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.
ethifi
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.
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.