Does anyone have any experience using a choke-to-ground on the cathode instead of a cap/resistor to ground?
I tried it twice. In both cases the current of the choke matched the current going through the tube so that it did not overheat and I made up the resistance required by the original cap/resistor combo by adding a resistor in series to choke. Bias was also correct.
I have tried it twice using two different chokes: one Ei and one toroid and they sounded the same. Both had a similar effect with a vocal and piano cd: the piano came out clear and sharp as usual; the voice was as if it was out of phase with itself: much quieter, recessed and barely audible relative to the piano.
What gives?
I tried it twice. In both cases the current of the choke matched the current going through the tube so that it did not overheat and I made up the resistance required by the original cap/resistor combo by adding a resistor in series to choke. Bias was also correct.
I have tried it twice using two different chokes: one Ei and one toroid and they sounded the same. Both had a similar effect with a vocal and piano cd: the piano came out clear and sharp as usual; the voice was as if it was out of phase with itself: much quieter, recessed and barely audible relative to the piano.
What gives?
A choke presents a very high impedance on the cathode. That lowers the gain appreciably. I can only see (big) disadvantages.
There is a reason nobody does this.
Jan
There is a reason nobody does this.
Jan
Thanks for your quick reply. Is it possible to still do this with a lower impedance choke and be successful?
If you use inductor instead of decoupled resistor, the cathode working with frequency dependent impedance:
Z= 2*pi*f*L
The bias was correct ... it's true only at very low frequency. When the signal frequency growing, the cathode "resistor" (DCR+Z) also growing.
Z= 2*pi*f*L
The bias was correct ... it's true only at very low frequency. When the signal frequency growing, the cathode "resistor" (DCR+Z) also growing.
Putting a choke there could cause the maximum safe heater-to-cathode voltage to be exceeded on program material with loud high frequency content in it. You would also have very attenuated HF sound, like a treble control turned way down. Not recommended. Chokes have been successfully used for plate loads, however.
Is it a single ended or push pull amplifier?
Did you have a capacitor in parallel to the choke?
What do you want to achieve?
In a push pull amplifier one can join both cathodes and use a choke from the joined cathodes to ground. The RDC of the choke can be used to auto bias the output tubes (possibly with some resistance in series), while the high impedance (at AC off course) forces the tubes to operate in a balanced fashion (as the chokes opposes variation of AC current, so the sum of the current of both tubes is kept constant). Therefore it also will stay in class A. I am quite sure that Thomas Mayes uses this in his PP amplifiers, and i have seen mentions of it elsewhere.
Best regards,
erik
Did you have a capacitor in parallel to the choke?
What do you want to achieve?
In a push pull amplifier one can join both cathodes and use a choke from the joined cathodes to ground. The RDC of the choke can be used to auto bias the output tubes (possibly with some resistance in series), while the high impedance (at AC off course) forces the tubes to operate in a balanced fashion (as the chokes opposes variation of AC current, so the sum of the current of both tubes is kept constant). Therefore it also will stay in class A. I am quite sure that Thomas Mayes uses this in his PP amplifiers, and i have seen mentions of it elsewhere.
Best regards,
erik
It is single ended.
My understanding is that having a capacitor in parallel with the choke defeats the purpose of having the choke there at all. Then the choke is like a very long resistor.
I would like to have a choke on the cathode without having a capacitor and attain full range hifi at the same time... Is this possible?
My understanding is that having a capacitor in parallel with the choke defeats the purpose of having the choke there at all. Then the choke is like a very long resistor.
I would like to have a choke on the cathode without having a capacitor and attain full range hifi at the same time... Is this possible?
How do I use a choke for the plate load? Is it a regular choke? RF choke? or a high inductance plate choke? What are the advantages of doing it this way?
Thanks for the reply, the cap in parallel would indeed defeat the purpose of the choke.
What do you want to achieve with the choke in the cathode? The impedance in the cathode circuit is multiplied by the mu of the valve, and this value (in ohms) is added to the plate impedance, which most people try to actually reduce (and not increase, as you are doing). With the choke you are applying a lot of local feedback, will reduce the distortion and gain, and increase output impedance at the plate.
I looked into a choke loaded LTP. It indeed would take a large inductance but it has advantages there in balance and lowered B+ requirement. I don't see a particular advantage for common cathode stage.
I've done this with guitar amps in the phase inverter. Both in long tail pair and double choke loaded cathodyne. The result was breakup occurred much sooner, but had a larger sweet spot before farting out. In hifi, I see no advantage to a choke on the cathode, only disadvantages. A bjt ccs would perform far better in this position in every respect. Even a lowly LM317 would fair better, in my opinion.
All you r answers are interesting and curious in light of the fact that the Altec 1570b used one on its cathodes. Can any one explain this?
Do you mean the transformer on the cathode side of the 6w6s? It does not have a secondary but should not be thought of as just a choke.
In that case the circuit is a cathode follower. One of its jobs is to keep the cathode voltage close to 0V to have proper bias for the 811a grids.
In that case the circuit is a cathode follower. One of its jobs is to keep the cathode voltage close to 0V to have proper bias for the 811a grids.
You mean in the 6W6GT cathodes? These are being used as cathode followers, there the choke makes perfect sense.
First example I can think about is the MoFo, where a FET is used as a source follower, loaded with a choke.
https://blog.audiomaker.tech/download/build-this-mofo/
First example I can think about is the MoFo, where a FET is used as a source follower, loaded with a choke.
https://blog.audiomaker.tech/download/build-this-mofo/