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Old 6th Feb 2016, 2:17 am   #141
DangerMan
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Join Date: Jul 2009
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 700
Default Re: EL34 power amp project

Quote:
Originally Posted by bikerhifinut View Post
By the way I only spotted the hot spots because I had the amps on while the room was dark. In daylight or under normal artificial light I cant see any problem even close up to the valve bottles.
Now then...

That's different, as they say. It's not unknown for EL34's to do this, even Mullard ones. I guess it depends on the brightness of the hotspots.
I've certainly got one or two originals that are completely stable and which measure within "new" parameters (even as measured hot whilst working in the amplifier) that show a very dull but even red glow on one side of the anode in the TL25plus in a darkened room. i.e. You cannot see small rectangular blocks corresponding with where the electron beam hits the inside of the anode. (Corresponding to the blue patches seen on the inside surface of the anode).

I guess the Leak DC conditions just push the valve towards dissipation limits, and if there's a shortfall of current on one side of the cathode then the other side takes up the slack. The rating of the valve presumably accounted for this possible imbalance.

However, if you can see discrete (especially just one or two) individual bright blocks, I'd suspect it was due to poor valve construction.
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