DIY tube gear and risks to life and limb,

[mikael__]

I guess this is one of those “if you need to ask you shouldn’t be working with this”-topic, but…

i’ve been increasingly interested in tube designs lately. I have doing many electronics projects over the years, but I have no formal training in this field whatsoever. For tubes the high voltages involved concerns me. For “normal” audio equipment I can keep it all connected to my trusty bench supply until I can verify that the circuit works as intended. For tube voltages it seems more difficult.

Can anyone share som general recommendations? Is this an area people with formal training should stay away from or is it generally safe if one takes some safety precautions while working ?

 

[tk@halmi]

You need to follow mostly the a same safety guidelines as residential electricians do. There is ample information on this available on the internet or your local library. When I work with high voltage circuits making sure that I am isolated from ground, as to not allow electrical current to flow through my body, is top priority.

 

[JohnRoberts]

Always keep one hand in your pocket. That makes it harder for an electrical shock to pass through your core and stop your meat ticker.

JR

 

[Ó Draighnáin]

What John said, I got into a habit of putting one hand behind my back. I only ever want to have one hand over the chassis. Remember to drain the capacitors.

Typically, valve circuits are pretty forgiving and valves are pretty robust. I have built a reasonable amount of valve gear. I have only ever cooked one valve and one transformer (see below).

My initial concerns have always been around making sure I don't fry any transformers when firing up for the first time. Check the circuit, check continuities. If you can't check your circuit against a schematic, then you shouldn't be doing it.

Obviously, make sure you know the voltages you expect to see, sometimes you need to wait for the HT to stabilise after the initial inrush. There often seems to be some tuning to (or taming of) the HT supply required.

There is plentiy of online guidance in respect of measuring voltages with and without valves and what you expect to see from a loaded vs unloaded transformer.

The only time I fried an OT was because I wasn't careful enough. I accidentally created an arc between two socket tabs with the multimeter probe tip. I recall it was between the HT and the heater supply in a JTM50 I was building. As I was removing the probe from the HT of one of the EL34s, I must have somehow just brushed the heater tab for a microsecond creating a bridge. As a result, one of the valve socket tabs ionised (vaporised) in a quite spectacular display of blue light.

This cooked the EL34 (a nice NOS brown base Tesla from the 60s) and the expensive output transformer.

 

[KrIVIUM2323]

Multiple random thoughts:

Use hook probe or anything giving secure connection to avoid shorts ( crocodile clips).
Use rubber glove ( put ring asides) and rubber soled shoes, no bare feet or socks.
Be careful with reflexes and habits from low voltage: rubbing a 'high voltage' rail can arc badly and reaction can be bad ( keep your hand into your pocket).
If possible don't use screwdrivers or metal tools when probing around a live circuit.
Assume anything exposed is hot and live, don't rely on insulation, unplug and discharge caps, works slowly.
Use wear eye protection...

Isolation transformer with automatic ground fault switch on your workbench.