Hi all,
One of my 300B valves just exploded from inside. Initially there was some noise, then a white/yellow flash inside, followed by loud ping!
A chunk of glass blew out inside the valve at the base (see pic below). The valve is about 6-7 years old, with on-off usage. I swapped the working valve into the other side and seems to work fine. Operating voltages measure fine, however I did just modify the driving 5842 (with interstage transformer) to battery bias on the cathode.
Coincidentally I bought an hour before a pair of EH Gold Grid 300Bs and would like a bit of reassurance that the problem is with the old valve and not the amp
WP_20140728_001(1) by andrew.randle
Andrew
One of my 300B valves just exploded from inside. Initially there was some noise, then a white/yellow flash inside, followed by loud ping!
A chunk of glass blew out inside the valve at the base (see pic below). The valve is about 6-7 years old, with on-off usage. I swapped the working valve into the other side and seems to work fine. Operating voltages measure fine, however I did just modify the driving 5842 (with interstage transformer) to battery bias on the cathode.
Coincidentally I bought an hour before a pair of EH Gold Grid 300Bs and would like a bit of reassurance that the problem is with the old valve and not the amp
WP_20140728_001(1) by andrew.randleAndrew
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I would NOT swap any more valves to the "bad side" without doing some measuring.
Even if it's just simple resistance at each point comparing Lh with Rh.
Measure the resistance of your output transformers and check for shorts.
You are not yet looking for absolute readings just any difference between the two sides.
Even if it's just simple resistance at each point comparing Lh with Rh.
Measure the resistance of your output transformers and check for shorts.
You are not yet looking for absolute readings just any difference between the two sides.
I was thinking about this, this morning...
The only thing I'd think would cause the glass to snap would be some extreme heating..
If it happened immediately after power up, it seems like it would be something easily measurable.. And he already put a good tube on that channel and it worked.
It does throw a red flag that he just modified the amp, but my thought is the tube probably had a small fracture there that finally failed. Maybe the tubes were pulled out and moved around and pushed back in while the amp was being worked on, and that was enough to weaken the small fracture.
Triple check all your voltages, and if they're ok, I'd power it up and hope for the best.
The only thing I'd think would cause the glass to snap would be some extreme heating..
If it happened immediately after power up, it seems like it would be something easily measurable.. And he already put a good tube on that channel and it worked.
It does throw a red flag that he just modified the amp, but my thought is the tube probably had a small fracture there that finally failed. Maybe the tubes were pulled out and moved around and pushed back in while the amp was being worked on, and that was enough to weaken the small fracture.
Triple check all your voltages, and if they're ok, I'd power it up and hope for the best.
Many thanks for your comments and support. On the working valve (which I'll be replacing anyway) the channel-under-investigation is exhibiting usual characteristic of Va = 350V and Vc = 69V. Battery bias on driver giving 1.28V
I'll leave it a few days and try it again. It could well be some-kind of structural weakness in the valve and vacuum as on switch-on (when it happened) I heard some rustling / cracking sound before the massive fireworks in the valve, then a massive ping as the internal envelope shattered
The battery bias is new and a couple of days earlier I had an amazing 3 hour session with the amp. Really improved the dynamics.
The amp has gone through many many design and build iterations and the valves have been pulled/replaced many times during the past 6-7 years.
Andrew
I'll leave it a few days and try it again. It could well be some-kind of structural weakness in the valve and vacuum as on switch-on (when it happened) I heard some rustling / cracking sound before the massive fireworks in the valve, then a massive ping as the internal envelope shattered
The battery bias is new and a couple of days earlier I had an amazing 3 hour session with the amp. Really improved the dynamics.
The amp has gone through many many design and build iterations and the valves have been pulled/replaced many times during the past 6-7 years.
Andrew
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I suspect from the description of the events leading to this that wicked1 hit the nail on the head.
I have had a few similar experiences with cheap Russian and Chinese made 300Bs over the years, as well as a few US made 6SN7. Careful testing subsequent to this revealed no issues short of a tube defect that would account for this behavior. The amplifiers in question functioned reliably for many years subsequent to these events.
I have had a few similar experiences with cheap Russian and Chinese made 300Bs over the years, as well as a few US made 6SN7. Careful testing subsequent to this revealed no issues short of a tube defect that would account for this behavior. The amplifiers in question functioned reliably for many years subsequent to these events.
