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Lux91 monoblock running very hot

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Posted on July 16, 2013 at 07:12:26
henryhleung


 
Hello

I have a problem with my Lux91 300b monoblock power amp. Not sure if anyone with the same amp has experienced a similar problem and could help me. One of the monoblocks gets much much hotter than the other one. For example, after playing music for about one hour, the power transformer and output transformer is very hot to touch, and the tubes all tend to feel hotter too. Comparatively the other monoblock is only warm to touch at least on the transformers. I guess this shouldn't be normal.. Obviously both left and right monoblocks have been turned on for the same amount of time, and both were connected to speakers while playing music from the same source. For the record, I'm using the same cables etc, none of those seem to be shorted.

One other symptom, that monoblock vibrates.. Yes, it feels like its coming from either the power transformer or output transformer but not sure which one. It physically vibrates the moment I turn it on, and I can also hear it emit a ticking sound..

Does anyone have any ideas or troubleshooting tips? Thanks!



 

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RE: Lux91 monoblock running very hot, posted on July 17, 2013 at 04:12:21
Thorsten
Manufacturer

Posts: 4172
Location: Somewhere nice on planet dirt
Joined: September 25, 1999
Hi,

Sounds to me if there is either too much current in the output stage or damage to the mains transformer.

Ciao T

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?

 

RE: Lux91 monoblock running very hot, posted on July 17, 2013 at 08:21:17
henryhleung


 
Hi Thorsten

Thanks for your reply. Are you still in southern China these days? I'm glad to find you here again.

Would you be able to elaborate how I could check the Lux91 monos? What measurements could I make to determine if the problem is too much current at the output stage? Or whether it could be damaged mains transformers? I've tried the DIYHifi website and have spoken with Simon, but as the Luxes were all fully assembled, the prior instruction manuals didn't elaborate what voltages to check etc.

The only troubleshooting I've done so far was to swap the tubes from left to right, that made no difference. So that rules out faulty tubes. I could try swapping speakers but not sure if that would make a difference. Next step, should I buy a multimeter and poke around with it?

Thanks much!

Henry


P.S. off-topic, but I've recently bought your ifi iDAC and iUSB combo and have been enjoying the music very much.

 

RE: Lux91 monoblock running very hot, posted on July 17, 2013 at 08:55:02
Thorsten
Manufacturer

Posts: 4172
Location: Somewhere nice on planet dirt
Joined: September 25, 1999
Hi,

You would need to measure the Voltages around 300B Tube (Pins), with the Amplifier bottom open and the amplifier flipped up, with something under the transformers to make sure there is no pressure on the 300B.

You should see around 400V on the Anode, -80V on the grid and a few 100mV on one side of the heater and 5.something Volt on the other...

Ciao T

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?

 

RE: Lux91 monoblock running very hot, posted on July 18, 2013 at 08:44:14
Henryhleung


 
Ok thanks Thorsten. I gotta get a multimeter and test it out.

Regards
Henry

 

RE: Lux91 monoblock running very hot, posted on July 19, 2013 at 07:03:42
Henryhleung


 
Incidentally, while on the topic of 300B plate voltage..

Thorsten, is it possible to lower the operating point of the Lux91 tubes? Ie reduce the voltage from 400v to 350v? I'm curious whether this would not stress out the tubes as much and increase the life of the 300Bs. Would this be possible with the Lux circuit and would it require much modification. Sorry if this is a newbie question or if I'm not making sense.

The reason for asking is because people are often surprised or disbelieving when I tell them my mono blocks are rated 16w. Some reckon its impossible for a single 300b implementation, others say my tubes will burn out very soon. I've read your previous posts about 300b suggested operating points being conservative. But any idea how much more quickly the tubes will be eaten up in the Lux91 implementation?

Thanks
Henry

 

RE: Lux91 monoblock running very hot, posted on July 21, 2013 at 01:19:40
Posts: 2
Joined: July 16, 2013



Have any other Lux91 owners experienced this?

Looking inside the mono blocks...

The left monoblock (i.e. the hotter one) has some brown goo / residue over the two chokes. I think those are the two reactive interstage chokes. It looks like leaking liquid.. There's also a little amount splattered in the chassis. The liquid is hard - a wax-like substance? Could this be related to the heat problem? See image below.

However, I also checked the right monoblock which doesn't run as hot, the chokes also exhibited some leakage although not as much as the other amp.

Still waiting for the multimeter to arrive so haven't taken the measurements yet. Will report back when measured.




 

RE: Lux91 monoblock running very hot, posted on July 21, 2013 at 11:49:53
Thorsten
Manufacturer

Posts: 4172
Location: Somewhere nice on planet dirt
Joined: September 25, 1999
Hi,

The residue is from the vacuum impregnation of the parts, they are basically dunked into a lacquer under vacuum to drive out air. When taken out the lacquer drops...

This is how they look new... ;-)

Ciao T

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?

 

RE: Lux91 monoblock running very hot, posted on July 21, 2013 at 11:59:05
Thorsten
Manufacturer

Posts: 4172
Location: Somewhere nice on planet dirt
Joined: September 25, 1999
> is it possible to lower the operating point of the Lux91 tubes?
> Ie reduce the voltage from 400v to 350v?

Not really.

> I'm curious whether this would not stress out the tubes
> as much and increase the life of the 300Bs.

Run at 300V/60mA the 300B should do 40,000 Hours, the way it runs in the Lux 91, around 10,000 hours. At 350V/80mA,the increase in live might be to 15,000 Hours, tops.

> The reason for asking is because people are often surprised
> or disbelieving when I tell them my mono blocks are rated 16w.

The standard operation of the 300B at 400V/80mA will give around 12 Watt.

http://www.westernelectric.com/products/300b_graphs.html

The rest is due to the driver circuit, being able to produce a greater input voltage to the 300B than the datasheet uses... Think of it as either turbo or afterburner.

> Some reckon its impossible for a single 300b implementation,
> others say my tubes will burn out very soon.

It is both possible, nor will the Amp "eat tubes".

If you look at the above link, you will find that WE lists operation with even nearly 18 Watt output power (more than the Lux 91 manages).

Of course, the tube experiences more stress and will live less long, but it is not a huge difference.


Ciao T

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?

 

RE: Lux91 monoblock running very hot, posted on July 24, 2013 at 08:46:40
henryhleung


 
I see.. thank you. If my 300Bs can last 10,000 hours I'm happy enough.

 

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