Selenium Rectifiers

[garykuster]

Hi all, I’m about to place my order at Justradios for all the resistors and Capacitors to refurbish my (not arrived yet) tube tester. One item on the Schematic has me wondering if I should replace it.
The diagram shows a 10 MA Selenium rectifier at the transformer going to a rheostat that probably controls the voltage to the tested tube socket. Am I understanding the set up correctly? Now. Is it practical to change it or is it incorporated within the transformer. I know I could wait until the tester shows up, but I’m anxious to get all the parts in one go so I can be ready. This tester will be my first project and I’m aware that I haven’t set the bar too high, but until my ‘Elements of Radio ‘book arrives, I thought it was a good way to get my feet wet. More Questions… Many types of caps available, which are best? I think I should get all 1% precision for test equipment. The Resistors are available in 1/2 Watt, 1 Watt, 25 and 10 Watt. Watts best to use (pun intended) Here’s a free link to the schematics..

http://bama.edebris.com/manuals/century/fc2

Thanks, Gary

• This topic was modified 6 years, 11 months ago by garykuster.

[Alistair Thomson]

I’m guessing your model is the code 127. The rectifier is a separate component, not built-in to the transformer, and doesn’t control the voltage to the valves at all. 1% caps unnecessary. Normal 20% caps will work fine.

The variable resistors are wirewound (WW), and they can take a bit of current, but the diode is operating at a low voltage so the currents will also be low. I don’t think a tester will apply high currents anywhere, so your resistors don’t need to be high wattage. However, it is always a good idea to check the original components. Large resistors will of course require high-wattage replacements.

Read over the manual carefully and make sure you understand everything it says! 🙂

[garykuster]

Alistair, It is the code 127. I’m still unsure from your answer if I should replace the rectifier with an updated one, or stop worrying about it. Because the difference is pennies, I think I’ll stick to the higher tolerance caps.
Does it make sense not to order anything until the tester arrives in order to determine the wattage of the resistors? Higher wattage, bigger physical size? I have studied the color coding for the resistors, but wattage wasn’t mentioned on the chart and doesn’t seem to be indicated by the stripes. I will trace all the circuits on the schematic one by one and try to understand what I’m looking at. The learning curve is steep when you’re starting from scratch.

[Alistair Thomson]

The resistor wattage isn’t labelled. Your only guide apart from analysing the circuit is to look at the originals. So yes, bigger resistors = higher wattage, but modern manufacturing and materials means that what once was a big resistor now isn’t nearly so big.

For example, I removed a 26K, 2W resistor from the 1940 Marconi 207 I’m working on, and it measures 43mm long by 7.5mm diameter. The modern equivalent is around 10mm long by 4mm dia.

[garykuster]

Thanks for the Resistor explanation. I will wait until it arrives before ordering anything.

[Alistair Thomson]

Oh, and re. the learning curve, you’re right. There’s a lot to learn, and a lot to do. I’ve been working with analogue and digital electronics for most of my life, and I’m till learning. And I’m 73… so take your time, learn well, and enjoy the ride.

[Alistair Thomson]

Sensible, and of course I do know how keen-ness to get on makes us jump! However, it never does any harm to buy components – even if we don’t use them right away, they will be there for when we do need them. A couple of years ago I spent about $100 on getting a whole load of caps, resistors, PNP, NPN and silicon diodes of a variety of types and ratings, and stocked up with small plastic slide drawers each carefully labelled with the range, eg. 0.01µf – 0.09µF, 2K – 2K4 etc, with the heavier current / higher voltage values in a different set of drawers. Took a week to set it all up, but now, finding almost any component I want is a breeze, with very few orders having to be placed.

“Oh, so I need a 3K3 1/2W resistor. Here it is. And a 2µ2 cap at 35V, here’s one at 25V, no good, but here’s one at 50V, perfect.”

[garykuster]

Down the road, I plan to do the same. I’m just in the process of making some space in the basement for my ‘Radio room’ The winters are cold and dark in Montreal and now retired, I need something to tickle my brain and I think I found it. I just have to cool my jets and slow down.

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