If it happened immediately after power-on, it must have been caused by the filament itself. Immediately after power-on, you don't have any B+ available to do damage. If it happened, say, 30 seconds after power-up, I'd start suspecting your modifications.
I've noticed that the 300B filament and filament support structure seems to go through quite a bit of stress on start-up. In circuits where the peak current of the filament supply is limited electronically, I've had 300Bs "sing" for a second until the filament heats up enough to not trip the current limiter. The tubes don't "sing" that loudly, not for very long, and I have never had failures as a result, but it does point to the stresses involved with the inrush current on start-up. Yet another argument for soft-start, I guess...
In your case, the base was probably weakened a bit and finally gave up. The tube is probably fine... Unless you can measure anything wrong with it or the rattling glass bit bothers you, it's probably still usable. If nothing else then for prototyping and experiments...
~Tom
I've noticed that the 300B filament and filament support structure seems to go through quite a bit of stress on start-up. In circuits where the peak current of the filament supply is limited electronically, I've had 300Bs "sing" for a second until the filament heats up enough to not trip the current limiter. The tubes don't "sing" that loudly, not for very long, and I have never had failures as a result, but it does point to the stresses involved with the inrush current on start-up. Yet another argument for soft-start, I guess...
In your case, the base was probably weakened a bit and finally gave up. The tube is probably fine... Unless you can measure anything wrong with it or the rattling glass bit bothers you, it's probably still usable. If nothing else then for prototyping and experiments...
~Tom
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I have had this problem a few years ago with an older Svetlana 300b. I read that these have a wrong construction of the filament. It wears out after a certain period on stresspoints. Luckily my Caddock katoderesistor burned out and not my expensive toroid OPT.
The photo shows whitening of the getter, so it has blown to outside air, lost vacuum. It's a display-only model now. They look good as a place-holder in a candle stick (dud 300Bs fit perfectly in mine).
The only worry about the circuit mods: is the driver dc-coupled to the 300B grid? Or is a coupling cap with less-than-robust reliability record there (wax caps, Audio Note paper-in-oil)? An unexpected dc offset could overheat the 300B.
If you're worried about the amp, you could put some 10 ohm 500mW carbon film resistors in the "cathode" return, and the grid circuits. (not metal film, certainly not carbon composition). These blow very quickly in a short circuit - IME quicker than a F-rated fuse.
Take them out again, if the amp runs OK for a while. Or just leave them for protection duty.
The only worry about the circuit mods: is the driver dc-coupled to the 300B grid? Or is a coupling cap with less-than-robust reliability record there (wax caps, Audio Note paper-in-oil)? An unexpected dc offset could overheat the 300B.
If you're worried about the amp, you could put some 10 ohm 500mW carbon film resistors in the "cathode" return, and the grid circuits. (not metal film, certainly not carbon composition). These blow very quickly in a short circuit - IME quicker than a F-rated fuse.
Take them out again, if the amp runs OK for a while. Or just leave them for protection duty.
5842 driver? Are "stopper resistors" installed on all the grid connections? This tube will self oscillate under certain conditions. The HF oscillation coupled over may cause a subsequent stage to "red plate" but an explosion is unlikely. A internal short caused by a broken filament wire falling loose may be another issue. This failure often happens with transmitting tubes such as the 833A (commonly used in older 1 kw AM broadcast transmitters).
I don't think whatever AndrewR did (mods) could have caused the problem.
He obviously did the same to the other channel, and there has been no problem there.
The implosion is due to a defect on the tube due to inconsistency in manufacturing. This is a sad reality with things nowadays if quality control is not diligent. 300B tubes with "holes" in the glass in the area where the holder of the electrodes gets soldered to the glass envelope are relatively common, I have seen a few and expect they might blow just like this one (which is probably related to the conditions of use - although said conditions might be quite OK for a well manufactured tube without defects).
He obviously did the same to the other channel, and there has been no problem there.
The implosion is due to a defect on the tube due to inconsistency in manufacturing. This is a sad reality with things nowadays if quality control is not diligent. 300B tubes with "holes" in the glass in the area where the holder of the electrodes gets soldered to the glass envelope are relatively common, I have seen a few and expect they might blow just like this one (which is probably related to the conditions of use - although said conditions might be quite OK for a well manufactured tube without defects).
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