[Amps] Outgassing tubes.

[Ian White G/GM3SEK]

Trying to wrap this up...

Contrary to what I'd said previously, apparently microscopic surface 
spikes or "whiskers" *can* re-grow on some metals in a vacuum. This can 
affect NOS and long-stored vacuum variables, but the spikes can be burnt 
off by controlled application of high potential.

Returning to tubes,  "conditioning" or "burn-in" of new, NOS or 
long-stored tubes needs to be seen as a big picture involving both 
cleanup of the vacuum and removal of any surface spikes. Gas releases 
into the vacuum space provide the current carriers that can start an 
arc; surface spikes provide locations where an arc can strike at reduced 
voltage.

Conditioning can only help remove very small quantities of gas that have 
been released by out-gassing of the tube's constructional materials. But 
if the tube is gassy because of a leaking seal, there's no way back.

>>What hasn't been mentioned much in this thread is the damage to the rest
>>of the amp when there is a large current surge in a tube. Sometime in
>>their careers, most amps WILL suffer these current surges, for one
>>reason or another - but it doesn't have to wreck the meters or anything
>>else.
>
>Sure I did. I said it could harm the rectifiers and capacitors where 
>the arc is similar to touching the B+ to ground quickly. It's a good 
>sized current surge with many joules of energy. If it was bad enough, 
>or excessive, it could even cause a failure in a HV winding of the 
>plate transformer. I recommend that any new tube be baked in the way we 
>were talking to prevent any arcing at the get go to save the power 
>supply. One never knows that one may or may not arc. This especially on 
>NOS tubes I'd think.
>
Apologies to Will for any implication that I'd ignored his contribution. 
However, I don't agree with his pessimistic analysis. Certainly it's 
good to condition the tube(s) to reduce the risk of arcing - but if an 
arc does happen, major damage to the rest of the amp is totally 
preventable.  The worst that should happen is a blown mains fuse.

A basic upgrade package for a low-cost or older amp is:

1. A surge limiting resistor ("glitch resistor") in the B+ line. This 
resistor is NOT intended to blow - quite the opposite. It's intended to 
hold together and absorb the surge energy for the first few milliseconds 
until something else (the mains fuse) disconnects the power source.

2. Meter protection diodes in the cathode/grid/B-minus area.

3. A step-start circuit, which has many other benefits, but importantly 
it allows you to...

4. Change the mains fuse or breaker to a lower-rated quick-blow type. 
For the most protection, use the lowest rating that will allow the amp 
to start up and operate under normal conditions.

There is more detailed information on both Rich's website and my own.



-- 
73 from Ian G/GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek