THE GREAT MULLARD MAGIC BLOG

I've got this terrible pain in all the diodes down my left side, good old Marvin and, doesn't the symbol for an indirectly heated thermionic diode look similar to Marvin 1.0's face? Who can forget those fateful words by the laconic and lugubrious Marvin, the Paranoid Android from The Hitchikers Guide to the Galaxy who was also described as having a brain the size of the planet as the video embedded below will remind you.     Of course, Marvin featured large in my formative years, because when I met the yet to be Mrs Mullard Magic, when she was a pert PhD...

With the birth of the Superheterodyne radio and the means to keep valve count and commensurate valve cost low, it turned to the valve manufacturers – particularly Mullard to come up with advanced valve designs to accommodate this requirement.    In early Superhets, the heterodyning frequency was generated by a local oscillator valve and was then super-imposed on the signal by a 'mixer' valve. A handy dodge was to incorporate both of these functions in a single envelope and developments saw successively the double-grid valve, the screened tetrode and the HF pentode being used where injection of the heterodyning frequency...

Today, I would like to raise the topic of SCREENS.  As we discussed last time, internal carbonising only decreases the risk of secondary emission but cannot prevent this unwanted electron straying.     As we have noted, secondary emission can play havoc with a valve's preformance, particularly in multi-electrode cages due to unwanted  coupling effects.   For some multi-electrode cages, metallic screens are fitted to keep the electrons on their true path.  Below, we have a photo of just such a deflection screen shown on the electrode cage of a dismantled EF91 pentode where you can clearly see a metallic...

Today, I want to discuss INTERNAL CARBONISATION sometimes also referred to as screeding or colloquially as grey glass.   From preceding blog articles, I have already explained that electron emissions  are drawn from the cathode to other positive electrodes within the cage and sure enough, the majority of emissions do travel within the electrode cage, however, like recalcitrant small animals and wilful children, a small proportion don't do what they should and escape from the electrode cage striking the glass envelope walls. These 'loose' secondary electrons can build up a potential charge on the glass which can cause audible distortion and sometimes a...