03-08-2015, 03:45 PM
(31-07-2015, 04:47 PM)Murphyv310 Wrote: I have never been a fan of either directly heated valve rectifiers or silicon rectifiers. This is a personal thing as neither are particularly elegant. With these types of rectifiers the off load voltage rises very quickly before the rest of the valves conduct. It's sore on the smoothing caps and even old paxolin valve bases.
Adding series resistors can reduce the the switch on surge but they are a compromise, insufficient current can be a problem with a warm set and of course series resistors like this get hot!
If at all possible I will swap a directly heated reccy for an indirect one, eg 5U4 - 5Z4. I've even replaced 4 volt reccys with a 5Z4 with no problem. My 1936 Pilot has an EZ80 in an adapter base and it's heater is 2v under run. It works 100%
Any idea how hot the series resistor gets? Today I decided to test the anode on my output valve for voltage. At the moment I'm finding it to be about 245 DC. The manual gives around 290 and points out the test was with an Avo 8 on Medium waves and at full volume. Of course, I expected the voltage to be a bit low as I started with a 1K resistor. However, I should point out this wasn't resistance in series with each cathode but just one resistor at the bonded cathode ripple output. Plus, my grid bias resistor on the output tube is 50 ohms out on the high side.
I can say, I'm really not so keen having the diodes soldered to the valve pins as it's a bit "rickety". The soldered joints seem really O.K. but you never know if it takes a knock and something comes off. Maybe one more reason why it seems a good idea to put a 1 amp fuse in the supply hot line (something I have to do). I guess you could say now I know I have a working set, maybe it's worth considering putting the tube rectifier in should one turn up at an affordable price.
However the case may be, I have a feeling my next project may involve either a metal rectifier or the silicon variant. I have a thirties TRF radio that actually came with a metal rectifier. My theory is someone at some time modified the radio around the fifties in order to do without the battery. Whichever way I go, it will involve a 120 volt HT supply to the Smoothing cap. All the valves are present this time and they're as big as pepper pots.
