[Amps] QST, diodes and equalising resistors

[Peter Chadwick]

Tom, W8JI, commented on the suggestion in the QST article (which I haven't seen yet) of using small  470k resistors across the diodes. I  seem to remember having this discussion previously, and having spoken at the time to the group in our company who made diodes (and SCRs and triacs and IGBTs in the several thousand volt, several thousand amp range for traction purposes) their advice was that unless all the diodes are from the same batch, you couldn't be sure that they'd all avalanche at about the same voltage. Avalanching can be controlled, but only to a certain extent, so if you are replacing one diode in a stack with equalising resistors, it's worth keeping the resistors. Or better, is to replace the whole string with new controlled avalanche diodes from the same batch and no resistors. But if using the resistors, they should be rated for the voltage, which small resistors are unlikely to be. If they are old carbon resistors, throw them out - more than 10 years old, or been heated, they're unlikely to still be within 20% of nominal.
A final point about QST. Like all amateur magazines, it has to appeal to a generally less technical audience than of yore. But the League is about a lot more than just QST, and without the League's support of IARU, you wouldn't have any need for amplifiers, because you wouldn't have any frequencies to use them on. My gripe about the ARRL (and I am a member) is that they don't blow their own trumpet enough about the work they do with IARU at the ITU and CITEL. Of all the national societies, only the JARL come close to doing the amount of work ARRL does in supporting IARU at the ITU - where it really matters! So in many ways, foreign amateurs are better represented where it matters as ARRL members than as members of their own national society.
73
Peter G3RZP 
.