I've come across some posts in the past claiming worn out tubes draw more power. I can't see how is that possible. But when I put old tubes in my amp, the trafo gets warmer. I put this down to the tubes simply being a different brand. But I still wonder about it.
Did you measure any of the relevant currents or just use your scientifically calibrated thermometric fingers?
Indeed without any measurement it's hard to say. It could be as simple as the line was stiff that day.
My only experience here relates to restoring and retubing old cathode biased broadcast amps with current measurement jacks for each tube. The average modern 6V6's and 6L6's always draw more than the stated spec, and I've made it habit when ordering matched tube sets to request the lowest current numbers of what's available. That gets things more in the ballpark. There is easily a factor related to modern wall voltage in this case.
I didn't bother with measurements, just trying to very what I've read. I noticed that the power trafo gets warmer with old worn out tubes which would indicate higher current being drawn. If it's not the case than it's simply a tube that draws more that's all.
If anything you expect the opposite. A worn tube heater should be a bit higher resistance as it is thinning and cathode emission will be down, reducing anode current
If just one tube drew enough extra power to make a power tranny noticably, significantly, warmer, it would be red plating long before you felt the difference. Why not check the biasing and current if you have a basic question about tube conditions? Being "old" is not a reason for a tube to draw less current. Being old is not a reason why it couldn't draw more current. Reduced emission and lower current flow is not the only thing that can happen to a tube as it gets used. It's easy to check for problem tubes and or other related components that may be failing and wrecking the good tubes you have. ... yes?
If you haven't been measuring anything during the life of your amp or tubes then how do you know whether or not your old tubes are drawing the propper current and it's your new tubes that are not drawing enough?
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Off course I test my tubes every now and than, The question was about old tubes, tubes which I know are worn out, around 30% life in them, at least that's what my tube tester shows and my ears can hear. I've just tried it today again. Took out four good tubes, put in four crappy ones, PT runs hotter. No point taking measurements on what they draw if I don't know what they should draw when they were new (50 years ego).
Assuming the valves have a normal grid resistor to ground, measure the DC voltage across this with no signal. It should be less than about 0.5V; an ideal valve would give near zero or even a slightly negative voltage. You may find that the old valves give a higher voltage, which indicates gas inside.
This is the way your experiment should be run. Put the amp on a regulated PS so the line voltage doesn't change as the experiment progresses. Put in your new tubes and run a 1K-2Khz drive signal set a fixed level. Allow the tranny to fully heat for 1-2 hours and check it with an IR thermometer on a stand that points to the same spot throughout the test. When the temp has stabilized at max, take the bias and grid voltage, screen voltage, plate voltage readings and calculate the currents through each tube. Turn off the amp and swap tubes. Repeat the procedure with the old tubes and write down the differences and report. Do the old tubes have cathode/grid leakage that could increase the idle currents? What is going on with them individually? Was the temperature difference from impropper procedure... this is what it takes to get to the bottom of something you think you see in amps. Measure. It takes a while to learn all the ins and outs. Or you could just skip the PT grabbing and check the voltages around your tubes and see if one or more is going whacky.
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Whacky tube yes that's a possibility. I just wanted to find out if what I've read was true, that's all. When I get some time to mess around with tubes I'll do measurements out of curiosity to see what's causing it. But for now I'll take the answers as "no, old tubes don't draw more power".
It's not. Old worn out tubes lose cathode emission: that's what wears them out. Back in "the day" it was a common practice to add small xfmrs to the heater leads of TV CRTs to overheat the cathode to get a bit more life from them. These were plug-in modules, so no circuit surgery necessary.
Does it really? Old tubes can also go gassy and pull higher than design nominal plate currents. This is seen more frequently with fixed bias. It doesn't take much gas to cause "bias creep" when using fixed bias, not even enough to manifest as a glow. (Any decent tube tester will register that creepy tube as gassy.) Some become gassy enough to begin to act like thyratrons that can pull poofage current. In that case, the gas discharge fills the entire inside between the plate and cathode, and it will probably red plate. I had an 807 go that way.
I'd expect that if the old, gassy tubes were pulling enough extra current to heat the PTX excessively you'd see red plates. Either that, or the PTX has no margin of safety at all.
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