I just recently finished putting together a VTA ST70. Sounds great, no hum. I’ve only got about 2 hours on it so far.
During my last listening session, about 20 minutes in, I saw a bright white flash in the top of the left rear EL34, accompanied by a moderate pop from the speaker. I used it for a few more minutes and it seemed to otherwise be fine.
Could it be a bad tube? I visually inspected the tube, and it looks like one of the grid cooling fins is too low and off center. I’ve attached a photo. Is it reasonable to assume that is the source of the problem? For reference these are the stock tubes - no name “made in China” EL34.
I’d like to try switching the tubes to different positions to see if the issue follows the tube, but I’m unclear what the risks would be. I don’t want to risk blowing up an output transformer or damaging my speakers.
Any advice would be much appreciated!
- Jonathan
During my last listening session, about 20 minutes in, I saw a bright white flash in the top of the left rear EL34, accompanied by a moderate pop from the speaker. I used it for a few more minutes and it seemed to otherwise be fine.
Could it be a bad tube? I visually inspected the tube, and it looks like one of the grid cooling fins is too low and off center. I’ve attached a photo. Is it reasonable to assume that is the source of the problem? For reference these are the stock tubes - no name “made in China” EL34.
I’d like to try switching the tubes to different positions to see if the issue follows the tube, but I’m unclear what the risks would be. I don’t want to risk blowing up an output transformer or damaging my speakers.
Any advice would be much appreciated!
- Jonathan
Attachments
Yes, it is reasonable to assume that the tube is the problem. Switching the output tubes around poses no more risk of damage then leaving them where they are. The real problem is that you are using Chinese tubes. They are more troublesome then any other makes available. I would get a replacement for that one.
If your in the UK or EU I'd recommend Langrex Premiun EL34's - EL34 LANGREX PREMIUM MATCHED PAIR VALVE TUBE | eBay yours is kaput from the sound of it,if bought from a reputable supplier return it.
Andy.
Andy.
Any tube that has flashed is suspected and should be junked.
Control that the 10ohm resistor is still 10 ohm.
Do yourself a favor and get a matched quad of quality tubes from a reputable dealer.
Recommended current of EL34 in ST70 amp is 40mA, so (via 10R) must be measuring 400mV in the front panel "socket" (if it is "real' ST-70).
Try to measure all of power tubes current.
p.s.
This video shows "new" VTA panel bias setting method:
Dynaco VTA-ST70 Tube replace and bias set - YouTube
See carefully from 08:00.
Try to measure all of power tubes current.
p.s.
This video shows "new" VTA panel bias setting method:
Dynaco VTA-ST70 Tube replace and bias set - YouTube
See carefully from 08:00.
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