may have something to do with the overall bias shift. We have a local co-op electrical company and I know they bump power up and down based on demand. I have the Backert Labs and the Luxman plugged directly into the wall AC, but I am going to get an IsoTek Corvus next week and see if that helps with stability.