Who knows anything about Thermionic Valves these days?

Thread Starter


Joined Dec 9, 2009
Hi guys,

I'm about to build my first valve circuit! I needed a high voltage amplifier, and found a great application note:


I'm going to build the '300V Output Booster'. Would anyone care to explain how exactly this circuit operates? I will be doing my own reading in the meantime, just thought I'd create the opportunity for some knowledgable soul to show off some of their expert valve kung fu ;)


Joined May 23, 2013
Hi, the applications notes seem to describe it very well. Just remember to take extra care when working with high voltage circuits, they can bite!! :) As stated in the notes.........The reader is advised that the construction, testing and use of this circuit must be approached with the greatest care. The output potentials produced are many times above the level which will kill. Repeating, the output of this circuit is lethal.


Joined Mar 10, 2018
Most amateur radio operators (the tech ones) are very involved with
tube amps, because of operating legacy equipment and RF power amps
for transmitters.

Hook up with a local group and you will find some white hairs that know
a lot about tubes.

Here is a wealth of info in the form of databooks, magazines, etc.. Many of
the older mags, like Electronics World, radio electronics, QST have extensive
articles on tube stuff. The tube databooks some have many articles on tube


Regards, Dana.


Joined May 23, 2013
Having been an amateur Radio operator (Ham) for many many years, and apart from my career in high voltage electronics, (early years all tube technology) as a hobby I have built several H.F and V.H.F linear amplifiers using valves and also later on solid state. The Valve linears are more tolerant of tune up errors, wrong antenna matching etc before they let out the magic smoke, whereas solid state seem to have a pre-disposition to self destruct at any opportunity unless very good protection/shutdown methods are employed.
This does not mean you can have a carefree attitude with the tube amps, and damage to expensive components can still occur if mistakes are made. Some of the tubes can cost as much as all the other components in the amplifier combined. Good operating techniques need to be applied and care taken when operating and or building, testing, repairing etc. all high voltage equipment.
I am wary of giving any more advice, as I do not know how I can put over 65 year experience (I was playing with valve circuits at age 6) in a few pargraphs here other than to say that the project you are looking at, really needs careful consideration regarding safe working practices, so I urge you to read up on safely working with high voltages and please take care. Best of luck, :)
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Joined Aug 27, 2009
I still work on a few tube devices today, mainly in high-voltage deflection and electron suppression systems.

Rule #1: Troubleshooting a circuit like the booster is extremely dangerous for a novice. Use the one hand only rule for probing live signals so when you do get shocked it will only hurt like hell, not kill you.
When possible, employees should perform energized electrical work following the “One Hand Rule”. This requires the employee to place one hand in a pocket to prevent contact with grounded work surfaces while performing energized electrical work and to prevent electrical current from flowing through the heart.