• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Cherry red plates

Was it common practice for amp makers to push output tubes beyond their specs?

I'm fairly new to tubes, but I have restored a half dozen pre-war AM radios. I've recently done a couple post war HiFi amplifiers. The first was an 1962 AMI Jukebox amplifier. The 6973 output tubes were missing, but a quick Google search suggested 6CZ5 as a substitution. These over heated and made the plates glow red. :( It was evident from the spec sheet that the plate and screen voltage were too high for this tube. BUT, it was also some what beyond the max for a real 6973. I ordered a set of Russian 6973s and they play fine.

The next amplifier I worked on is a 50's Pilot AA-903B. I got this working with the original tubes. The output tubes are PP EL84s. The two tubes were different brands (GE and "M Valve Art"). I inherited a tube collection that has a dozen NOS Raytheon EL84s, so I figured what the heck, two of the Raytheon would surely match better than what's in there. I put two of the NOS Raytheons in, and they played, but after a while the plates got red hot. :( The plates and screens are running at 324V the max spec in the data sheet is 300V. Grid one is -10V just like the schematic shows. I put the original tubes back in, and it's been playing just fine for the past couple days.

Back when I was a teenager, I had a 60's vintage Fisher stereo amplifier. I don't remember the model number or the tube numbers. For some reason, I decided to change the output tubes. I didn't know what I was doing back then, but I do remember that I replaced the old ones with the exact same tube number. Guess what, all four got cherry plate. :( I put the old tubes back in and every thing was fine again.

So, I have not had good luck with output tube replacement. Do these things have to be hand picked? Does everybody run into this? What gives?

BTW, the AMI and Pilot amps both have fixed non-adjustable bias, and the coupling caps are new.

Bobby Dipole
I'd say that the thing to do is to set the bias, even if the bias is "not settable". Check the current through tube by measuring across either a cathode or plate resistor, as needed.

Sometimes tubes that seem otherwise good are not and the current "runs away." I've seen this quite often with EL-34s/6CA7 tubes - over time the quiescent current will creep up and up and up, and after a while you can no longer set the bias so that the current falls within limits.

Since you say the coupling caps are good (are they really? check the bias voltage on the grids) a leaky cap will cause the bias voltage to go the wrong way.

Something is not quite right with what ur experiencing.

Leaky coupling capacitor to the output grid is the most likely cause of red anodes. You will find that this capacitor needs replacing on almost all old radios. Otherwise you will just have to keep replacing the output valve, so using up remaining stocks.

When you say the coupling caps are new, do you mean replaced with a modern component or replaced with NOS caps which may be as leaky as the ones you took out?
I think Anatoliy is being a bit optimistic about the bake potential in an oven... you'd need an induction heater to do the job usually... running the filament a bit hot for a while may work in some cases... but might harm the emission too... I kinda think that if you got the tube hot enough for the remaining getter material to have an effect you'd do harm to the bakelite base... otoh if you desolder the tube from the base OR if it doesn't have a base ur in business... :D Keep in mind that you had best let it cool very slowly so that you don't crack the glass.

Yeah, how hot... maybe on clean cycle?? Guessing...

There is another factor that isn't mentioned here. When many of these vintage amps were made the average line voltage was 110 to 115 volts. Now 122 volts seems to be the norm.

There are also a lot of different things being sold as EL84's. I run mine at 355 volts plate ans screen with 12 to 14 volts on the cathode. Vintage Sylvanias and modern JJ's seem happy. Many others develop the glow of death after a few minutes.


2007-11-13 4:03 pm
"I think Anatoliy is being a bit optimistic about the bake potential in an oven... you'd need an induction heater to do the job usuall"

Not at all
last christmas I baked 8 gassy 13e1 tubes over night.They were all running away before a night in the oven.Afterwards 7 has behaved well.look at vacuumstate website for MJ 's article on the subject