hi all i have a valve radio i am going to recondition
i have the schematic of it but unsure about the electrolytic capacitors
pic attached
the caps marked with a * are the electrolytic
are they 25 32 and 16uf capacitors ?
cheers
marko

 

Electrolytic 25,32,16μf.
The cathode by pass is much lower voltage rating, ~25vdc.
Max.

 

also, the wax paper caps are probably leaky too. might want to replace them. might want to check the resistors to see if they have changed resistance with age.

 

might want to check the resistors to see if they have changed resistance with age.

Does it happen (or used to happen with old type resistors) even if they are not powered?

With modern ones is the same?

 

Not usually if not powered, but the older carbon style used to change over time with temperature changes.
The modern carbon film do not seem to suffer the same way, except when heated over their wattage limit of course.
Max.

 

Humidity will change (Edit: the old) resistor values, too. Most often in the larger values like 470K ohm and up. They resistance usually rises.

 

The old carbon composition resistors (cylindrical, usually have a brown body) will indeed change resistance over time, even if they are in a sealed package. I had several packages of mil-spec carbon composition resistors that were sealed in aluminum foil and plastic back in the 60's; many of them had as much as doubled in resistance.

The newer carbon film resistors (they usually have light brown or tan bodies and are dumbbell shaped) are far more stable; I have some that I bought in the 70's and they are still within tolerance. Metal film (usually a light blue dumbbell-shaped body) are yet more stable than carbon film, and are less noisy.

 

Yes. What Wookie said. The higher the resistance, the worse they change. Anything over 100k is a likely suspect.

 

https://www.google.com/search?q=orange+drop+capacitors

replace ALL the caps would be my suggestion for a restoration. I've done the same thing with vintage vacuum tube car radios.

And be careful inside the equipment. The B+ voltage is dangerously high in tube circuits.

 

would it be safe to use a 33uf to replace the 32uf and for the 25uf use a 22uf.
as most capacitors have a 20% tolerance

 

Many electrolytic's state +100% -0 tolerance.
Max.

 

would it be safe to use a 33uf to replace the 32uf and for the 25uf use a 22uf.

In this case, they are filter capacitors. Better that you go over-sized than to get done and find out you have a hum problem. They aren't all that expensive for the next size bigger. In this case, you'd probably end up with a pair of 33uf caps.