It depends on their relative size. If the plate load is high compared to the plate resistance, it will dominate, just like in the gain equation.
In any case, if you think of the equivalent circuit for the tube, the plate resistance and the load resistance are in series, so that's what gets reflected (divided by mu + 1) at the cathode.
In any case, if you think of the equivalent circuit for the tube, the plate resistance and the load resistance are in series, so that's what gets reflected (divided by mu + 1) at the cathode.
Much thanks for your help. I'm afraid of equivalent circuits (in this case, given my vast ignorance) so I'm going to try to derive an answer from the one thing I know fersure: that current is the same though both anode and cathode. Maybe I can work with the relative sensitivities of RL and RK values and then back the RL and RK out. Anyway, will report if successful.
Thanks again,
Chris
Thanks again,
Chris
The logic is simple:
- Low distortion and wide bandwidth are essential for good sound quality.
- Transformers add distortion and restrict bandwidth.
- Therefore Transformers improve sound quality.
Oh hell ! It seems that since 1851 already knew that paper capacitors are more linear. 
On the Question of Dielectric Hysteresis
On the Question of Dielectric Hysteresis
Oh hell ! It seems that since 1851 already knew that paper capacitors are more linear.
On the Question of Dielectric Hysteresis
Paper dielectric in signal cables next then...LOL
Perhaps with linseed oil to prevent water ingress
With silk fibers for damping..
Back to paper/wax insulated OP Tx's...sigh
Regards
M. Gregg
Last edited:
If you are sitting at the anode, the intrinsic anode resistance and the external anode load are in parallel. If you are sitting at the cathode (looking 'up' to the anode) then these two are in series (and divided by mu+1).
The same applies (although the other way round) if you want to determine the effect of Rk on anode impedance. At the cathode Rk and 1/gm are in parallel. At the anode the effects of Rk and 1/gm are in series (and multiplied by mu+1).
These subtle effects are often overlooked, but fortunately they are often small enough that they can be safely ignored. Unfortunately this can lead to people not being aware of them even in situations where they may dominate the circuit operation (and hence ought to dominate design calculations).
The same applies (although the other way round) if you want to determine the effect of Rk on anode impedance. At the cathode Rk and 1/gm are in parallel. At the anode the effects of Rk and 1/gm are in series (and multiplied by mu+1).
These subtle effects are often overlooked, but fortunately they are often small enough that they can be safely ignored. Unfortunately this can lead to people not being aware of them even in situations where they may dominate the circuit operation (and hence ought to dominate design calculations).
And the best amp is...no amp
Which is one of my pet peeves. I like to go to concerts of classical music, but these days the orchestra is miced within an inch of its life and fed through some dodgy PA mixer into some enormous horn speakers covered in black plastic. It is becoming increasingly difficult to hear the real thing.
Cheers
Ian
And people tend to forget that he was using 70V rms across the capacitors in his tests.
Cheers
Ian
Quite correct.
The cathode bypass capacitor creates a pole-zero pair, not just a pole.
The zero is given by the oft-quoted simple formula:
f = 1/(2 pi Rk Ck)
And the pole is given by the longer formula:
f = 1/ (2 pi rk Ck), where rk = Rk || [(Ra + ra)/(mu+1)]
But usually Rk << ra, and triodes don't have much gain, so the pole is rarely very far from the zero, so it is indeed fine to use the simple formula provided you set it very low, with the understanding the the pole will be just a little bit higher.
For once I get to disagree with DF96!
The simple formula is not "wrong", it is simply the formula for the zero, rather than the pole.
Perhaps the definitive study of audio distortion is capacitors was done by C. Bateman in Electronics World (6 parts from July 2002 onwards). This work is built upon and extended by Doug Self in his excellent book 'Small Signal Audio Design'. He used 20V rms across the capacitors in is tests. He found polyester types gave significant distortion (his words), entirely 3rd harmonic at levels up to 0.003%. He also tested Polystyrene and Polypropylene types and found they produced no distortion at all. He lalso found the distortion was directly proportional to level. So I think properly sized coupling capacitors of this type are going to have zero effect on distortion.
Electrolytics were different. He found they produced much greater distortion in coupling circuits. His test circuit was a high pass coupling circuit with a 47uF cap and a 1K load which has a -3dB point at 3.38Hz (0.2dB down at 20Hz). With 10V rms fed into it, this gave 0.008% distortion at 20Hz. His solution is to raise the value of the cap in order to reduce the ac voltage across the cap. At 100uF in the same circuit the 20Hz distortion dropped to 0.0017%. This is directly relevant to this discussion and suggests we should size cathode decouplers to be 3dB down at less than 1Hz.
Even so, the ac voltages we are talking about are a lot less than Self used so the distortion should be below the noise and below audibility.
Cheers
Ian
Electrolytics were different. He found they produced much greater distortion in coupling circuits. His test circuit was a high pass coupling circuit with a 47uF cap and a 1K load which has a -3dB point at 3.38Hz (0.2dB down at 20Hz). With 10V rms fed into it, this gave 0.008% distortion at 20Hz. His solution is to raise the value of the cap in order to reduce the ac voltage across the cap. At 100uF in the same circuit the 20Hz distortion dropped to 0.0017%. This is directly relevant to this discussion and suggests we should size cathode decouplers to be 3dB down at less than 1Hz.
Even so, the ac voltages we are talking about are a lot less than Self used so the distortion should be below the noise and below audibility.
Cheers
Ian
That is the sort of 'disagreement' I can cope with! I will leave the pedants to argue over whether the right formula for the wrong parameter is in fact the wrong formula for the right parameter. The popular practice of using a rather large decoupler is a good practice, although perhaps not for the reason some people think.Merlinb said:
Yes. This is why an electrolytic, whether coupler or cathode bypass, should not be used to set the dominant LF rolloff. A large decoupler ensures that it can't create distortion, and it can't create channel LF phase imbalance. If people choose to believe that the real reason is that they can detect audio down to 5Hz (or whatever) in a room which probably won't support modes below 20Hz and speakers which rolloff at 50Hz then that is their privilege; at least they are choosing the right component value even if for the wrong reason!ruffrecords said:
There is some excellent info on capacitor distortion etc. to be found on this website:
Capacitor Voltage Change
Capacitor Voltage Change
Interesting to say the least. Fascinating how small the signal needed to cause significant distortion.
Shoog
Paper dielectric in signal cables next then...LOL
Perhaps with linseed oil to prevent water ingress
With silk fibers for damping..
The distance to the cultural centers, and wallet, make it difficult for me to attend a concert these days.
I'm also a lover of classical music, in the last concert I attended, "Il Barbiere di Siviglia", the orchestra played in the pit below the stage, unaccustomed to the opera, initially suspicious, but the sound was great, and the restraining effect of the air column produced the best bass I heard in my life.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.