White glow in EL34 tube

[pch64]

Hello,
I'm testing some EL34 tubes in a cathode biased amp with shared Rk and some of them have problems - the plate current start to rise. I have managed too cure some such tubes before - when the current get close to 100mA I turn the standby switch off. After some time I turn the standby switch on if the current is still high I turn it off again. I think this high current is due to the presence of some gas in the tubes (usually NOS ones) and after repeating the procedure several times (after an hour or two) the tubes start to work properly.

But today I have one NOS RFT that starts glowing white so I increased the Rk to 11KOhm and that limited the plate current to 8.8mA but after an hour it only dropped to 8.5mA. Is that white glow caused by a gas in the tube or something else is wrong?

Is it possible to cure such a tube or not? Any help is appreciated.

Peter

 

[Blue Strat]

I've never seen a tube that draws too much current "be cured".

Sometimes you just get/have a bad tube. Time to move on.

 

[pch64]

I have succeded several times and I think those tubes have had a small amount of gas which had disappeared after some heating.

But this tube is a worst case - I reduced Rk to 2K2 (B+ is about 430V) and something inside it start to glow white although the plate current is limited to 39.5mA. Is this the cathode or the screen grid?

 

[pch64]

It seems that this tube really can't be "cured". It draws constant current and glows like a incandescent bulb. I took a picture of it:

But I succeded to cure a Tesla EL34 with a hole in the guiding pin and single O-getter (from 1966). It was drawing 120mA with the same Rk and had a strong blue glow. After several times switching on and off the current started to drop but then was raising again slowly till reaching 112-120mA.
I almost gave up but finally it settled down to 40mA and then the current started to drop slowly till it reached 10mA and stopped there. I will do some more tests later but I think it's OK now.

 

[Blue Strat]

That might be the screen glowing. Nothing will cure that.

 

[pch64]

Mike, I think you are right about this tube - it draws a constant current and it's not changing.
But it is possible to "cure" the ones that I think have some gas - now I succeded with another Tesla EL34. The current was rising up to 150mA (instead of 35-45mA) and it was starting to glow blue. After some training they stop doing that and are fine.

 

[smk001]

That might be the screen glowing. Nothing will cure that.

I know it's 8 year old thread... but I haven't found much info on that elsewhere. I noticed while troubleshooting my Bugera 6262 amp on the bench (issue is unrelated to the output tubes) that when I crank the amp up to "11" on a dummy load, screens in all four EL34s start to glow from red to bright orange according to the input signal, I guess they'd make great signal meters. Is it normal in 120 W PP power section or maybe it's a sign of tubes reaching their EOL or other issues with the amp? Otherwise they bias to a healthy 38 mA plate current and work well, although with Volume I cannot go past "2" with a speaker connected even when I use a dummy load which is dissipating 20 dB from the output.

Power amp section schematic:

 

[jthomas666]

https://www.ampvalves.co.uk/red-plating-guitar-amplifier-valves/

 

[pdf64]

Screen grid dissipation increases significantly at high power levels, especially so for true pentodes such as EL34, hence its wire will tend to glow brighter than it does at idle.

So the situation described seems normal. And seemingly unrelated to the thread.

 

[HotBluePlates]

... when I crank the amp up to "11" on a dummy load, screens in all four EL34s start to glow from red to bright orange according ...

Depends on where the "glow" comes from. If it's in the gray plate, as shown in the photo below, that's bad.

#1 cause tends to be idle current is too high, because bad internet advice is being followed (Class AB idle bias is not one-size-fits-all). If plate voltage is high to squeeze high output power from a given tube-set, and/or load impedance is too small (because of designer choice, or because you're monkeying with speakers), then yes the tubes will overheat & redplate when driven hard.

If the glow is inside the space bounded by the gray plate structure, you'll have to figure out if it's the heater/cathode, or if the screens are glowing (because they're too hot). Overheat your screens and they'll soon melt; afterwards you might have a decent low-mu triode.

 

[soundchaser59]

Depends on where the "glow" comes from. If it's in the gray plate, as shown in the photo below, that's bad.

#1 cause tends to be idle current is too high, because bad internet advice is being followed (Class AB idle bias is not one-size-fits-all). If plate voltage is high to squeeze high output power from a given tube-set, and/or load impedance is too small (because of designer choice, or because you're monkeying with speakers), then yes the tubes will overheat & redplate when driven hard.

If the glow is inside the space bounded by the gray plate structure, you'll have to figure out if it's the heater/cathode, or if the screens are glowing (because they're too hot). Overheat your screens and they'll soon melt; afterwards you might have a decent low-mu triode.

