thermal runaway in valves

[spuffock]

I have seen some valves, all of Chinese manufacture, go into a state of thermal runaway, in which the anodes go from just visible red, through orange to a damn great hole in about 2 seconds. The first was a 6146 in an hf transmitter, and subsequently several el34 and 6l6 in guitar amplifiers. The best theory I have heard to date is that the Chinese are happy if they get most of the oxide coating on the cathode, and are in too much of a hurry to care where the rest goes. That which ends up on the control grid will begin to emit electrons if the valve is run anywhere near its maxmum dissipation, turning the valve further on and getting it hotter.
Has anyone any better ideas? I bet Nigel has something to say on this one

 

[Nigel Goodwin]

Has anyone any better ideas? I bet Nigel has something to say on this one

Well I wouldn't have had :lol: but seeing as you asked!.

Generally Chinese made gear is absolutely crap, but I think you already know that!. Luckily I don't have any need or use for valves, I consider the modern fashion for valve audio gear amusingly stupid - with the exception of valve amplifiers for lead guitar use, where the soft distortion of the output stage clipping is often desired. The bizarre fashion of adding a single ECC83 in a guitar preamp and tripling the price is beyond belief, but people still pay it?.

 

[spuffock]

Unfortunately I have seen the practice of adding a single ecc83 to a transistor amplifier and fitting a string of big wirewound resistors from the 60v rail so that it lights up. A collection of passives are grouped around the valve to make it look good, but there are no volts on any of the electrodes. Removing the valve stops the smell of burning from the pcb under the resistors but has no other effect on the operation of the amplifier.
This amplifier is in current production by a well known and respected? manufacturer of guitar amplifiers but I can't remember who until I see another one. When I'm sure who it is I will shout their name.

 

[Nigel Goodwin]

This amplifier is in current production by a well known and respected? manufacturer of guitar amplifiers but I can't remember who until I see another one. When I'm sure who it is I will shout their name.

There's more than one that do that!, some don't even bother adding components round the valve - just feed the heater to make it light up!.

 

[spuffock]

It was FENDER! so go on, FENDER, sue me for letting on!!