I’m not much on the smell of rotten eggs.
I got all the parts I ordered to replace the rectifier and the electrolytic capacitors in the Granada a few weeks back. I replaced all the parts and reassembled the radio. It worked, and had less noticeable hum, but I checked the voltage on the B+ and discovered that replacing the ancient selenium bridge rectifier raised the DC output. The B+ was at about 309 volts instead of the 250V with the original Siemens B250C100 selenium bridge rectifier.
Tubes are rather tolerant of over voltage, so I put the radio in the living room so we could listen to it and then I ordered some Zener diodes to reduce the B+.
I didn’t want to leave the radio operating on high voltage for too long. The tubes might tolerate it, but I’m pretty sure it wouldn’t do their lifespan any good.
The Zener diodes came in a couple of weeks ago, and I finally got a chance to finish up the Granada this afternoon.
Here’s the final circuit I ended up with to replace the old diode bridge:
|B250C100 replacement circuit|
For comparison, here’s the original circuit:
Given the 100mA rating, dropping those 54V means dissipating 5 watts. The 1N5355B is rated for 5 watts, so that’s plenty of a safety margin - each only has to handle 1/3 of the full dissipated power.
The 50µF capacitor is simply a modern replacement for the existing part. I added the 100nF part because I had one here. I don’t know that it helps, but it doesn’t hurt.
The final output voltage is 256VDC. Still a bit high but I don’t think it’ll bother things in the long run.
The ripple is better. It isn’t dramatically lower as seen on the oscilloscope, but the hum is noticeably quieter - or rather, not noticeable any more.
Here’s what the ripple for the new rectifier looks like in comparison to the old rectifier:
|Rectifier replacement||Original B250C100|
There’s only about four volts difference between the two, but the frequency content is different. Between the lower voltage and the less zig-zaggy ripple, the hum is now low enough that the magic the Blaupunkt engineers did with negative feedback through the transformer can clean it up to inaudible levels.
If you have to replace a selenium rectifier, keep that voltage difference in mind. Check the specified DC output voltage for the device, and compare it to your replacement. Zener diodes are available in many voltage ratings. Find ones that’ll handle the power at the rated current, then stack Zeners to get the needed voltage. Multiple Zeners are better than one rated for the exact voltage - with multiple diodes you can spread the power dissipation (and heat) over several diodes. They’ll run cooler and last longer.
You’ll often find recommendations to insert a series resistor. Don’t do that. That depends entirely on the load for regulation. At low load the voltage will be high, and at high load the voltage will be low. The Zeners can regulate the voltage better - they maintain the voltage better despite the load changes. “Load changes” means “volume” when dealing with tube amplifiers.
At any rate, the old Granada sounds good. It’s going back to the living room in just a few minutes - I’m about to head downstairs to watch Endeavour Morse on TV, and I’ll take the radio with me.
About the rotten eggs:
Selenium diodes have a tendency to quit suddenly and stinkily with a stench of rotten eggs. The diodes in my old D43 have done it before, and I’d just as soon not have that smell in the living room.
The selenium diodes in the Granada are OK - for now. I just don’t want them to go “bang” and stink up the living room some day.
I left the old B250C100 physically inside the radio because taking it out would be a mechanical challenge - the B250C100 is in a flat metal package that is riveted to the chassis. Maybe somebody in 50 years will appreciate having the original part at hand. Or not.
Any way, I don’t have to worry about the selenium diodes burning out anymore.