Seen that many times before in my amps, but only in EL84 Mesa amps. The only tubes that did not do that in those amps were JJ.

 

[TL;DR]

You probably bought the “soft white” version. Next time get the “standard”

 

[HotBluePlates]

Seen that many times before in my amps, but only in EL84 Mesa amps. The only tubes that did not do that in those amps were JJ.

Mesa-branded EL84s overheated in your Mesa amps?

I never have redplating in any of my amps, including the 3 Vox amps I have.

 

[oneblackened]

Mesa-branded EL84s overheated in your Mesa amps?

It's not as uncommon as you think. Mesa uses very hot fixed bias for a lot of EL84 amps, so you don't get the automatic current limiting you get with a cathode bias resistor.

This is a DC-3 for example - an amp that is known to eat EL84s for breakfast, even the uprated Soviet 6P14P-EVs. Even with a reduced screen voltage vs plate, it's still way hot. It runs the tubes way over 100% at idle.

 

[HotBluePlates]

... Mesa uses very hot fixed bias for a lot of EL84 amps, so you don't get the automatic current limiting you get with a cathode bias resistor.

This is a DC-3 for example - an amp that is known to eat EL84s for breakfast ...

Cathode biasing is not a free lunch, as any tube can be cathode-biased to redplate at idle.

I just assumed Mesa would pick low-idle-current EL84s to use with their too-hot amp. The EL84 data sheet (Page 9) shows with 300v on the screen & -11v of bias, the tube is idling above 100% dissipation at 400v on the plate.

Dunno what the "bright idea" is here, since even Philip's data sheet (Page 14) shows the EL84s oughta be run down around -15v (and maybe even cooler if high output power is the goal).

 

[oneblackened]

Dunno what the "bright idea" is here, since even Philip's data sheet (Page 14) shows the EL84s oughta be run down around -15v (and maybe even cooler if high output power is the goal).

I have no idea. They sound better when it's lower anyway. Less mushy and flabby.

Cathode biasing is not a free lunch, as any tube can be cathode-biased to redplate at idle.

My point was more that a cathode biased amp can be biased hotter because the cathode resistor to some degree limits how much current can flow - as the cathode draws more current, the voltage drop across the resistor increases. So even though Voxes run around that 100% idle dissipation point, the cathode bias keeps it from going too too far in operation, whereas the grid bias in these amps doesn't do that.

 

[HotBluePlates]

... My point was more that a cathode biased amp can be biased hotter because the cathode resistor to some degree limits how much current can flow - as the cathode draws more current, the voltage drop across the resistor increases. So even though Voxes run around that 100% idle dissipation point, the cathode bias keeps it from going too too far in operation, whereas the grid bias in these amps doesn't do that.

You're not wrong but what you describe only happens if the cathode resistor value is picked well for the voltages applied to the output tube. It also happens a little differently than you appear to be describing.

- For instance, early AC30s used an 80Ω cathode resistor, and few people gripe about those amps running too hot. But later AC30s (still in the 60s) were using 50Ω cathode resistors, and modern copies tend to use 47Ω (often with higher supply voltages than the original amps). And now people tend to complain about how hard the amp is on EL84s or excessive heat...

- Take any 5E3 Deluxe and install a 100Ω cathode resistor, and the 6V6s will redplate at idle and the whole time you're playing.
​

So the "current limiting" is in proportion to the resistance used to bias the amp. But where it really comes into play is when you try to push a cathode-biased amp far into Class AB (there's a whole spectrum between a tube conducting 100% of the input signal and during only 50% of the input signal).

- When the push-pull stage cuts off on one side and the remaining side tries to go beyond 2x the idle current (of that side), that's when the cathode resistor tends to fight back & increase effective bias to constrain plate current.

- High output power Class AB amps have average plate currents around 2-3x their idle current (for the entire output section), and the peak plate currents are higher than that. It's why all the high-power Class AB amps had fixed-bias, even if the "slightly Class AB amps" (like the 5E3 Deluxe and AC30) operated with cathode bias.​

And you can still mis-load and/or mis-bias a cathode bias amp so that it's redplating at idle or with signal. The cathode resistor is only sensing & holding back cathode current vs grid-to-cathode bias, and doesn't react to the fact there is a too-high plate voltage present (since plate dissipation is plate current * plate voltage).

- Also, if screen voltage changes, the right grid-to-cathode voltage for a specific amount of plate current changes, so a resistor copied from one circuit with a low screen voltage is a wrong-value (idles too hot) in another amp with a higher screen voltage.​

There was a thread on here a couple months ago where a guy was experimenting with larger cathode resistors to keep his modern Vox amp from running too hot & redplating.

