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Do failing tubes give off burning smell?

I am new to tube amps. I own a tube dac and tube pre-amp, and they have been problem free. Recently, I switched over from a Pass Labs X250.5 and bought my first tube power amp - a used ASL 1009 monoblocks - they use 2x845, 2x12AU7, and 2xEL34 per monoblock. These amps require manual biasing. The recommended bias setting for the 845 tubes is between 800 to 850 mV. I try to keep the bias between 800 - 820.

This week, one of the 845 tubes (right monoblock) show bias readings between 730 - 765 mV and could not go up any further. The other tube is fine. Today, after warming up the amps for about 10 mins, I manually adjusted the bias dial of the "problematic" tube a little with the aim of pushing up its bias, but there is no change in the bias reading. Within a minute, I noticed a strong burning smell coming from the amp / this tube. I switched off the amps immediately. Is this normal because the tube is failing or is there something wrong internally with the circuits or components?

Another observation is that both tubes on the other (left) monoblock are displaying bias readings of over 930 mV upon startup, but the readings would gradually drop to between 800 - 830 mV after 10 minutes of warm up. As of last week, they read between 800 - 820 upon startup and stayed constant. Is the big swing in bias reading abnormal behaviour?

Your feedback and comments are highly appreciated. Thanks.
128x128ethifi
ethifi's avatar
ethifi OP
75 posts
 
Thanks guys for your prompt comments.
Looks like I had bought myself some problem amps and lots of trouble. As a recent buyer, I didn't expect a trip to the tech doctor so soon.
Is there any other test I could do? I was thinking of swapping tubes on the same monoblock to see if the bias is stable, but I am actually afraid to turn the amps on. Things may go from bad to worse. Any advice? Thanks.
spatialking's avatar
spatialking
610 posts
 
Most likely what is burning is a resistor due to over current. In general I suggest a trip to the local stereo repair house that specializes in audio repair. Here in Dublin CA we have the Stereo Doctor. They are good and do all of my repairs when I don't have the time or inclination to do it myself.

Be advised, if it is a tube that is causing something inside the amp to burn, then swapping tubes will cause the other channel to burn! I suggest you simply purchase some new tubes and give that a try. If you don't need them now you will need them eventually, so the money is not wasted.

If it really was a resistor starting to smoke, then a short burn (seconds) may not have hurt anything other than cosmetics. Extended burning, for say several minute(s), may permanently damage the part.

Another possibility for burning is some previous owner put some damping rings or compound on the tubes to squish microphonics and that is what you are smelling. If that is the case, then the odor is no big deal. I would just remove the rings or compound and clean the tubes. As mentioned above, the dust that collects on tubes may smell a bit as it burns off. This is also a no big deal.

The fact you can't adjust the bias properly does indicate a problem with either the tube or the associated circuitry, though. Most likely that is the source of what you are smelling and it is the first place I would look for problems. If something was really damaged from over heat, then you should be able to see it.
ethifi's avatar
ethifi OP
75 posts
 
Thanks Spatialking for the clear explanation.
hotsauce
108 posts
 
I would try swapping the tubes left/right.

This will show if its the tube that won't bias, or the amp can't be set correctly.

John C.
ethifi's avatar
ethifi OP
75 posts
 
Hi Hotsauce, I swapped the left/right tubes today and noticed that the lower bias reading switched sides. I would conclude that the tube is failing and won't bias.
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