Slight red plating on EL34

[EF80TVVALVE]

Hi all, just a quick question here. I've recently brought this old AEI/BTH amplifier back to life after changing a fair amount of the components in it (all caps changed along with quite a few ERIE resistors that had drifted out of spec). It's all playing nicely now but I noticed today that there is some very slight red plating on the EL34s, both of them in fact.

I just wonder whether people would consider this to be normal in this amplifier? I know of some newer valves (KT150 for example) that do have a bit red plating when working at their maximum conditions but this is the first time I've seen it in an EL34 that isn't operating under fault conditions.

I've attached a photo which makes it look a little worse than it is, but I presume that my camera is also picking up some light that would otherwise be invisible to the human eye. Both valves are doing it in the same spot which makes me believe that it's a combination of them being ran a little hard along with construction of the valve.

Everything checks out okay and the amp works and performs as it should, I'm getting a nice 20.8w of undistorted output from it which is great.

I'm not going to start looking for a fault that doesn't exist but equally this is the first time I've seen an EL34 do it.

[GrimJosef]

You can check the quiescent anode-cathode voltage and the current and calculate the power dissipation in the anode (you might want to check the screen grid too). If they're within the valve's spec then I don't think I'd worry too much, although I wouldn't want it to be running much redder than that (I accept it's very hard to be quantitative from a photo though). Something else that might be worth checking is whether the high temperature is causing any outgassing and consequential rise of the control grid voltage by more than a few hundred millivolts. If it is then there might be some risk of thermal runaway, which would be really unfunny.

Cheers,

GJ

[EF80TVVALVE]

Thanks for your advice, I'll give it all a double check just to be sure. Previous measurements were okay but there's always the possibilty that something has changed within the few hours of use it has had.

I get where you are coming from regarding the photo and it does make it look much worse than it really is, to the naked eye it is nothing more than a very slight red patch, a passing glance over the valve and it isn't even noticed. I've seen EL34s and KT88s run away taking out the grid resistor with them so I know what real red plating looks like but this is so slight it isn't causing me any concern if everything checks out okay.

[Radio Wrangler]

The valve is designed so that the formed beams aim at the anode where the seams and the mounting rods lie so that they give an increase in radiating area. It is possible that <ahem> cost pressures have caused thinner anode metal to be used and that will reduce the thermal conductivity in the lateral direction, so the trick of using the seam flanges is less effective. I rather suspect that numerous little cost savings might have happened compared to the original devices.

Is the other side the same?

David

[EF80TVVALVE]

You could be right, both valves are doing it on the same spot but on the opposite sides they don't show any red plating. I have found Shu Guang valves to be quite reliable, their EL34s at least seem good. I've seen ones that have had 10 years worth of use, their bases black and cooked from the amount of hours they've ran but they still test up to spec, of course that doesn't mean that cost cutting hasn't been involved in their production.

[EF80TVVALVE]

Hi all

So I've checked the amplifier this afternoon and indeed the EL34s are running a bit too hard. From my calculations they are running at 29.3 watts although I have also done a calculation with consideration that the 250R cathode resistor measures 260R on my multimeter, with this the result is 28.2 watts.

Measuring voltages once it has been given time to warm up I have 462v on the anode and 34.3v on the cathode.

[GrimJosef]

If the valves are running a little bit hot then the small amount of red-plating looks consistent. Does the mains transformer primary have a voltage selector ? Is your mains voltage actually running a bit hot ?

I've wondered in the past whether the EL34 design relies to any important degree on the mounting rods being gripped tightly by the anode plates. I know that grid rods sometimes have little fins on top to try to keep the grid cool (indeed you can see them in your picture). I guess that conduction down one of the anode rods through the button base and into the external pin might also shift a little bit of heat. But this will be less effective if the anode seam has worked loose and the plates are only brushing the rod. That might explain why the seam isn't lighting up along its full length (somehow they never seem to). On the other hand that could also be caused by non-uniform emission from the cathode I guess.

Cheers,

GJ

[bikerhifinut]

Am I to assume its a shared cathode resistor with cathode bias?
if so each EL34 is passing just a bit less than 66mA which is just fine. If the cathode resistors are individual ones then its 131mA...………… ouch.
One of my EL34 push pull builds showed a bit of red plating, only visible in a fully darkened room. That amp was pushing out 37W at clip from an HT of 450V in Ultralinear PP. I was using 470R cathode resistors as per the Mullard specs for the 5-20, admittedly with a slightly different Long tailed pair driving it.
I shall turn the lights off and bung in my Brimar branded EL34s which I reckon are selected matched Shuguangs. I am running a mere 408V HT these days with a 470 cathode resistor per valve and they are drawing around 68mA per valve, have done for the last 2 years of regular use and show no ill effects,33W at clip last time I scoped it.Neither did the same set of valves when the original mono amps were running at 450VHT.
Andy

[bikerhifinut]

Yep.
I needed a totally darkened room but spotted a red patch on the seam of an anode of an EL34. Mine are passing the thick end of 70mA each which is well within their capabilities.
Given that these valves have been in the system for the thick end of 4 years and have stayed reliable I wouldn't worry too much.
My amps are driving a pair of Quad ESL989 and at times are hard pressed.
I think its a feature of the current Chinese production, I have some "military" sovteks that did the same.
I have heard stories of original Mullards being run at 800V on the anodes in instrument amps and the valve envelopes going soft with the heat.
I suspect its the nature of the beast, along with its EL84 brother being in small envelopes they run hotter than their predecessors.

A.

[EF80TVVALVE]

Thanks both for the advice. GJ, the mains tx does have a voltage selector and it is set to 240v which is the highest tap, here it is not unusual to see the mains up to 250v some days although I doubt this would make a big difference. I do have a Mullard EL34 that came from it that I can compare it with and see if it redplates too. I suppose that would answer the question of whether it is the construction of the valve or the amplifier.

Andy, thanks for checking with yours and I suppose that further confirms the SG valves being the offending component. It is amazing to see how much these valves are being pushed in some amplifiers and it shows that they are resilient to some abuse.

Like I previously said it doesn't really worry me as everything checks out as it should and the amplifier isn't running with any faults. The circuit is still the same as it was originally with no alterations so I'm confident that this is the way the amplifier has always been and maybe some valves just cope better than others.