Tubes HOT!

[agithegreat]

Ok, so i fixed my volume pot problem but now after running the amp for a couple of minutes i felt the valves and they are much hotter than the used to be. I cannot touch them for even 1 second.

The heat seems to come from the middle of the tube, the tops and bottoms of the tube are relatively cool.
The heater voltage is 6.2v solid. The B+ voltage is 315v ish dropping to 275v This should be 250v i know, but would this result in hot running valves?

[TheGimp]

Are any of the plates glowing red (best viewed in a dark room)? If so you are dissipating too much power adn have biasing/supply problems.

[agithegreat]

No, one of the plates goes blue after about 30 seconds? This always used to go blue though.

Then after about a minute, some other blue appears (looks like its on the side of the tube).

Just the heater is red though. (In 2 parts by the looks of things) top and bottom(coil at top and some small linkage looking thing at the bottom).

[DF96]

If your HT should be 250V but is actually 275V then your valves will probably be dissipating 10-20% more heat than planned. Whether this is harmful or not depends on how close they were to the limits before. It is quite normal for valves to be too hot to touch.

[agithegreat]

I've pulled the russian 6p1's out and put back the chinese ones so if they break it wont matter too much.

If they were to break, would it break the glass catastrophically or just break the filaments?
I mean if it will be a hazard i'll try and lower the b+ but if it just stops working due to it melting a plate and forming a break in the circuit i really don't mind as it's only the chinese tubes.

[DF96]

Hot valves don't generally fail catastrophically. The most likely outcome is shorter life. A very hot valve may crack the glass, usually by a pin, and then the getter goes white and the valve dies. It is just about possible to melt the glass and see it collapse - in some cases the valve may continue working for a while, I have heard of this happening in a TV. So if you are happy to replace valves as necessary then carry on. Just don't burn your fingers!

[agithegreat]

Great. A few years less life on some £10 valves doesn't really matter does it .
If they melt or split or crack, or even just fall apart i'm not bothered, i just don't want them shattering. I'll keep them running then. They play lovely.

[Miles Prower]

Originally Posted by agithegreat

No, one of the plates goes blue after about 30 seconds? This always used to go blue though.

Then after about a minute, some other blue appears (looks like its on the side of the tube).

This looks like gassy tubes. Once they go gassy, the bias starts to run away, and won't settle down. I had a gassy 807 that also did that, pulling way more current than it should have. In that case, you could see the blue glow filling the space between the plate and cathode.

I also had gassy 6BQ6GTBs. In this case, there was no visible blue glow, but the bias would never settle, and just kept rising until they went red plate at no signal. You could get rid of the color by playing them, but not settle the bias. A tube tester verified the gassy condition.

[Ian444]

Measure the voltage drop across each 6P1P cathode resistor, from that you can calculate how much power the tube is dissipating at idle, if its less than 10W you should be OK.

[TheGimp]

If the blue glow is along the screen support rods inside the plates it is no problem. I've run over 30 6P1P-EVs and every one has this. Kind of a nice touch in my opinion.

6P1Ps run hot at 40mA and above with 250Vak (10W plate dissipation) and will burn your fingers. One of my specs states the "Bottle Temp" as 220 C at full dissipation! I presume this is at max op ambient of 90C, so that is a temp rise of 130C and at 21C ambient it could be as much as 151C! Toasty!

For images of the blue glow look at my thread on the Sven 6P1P-PP amp post 11:

Sven 6P1P PP Amp

I've pushed 6P1P-EVs much higher than 10W without catastrophic failure, but don't recommend doing it long term. I've had a few catastrophic failures which I think were due to one bad tube that partially shorted a cathode resistor and subsequently destroyed two more tubes. See later in the same thread. There is a pix of one tube failing in a bright flash. No busted glass, just toasted guts.