Low voltage electrolytic, change or not

[Oldelectronics]

Thanks guys it would seem the best course would just be to change it with a 16v 47uf 20%? You can get a pack of 5 for just over a pound on e bay and seem nearest in spec if not size!

Kev

[Phil G4SPZ]

Originally Posted by Herald1360

When you're older than the part in question, you have a reasonable excuse....

Ha ha! Probably about the same age, in fact... but going back to the OP's specific question about C30 (Bush service sheet) in the AC34, at first glance 50uF does appear to be a rather large value of capacitance, as V3 is the detector/AGC/AF amplifier, not the output valve, and has a 5,600 ohm cathode resistor.

My 1953 edition of Fritz Langford-Smith's 'Radio Designer's Handbook' (which is the same age as me) states that the reactance of the bypass capacitor at the lowest frequency to be amplified should not normally exceed one-tenth of the resistance of the cathode bias resistor. Assuming the lowest audio frequency of interest to be (say) 100Hz, the reactance of a 50uF capacitor at 100Hz is approximately 32 ohms, so on the face of it, a capacitor of about 3uF would have sufficed.

Manufacturers didn't gild the lily and fit components any larger or more costly than absolutely necessary, so my guess, although I may be totally wrong, is that the value of C30 was chosen to maintain the gain of V3 right down to just a few Hertz in order to reduce the effect of rapid signal fading on the operation of the AGC, and to hold V3's cathode potential virtually constant to reduce volume 'pumping'. In other words, C30 also provides AGC smoothing.

As the service sheet quotes the cathode voltage as just 0.8 volts, if you do decide to replace C30 it would be wise to follow previous advice to use a replacement capacitor with a low-ish working voltage.

[Oldelectronics]

Hi Phil thanks for that thinking about it its really easy to take out an AC34 chassis if when together And after an out of case bench soak test it performs as well as my other two I am tempted to leave it if not it would be easy to change it for a 47uf 16 volt which is about as close as I can get useing modern capacitors. It does not seem to be a component under any great stress and is free of any bulges etc.