THE GREAT MULLARD MAGIC BLOG

Over-running output valves and rectifiers or allowing them to operate at extremely high envelope temperatures, can have another undesirable effect as the valve ages where electrolysis of the the envelope glass can occur.  The electrolytic action attacks the glass envelope in which lead (Pb) is extracted from the glass  as an anion which in turn is then attracted to internal wires having a negative charge resulting in metallic lead deposition around the negatively charged lead in wires -  especially where they enter the pinch or base button.   The effect of these 'Lead lead trees'  - which are often only...

This is an interesting one as a number of insulation failure modes exist for ageing valves.   Often poor insulation may be caused due to cathode emissive material or metallic deposits which have evaporated from the  electrode cage being deposited on various parts of the valves interior.  Deposits can build up on mica separators or the glass pinch where the connection pins/leads enter the envelope and even sometimes the inner envelope glass - remember those manky EL84 which had been run hard and hot and built up those yukky black deposits on the glass.........? In manufacture, the mica separators are...

Well just look at this, bet you have all seen 'phone zombies' like this mooching about and crossing roads without looking.   Nice Sennheiser headphones the lady is wearing and the iPhone looks cute too - very 2017.     You might think this is a new phenomenon but no, you would be wrong as the following picture attests:- Et voila, in 1924,  a young lady outdoors, complete with headphones and a rather much heavier piece of music making kit.    Like I said, nothing new under the sun!!!!  

We recently left our two part Star Trek blog wondering whether or not thermionic technology ever again appeared in episodes moving on from the pilot episode, The Cage and the answer is a resounding YES.     I want to bring your attention to Star Trek TOS, season 1, episode 28, titled The City on the Edge of Forever, from star date 3134.0 or by my calendar February 18,  2326, Time: 21:50:24.  This episode first aired on 6th April 1967. As in the prior posting, I could paraphrase the plot but I am not going to, instead, here is a...

If a valve passes an anode current of 1mA then this equates to 650, 000, 000, 000, 000 electrons a second.  When you consider that an output tetrode like a KT66 can pass up to 100mA in anode current then as Cilla Black would have said 'that's a lorra, lorra electrons!'    I think this little fact demonstrates quite well that over a period of anywhere between 2000 and 8000 hours on average, a valve may loose a proportion of it's emissive properties.  During the early life of a valve, the effect of emission loss is realtively imperceptible but after...