There has been on this forum some discussion about the advisability increasing the capacitance value of coupling and bypass capacitors. There are those who whose philosophy is to keep everything as original as possible, and for them capacitance values tend to be very important. At the other end of the spectrum, there are those who wish to end up with a beautiful and reliable tube radio that retains its original beauty and tube circuitry, with some liberties taken - but who not wish to buy or store a large number of different component values, and many times wish to use something they have on hand.

First of all, a rare or high-end radio shoud perhaps be treated differently. But for the majority of tube radios, efforts were made during design and manufacturing to address cost and competition issues; so they didn't spend any more money on capacitors than they had to. They also needed to keep 60 Hz hum to low and acceptable levels. Typically they didn't make the filter capacitors large, and used coupling capacitors, bypass capacitors, output transformer, etc. that cut down the 60 and 120 Hz response of the whole circuit. Larger bypass capacitors aren't likely to hurt anything; some might argue that they make the tube take longer to reach proper operating bias / operating voltage, and damage the tube; to them I say, give me a real story of a damaged tube, not just a theory. The same could be said for coupling capacitors. About the only thing you are going to do is increase the hum heard in the speaker. I must admit that there is a limit - but I see no problem from a reliability or operational standpoint of output tube cathode bypass resistors up to 100 mfd, other audio tube bypass capicitors up to 10 mfd, and coupling capacitors up to 1 mfd. (You do want a fairly short time constant to reach equilibrium, considering both capacitance and resistance in the circuit.) Ditto for any bypass capacitor in the IF or RF stage where the original is .005 mfd or larger. Anything rated for at least 2X the voltage the part will see in a DC circuit should be acceptable. (Manufacturers sometimes used higher, because they bought in volume, and used the same part numerous places.) The capacitance value of power supply filter capacitors can likewise be increased, and in many cases will reduce hum and improve overall performance; just if you go much larger than the original, go through a 22 or 33 ohm resistor off the cathode of the rectifier tube to limit surge current if the power to the radio in switched on and off after the tubes have warmed up.

EdM, former tube radio circuit design engineer.

Back in the day when I was doing design, and mostly solid state, a rule of thumb was capacitive reactance was to be no more than 10% of the resistor being bypassed at the lowest freq expected/desired. Don't know for sure if that works or even applicable to tubes anyway. Its been long ago, and I could be way off base.

marv

:There has been on this forum some discussion about the advisability increasing the capacitance value of coupling and bypass capacitors. There are those who whose philosophy is to keep everything as original as possible, and for them capacitance values tend to be very important. At the other end of the spectrum, there are those who wish to end up with a beautiful and reliable tube radio that retains its original beauty and tube circuitry, with some liberties taken - but who not wish to buy or store a large number of different component values, and many times wish to use something they have on hand.
:
:First of all, a rare or high-end radio shoud perhaps be treated differently. But for the majority of tube radios, efforts were made during design and manufacturing to address cost and competition issues; so they didn't spend any more money on capacitors than they had to. They also needed to keep 60 Hz hum to low and acceptable levels. Typically they didn't make the filter capacitors large, and used coupling capacitors, bypass capacitors, output transformer, etc. that cut down the 60 and 120 Hz response of the whole circuit. Larger bypass capacitors aren't likely to hurt anything; some might argue that they make the tube take longer to reach proper operating bias / operating voltage, and damage the tube; to them I say, give me a real story of a damaged tube, not just a theory. The same could be said for coupling capacitors. About the only thing you are going to do is increase the hum heard in the speaker. I must admit that there is a limit - but I see no problem from a reliability or operational standpoint of output tube cathode bypass resistors up to 100 mfd, other audio tube bypass capicitors up to 10 mfd, and coupling capacitors up to 1 mfd. (You do want a fairly short time constant to reach equilibrium, considering both capacitance and resistance in the circuit.) Ditto for any bypass capacitor in the IF or RF stage where the original is .005 mfd or larger. Anything rated for at least 2X the voltage the part will see in a DC circuit should be acceptable. (Manufacturers sometimes used higher, because they bought in volume, and used the same part numerous places.) The capacitance value of power supply filter capacitors can likewise be increased, and in many cases will reduce hum and improve overall performance; just if you go much larger than the original, go through a 22 or 33 ohm resistor off the cathode of the rectifier tube to limit surge current if the power to the radio in switched on and off after the tubes have warmed up.
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:EdM, former tube radio circuit design engineer.
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Agreed. ...Though there are those really well designed radios, like most Zeniths, where it is best to stick to original values first, before making any modifications. The tone circuits in those radios are quite genius, and modification of the capacitor or resistor values will alter the way the tone circuits respond--that certain buttons respond to certain frequency ranges. ...Also, with regard to well designed portable radios (again, Zenith), it is best to stick to original capacitor values. You will find that the amplifiers are often 'tuned' for best performance and maximum gain, while still maintaining good tone response.

...But it can be fun and interesting to modify the way a radio sounds, especially if you can make it sound better, and not paying strict attention to capacitor values (except in RF circuitry) usually causes no great harm.

T.