If I recall correctly, an unbypassed cathode provides negative feedback, which will lower distortion in all output stages, single or PP. In my radio engineering days, we had class "B" modulators,and it took quite a bit of unbalance before the distortion started creeping up. The main reason we kept the tubes balanced was they rean red hot, and they had a shorter life span if one was doing all the work.
Lewis
In push-pull one tube is on, the other off. In theory a bypass cap isn't needed since cathode voltage shouldn't vary.
In actual practice a cathode bypass cap still improves low frequency response. I've never seen a push pull circuit where cathode voltage didn't vary. Look at the cathodes on a scope.
Norm
::Some circuits I have seen bypass the cathode resistor of a push pull amp and some don't. Some say they can hear the difference others say no. Is it possible that the people that can hear the difference may have unmatched tubes? When possible (if I have enough tubes) I try to match the DC current draw of the tubes. I do this by only installing one tube and measuring the cathode voltage and then swaping it out with another tube and measuring the voltage. Granted this only gives the DC current, which may be different than the AC operation, but is seems to be close enough. What do you think
::Bob
:
:If I recall correctly, an unbypassed cathode provides negative feedback, which will lower distortion in all output stages, single or PP. In my radio engineering days, we had class "B" modulators,and it took quite a bit of unbalance before the distortion started creeping up. The main reason we kept the tubes balanced was they rean red hot, and they had a shorter life span if one was doing all the work.
:Lewis
This is true, but only for class "B" In class "A", both tubes conduct all the time, and there are classes of "AB" where both tubes conduct part of the time, and one cuts of on positive peaks and the other on negative peaks. In my radio stations, the modulators drew a small amout of current without modulation, and then the current went up as modulation was applied. In a class "A" the amplifier will work about the same with one output tube as two, only with less power and more distortion. After the modulator, the audio signal, 2,500 Watts worth, went through a modulation transformer, where the B+ for the final tube went through the secondary, and then to the final stage, an 892R, where 9,300 Volts of DC and 2,500 Watts of audio were combined with 580 KHz of RF, and the whole mess went to a 420 foot vertical antenna, where 10.5 Amps of RF produced 5Kw. of radio station WGAC. The power amplifier was biased well beyond cutoff, and operated at about 73% efficiency, 9,300 Volts and .740 Amperes of DC to make 5,000 Watts of RF. Would have made a Great Tesla coil.
Lewis
: In actual practice a cathode bypass cap still improves low frequency response. I've never seen a push pull circuit where cathode voltage didn't vary. Look at the cathodes on a scope.
:
:Norm
:
:::Some circuits I have seen bypass the cathode resistor of a push pull amp and some don't. Some say they can hear the difference others say no. Is it possible that the people that can hear the difference may have unmatched tubes? When possible (if I have enough tubes) I try to match the DC current draw of the tubes. I do this by only installing one tube and measuring the cathode voltage and then swaping it out with another tube and measuring the voltage. Granted this only gives the DC current, which may be different than the AC operation, but is seems to be close enough. What do you think
:::Bob
::
::If I recall correctly, an unbypassed cathode provides negative feedback, which will lower distortion in all output stages, single or PP. In my radio engineering days, we had class "B" modulators,and it took quite a bit of unbalance before the distortion started creeping up. The main reason we kept the tubes balanced was they rean red hot, and they had a shorter life span if one was doing all the work.
::Lewis