Replacing 'Waxy' Capacitors

[_Garak_]

Replacing waxy caps in a H.M.V radio.

The Grid Coupling Capacitor installed at the moment (and shown in the schematic) has a value of 0.1µF does that value only have to be installed? I'm sure that I read somewhere that the value was not that critical, I do have a few 0.01µF yellow Polyester Axial Capacitors would one of these be OK?

On the subject of Polyester Axial Capacitors, these tend to be at the higher end when it comes to price so can I replace waxy's with Orange Drop Polypropylene Capacitors instead?

[Maarten]

Polypropylene is even better than polyester for most, if not all, purposes.

[joebog1]

0.01 is ten times smaller !!! You will lose bass. If that's not a problem it will work.

Joe

[paulsherwin]

In practice the grid coupling capacitor isn't at all critical, as the impedance involved is very high. Anything from 0.001uF to 0.1uF should be OK, and I get the impression that manufacturers used whatever value they had in stock and wanted to get rid of.

That said, it's always good practice to replace caps with the same values as the originals.

[Sideband]

It would matter much more if you were dealing with a Hi Fi amplifier where frequency response is important. With a domestic radio, probably less so but if you are not pleased with the sound after reducing the value by 10 times, you might have to change it for something nearer the published value.

[Herald1360]

1nF (0.001uF) with a typical 470k output valve grid leak will be down by 3dB at around 300Hz. Fine for speech, maybe a bit lacking for music.

so can I replace waxy's with Orange Drop Polypropylene Capacitors instead?

Yes indeed, the axial yellow jobs are a bit easier to use as they have the same mechanical fit, the orange drops are radial and look a bit odd (as you know) perfect replacements though. The modern radial electrolytics are so much smaller and the leads being the "wrong way" do look very odd in a vintage set, not that it matters if it is working again.

[paulsherwin]

Originally Posted by Herald1360

1nF (0.001uF) with a typical 470k output valve grid leak will be down by 3dB at around 300Hz. Fine for speech, maybe a bit lacking for music.

Yes, I'm not suggesting that 1nF would be a good choice, but lots of commercial domestic radio designs did indeed use that value, possibly to reduce high volume rattles in small sets.

[mister valve]

Sets made by EMI often had relatively low value grid leak resistors, especially where high slope O/P valves such as 'KT61' & 'N78' were used. The resistance value would often be 250k ohms (220k ohms), anything higher in value, was considered a risk of 'grid blocking'. So in order to give a good Bass response, a higher than usual inter-stage coupling capacitor would be used such as '0.1uF' up to about '0.5uF' (0.47uF).

If you can afford the loss of the 'low notes', then a 0.01uF will be fine.