 

[oneblackened]

You're not wrong but what you describe only happens if the cathode resistor value is picked well for the voltages applied to the output tube. It also happens a little differently than you appear to be describing.

- For instance, early AC30s used an 80Ω cathode resistor, and few people gripe about those amps running too hot. But later AC30s (still in the 60s) were using 50Ω cathode resistors, and modern copies tend to use 47Ω (often with higher supply voltages than the original amps). And now people tend to complain about how hard the amp is on EL84s or excessive heat...

- Take any 5E3 Deluxe and install a 100Ω cathode resistor, and the 6V6s will redplate at idle and the whole time you're playing.
​

So the "current limiting" is in proportion to the resistance used to bias the amp. But where it really comes into play is when you try to push a cathode-biased amp far into Class AB (there's a whole spectrum between a tube conducting 100% of the input signal and during only 50% of the input signal).

- When the push-pull stage cuts off on one side and the remaining side tries to go beyond 2x the idle current (of that side), that's when the cathode resistor tends to fight back & increase effective bias to constrain plate current.

- High output power Class AB amps have average plate currents around 2-3x their idle current (for the entire output section), and the peak plate currents are higher than that. It's why all the high-power Class AB amps had fixed-bias, even if the "slightly Class AB amps" (like the 5E3 Deluxe and AC30) operated with cathode bias.​

And you can still mis-load and/or mis-bias a cathode bias amp so that it's redplating at idle or with signal. The cathode resistor is only sensing & holding back cathode current vs grid-to-cathode bias, and doesn't react to the fact there is a too-high plate voltage present (since plate dissipation is plate current * plate voltage).

- Also, if screen voltage changes, the right grid-to-cathode voltage for a specific amount of plate current changes, so a resistor copied from one circuit with a low screen voltage is a wrong-value (idles too hot) in another amp with a higher screen voltage.​

There was a thread on here a couple months ago where a guy was experimenting with larger cathode resistors to keep his modern Vox amp from running too hot & redplating.

Right, I understand all that. My point was more with properly sized cathode resistors, of course.

 

[soundchaser59]

Mesa-branded EL84s overheated in your Mesa amps?

All the Mesa amps I've had except one were bought used. The one I bought new had the Mesa tubes removed at the store and replaced, which I didn't know about until later because it was my first ever Mesa amp. I got hold of Mesa and they made the store ship me a full set of brand new Mesa tubes. But I never had any EL84's red plate in that amp, that was a Mesa Express 5-25.

The other Mesa amps I bought were all used.....two DC3, a few Subway 20, a couple of Subway 44, all of them ate tubes for lunch, most red plated, if they didn't red plate then they only worked for a few weeks, maybe 200-300 hours, something like that. Even the Soviet 6P14P red plated. But I did notice that the red plating stayed with the socket, it didn't follow the tube. I could switch the tubes around, but any tube I put in that socket would red plate. The Subway 44 had two sockets out of four that red plated tubes.

But when I bought brand new JJ the red plating stopped. A set of new JJ EL84 would last at least 6 months, maybe a year in those amps, and that was playing fairly loud maybe 15-20 hours a week every week. So at least twice as long as the other tubes. As I read more and more I realized the amps were designed to run hot, but man they sure were the best sounding amps. That Subway 44 is the only amp I've ever had that sounded as good as the Rivera K55 and R55 I have now.

 

[HotBluePlates]

... But I did notice that the red plating stayed with the socket, it didn't follow the tube. I could switch the tubes around, but any tube I put in that socket would red plate. The Subway 44 had two sockets out of four that red plated tubes. ...

Gotcha! That tells you there is either a failed component feeding those sockets, or an oscillation issue. So it's something in addition to Mesa being boneheaded with the design.

... But when I bought brand new JJ the red plating stopped. A set of new JJ EL84 would last at least 6 months, maybe a year in those amps ...

The particular JJ may have just been cooler-running than the other tubes you'd had. That's why I asked about whether you'd used Mesa tubes in the amps, as I wondered if they selected cooler-running EL84s for sale. Mesa usually uses non-adjustable fixed bias in their amps, and then says you have to buy/use their tubes or void your warranty. There are arguments on both sides of that issue, but in theory Mesa could be selecting the tubes that will run in their amps as-designed without problems.

For instance I have a pair of American made 5881s that IMO must be defective. when plugged into an amp, they pull about double the current of any other 5881 or 6L6 in the same circuit. So plug them into your amp, and it will redplate unless you go to extremes to tame their current.

Any individual tube may run at higher or lower current that the data sheet specs say they should.