Too much B+ for the poor tubes.

[JC@]

I believe that this is fact, as I have two boxes of twenty each.
And by any means I am trying to put them down, because other than this they sound good, work well, and are quite affordable.
But the chinese made EL34 Shuguang brand can't stand plate voltages near 500 Volts. I had no problem with them untill I tried them on an amp with 525 volts B+. more than 6 out of 10 red platted so badly that if the current is not cut off quickly, the tube is gone.
So, people in europe, with so many vintage amps that work with high B+ voltage, please remember. Many tube sellers carry these tubes under their own brand...

As a final test I installed a set of very used Siemens and the amp was happy again.

So I don't know what to do... I can't u$e vintage for a simple repair.

[Enzo]

But how much current were they biased at? They red plate from power dissipation, not voltage. Voltage problems would be arcing and sparking and shorting elements. Perhaps these tubes require a bias setting outside the range of this amp adjustment.

[JC@]

Good point, but I still believe voltage is the reason. Some of them showed a vivid blue through the plate "vent" openings for a couple of seconds before they inmediatly went to a vivid orange then almost yellow (and plates after cooled, showed that overheated dark color, on some even the getter got a hardened steel blueish tint on them!). This line of amps had quite a wide range bias setting as you can set it from a few mA per tube to way high up. Very difficult to set to something exact as just forcing a bit the screwdriver produce a huge jump.
And, two of the four power tubes could be iddling at 35mA while the other two were red platting as described.

I remember random red platting problems being caused by loosy contacts in bias trim pots of those metal open ones, not the more contemporary plastic encapsulated ones, but this is not the case.

Anyhow if I recall right I remember something similar happening to me in a MusicMan in the past.

[Bruce / Mission Amps]

Good point, but I still believe voltage is the reason. Some of them showed a vivid blue through the plate "vent" openings for a couple of seconds before they inmediatly went to a vivid orange then almost yellow (and plates after cooled, showed that overheated dark color, on some even the getter got a hardened steel blueish tint on them!). This line of amps had quite a wide range bias setting as you can set it from a few mA per tube to way high up. Very difficult to set to something exact as just forcing a bit the screwdriver produce a huge jump.
And, two of the four power tubes could be iddling at 35mA while the other two were red platting as described.

I remember random red platting problems being caused by loosy contacts in bias trim pots of those metal open ones, not the more contemporary plastic encapsulated ones, but this is not the case.

Anyhow if I recall right I remember something similar happening to me in a MusicMan in the past.

I just finished refurbing an old, fan cooled, YBA-1A Traynor with a pair of Svetlana (SED) El34s.
The final B+ at the plates was 580vdc!!! Yes 580vdc.
Screen voltage is 572vdc.
With the two EL34 tubes idling at 30ma each, this amp puts out an amazing 72 watts into a 100 watt, 8ohm dummy load at 83Hz with virtually no distortion as viewed with my O'scope across the dummy load.
I ran for about 2 hours in idle and played it for about an hour... tubes are running fine with no blue and no red plating.
Now there's a pair of EL34 tubes for ya.

[JC@]

But how much current were they biased at? They red plate from power dissipation, not voltage. Voltage problems would be arcing and sparking and shorting elements. Perhaps these tubes require a bias setting outside the range of this amp adjustment.

Enzo, the bias supply goes to up to -62 volts negative voltage, and as some of the tubes in the lot I have stand it well and bias properly at 35 mA most others red plate without apparent limit of current draw, I think I will just plug in a different brand of tubes.
If I had some time to experiment I would really love to SEE what would happend If I installed a voltage doubler or something like that for the bias supply and see at what point all the tubes will stabilize or if deffinetely it is a too high B+ problem.
Thanks for your help.