Hi there.

My current project is a 1947 Monarch valve radio. The parts list specifies a few 300 and 50 ohm wire wound resistors. I'm just wondering if the type of resistors is critical/important. Provided the value and power handling capacity was a match i'd have thought that it shouldn't matter.

The wire wound are all small values 1/2 and 1 watt ratings.

Can I replace these with metal film resistors of the same value and rating?

Thanks for your help.

Peter

Should be fine.

Although not critical but some electronic designers used the fact that wire resistors drawing close to designed wattage would increase in value when warm/hot.
This could be used to advantage, for example in the cathode circuit of an audio output stage.
When pushed hard, cathode resistor would increase in value to changed bias.
Thus the stage could be designed to have better “results” at different volume levels.
Carbon resistors could also do this, but would fail after a while.
JJ

Hi, in the Monarch and in those wattage sizes probably wont matter.

However if the resistor has current running through it like a HT dropper or output cathode or back bias purpose I tend to put in a WW type.
I use PW types from Jaycar because its just as easy for me to grab those from the components section while stocking up on other bits.
They may not be liked by vintage people as they are white rectangles, not round things. I just spray them black if needed!

At audio frequencies the inductance of the resistor winding will not matter.
At UHF a different matter, no radio goes that high.

Back in the day, the WW low value high wattage resistor was probably more reliable than a carbon unit when run hot..

Fred.

Thanks for your comments and suggestions concerning my post.

Given that the tolerance of these resistors is specified as 10% and the capacitors as 20% I'm wondering how valid a design based on relying that a component will change value as it gets hot is likely to be.

I can understand it if the rating of these resistors was higher

Did they hand pick components when they built these radios?

Cheers and again thanks.

Peter

More of a case of what components were available.
I know in some instances that certain valves were used to get rid of surplus stock, left over from B/W TV production.
Same with some values of capacitance, the bean counters telling the production managers to get rid of for example 10 thousand 0.012 400volt capacitors. So 50 years later enthusiasts would argue about the sound difference between a 0.01 and a 0.12 cap used in an audio coupling stage.
Just use the best you can find, bearing in mind operating voltage in caps and wattage in resistors.
Even then some factories got this wrong, or perhaps its what they had in stock.
JJ

Can I jump in here and suggest you don't need to change a wire would resistor unless it's open circuit or badly burnt up?

IRC used to make half and one watt, low ohm resistors in bakelite covers just like carbon resistors. These are normally VERY reliable.

Tolerances were very broad too, simply because the technology was not there to make close tolerance components economically.

When you were designing a circuit, you'd run the calculations (on your slide rule or adding machine) for worst case tolerances and if that broke the circuit you'd adjust a value or two. Valves also varied a lot so you had to design conservatively.

In many ways design is so much easier today.

One other thing to mention about wire wound resistors is that they are unsuitable for use in critical circuits where low inductance is required, but that doesn't generally apply to domestic broadcast band receivers.

Wire wound resistors can be bad news in the RF section, albeit that I did use them in recent STC where the resistors were parallel with the heaters and a couple of the carbon ones fitted had fried.

Wattage is not everything. I tend to stock mainly 1 Watt resistors,, not only to keep the inventory down, but to get the voltage rating.

Irrespective of it being a commercial fix here, or not, there is one standard; A resistor more than 10% out is liable to end up binned and any non polarised cap found leaking to any extent, will also go: Especially if its "wax paper", which I will normally toss without checking as the probability of it not leaking is remote.

Hi All

Many thanks all for you comments and wisdom.

Peter