WAD 6550 Kit Cathode Resistor Upgrade

[Revoli]

I am having occasional surge/flash (disconcerting at least|) seems to be in 6550 valves. Possibly rectifier surge? I have not measured anything as yet. Valves should still be ok.... replaced quite recently. I have notes that suggest upgrades discussed in years gone by for rectifier flash over surge resistors to 47R are a good idea. Wondering if this may be sensible....

There are some other outstanding upgrades I think may be worthwhile when I take the lid off including:

1. Upgrade of Cathode resistor on 6550s to 680 Ohm as per attached article from Oct 2001 re changing pin 8 resistor as per attached article (Hi Fi World Magazine I believe).

2. Improving DC Heater Supply. http://www.world-designs.co.uk/forum...ead.php?t=7643 The idea is the same but these amps are built on tag boards so with replacement of the bridge with 4 schottky diodes and a little re-arrangement of the components the following can be achieved (see attachment)


Any ideas/guidance welcome.

[TimN]

I've just fitted the DC heater mod on mine in an effort to address an annoying buzz that appeared after swapping the original input selector switch for the relay board fitted to modern WD kits. I mad a daughter board and fitted it to the tag board in place of the original rectifier. I haven't tested it yet so don't know how successful it will be. Photo attached.

Mine already had the other mods fitted it when I bought it, but I've never had the problems you mention so I assume they have worked.

[bob orbell]

To Revoli, yes, do the 47R resistors first to see if this stops the flashing, the flashing was always it the 5U4G tubes, as for the heater upgrade, I would not bother as I doubt you will hear any difference, in fact it will most probobly make the ECF80 voltage too high, if you ues shottky diodes, more trouble than its worth. Bob

[jdd308]

I also did the 47 Ohm resistor mod years ago on my KiT6550 and it has worked fine until recently. The mains voltage is now a lot higher than it was, 250v, and I'd also recommend the following:

Use 5U4GB valves rather than the 5U4G, so NOS American valves like JAN National, Amperex, GE as they seem a bit tougher and resistant to flashover.

Add a thermistor in to the main input (search thermistor on here for more) and this has been the best mod so far. I use a 10 Ohm 4 amp thermistor from RS, however this one might be a bit strong as the smart meter zips round to 12p per hour from 2p per hour when the amp is on! At least I'm not blowing fuses every time now.

Replace C6 in the power supply 4700uF and 10v. Maybe mine was dodgy after 20 years as the test voltages are now a lot more in line with what they should be, especially the heater voltages.