Assuming a triode with good linearity, say a 6H30, at its ideal operating point. Can we generalise theoretically that one is better than the other?
1. Single stage resistor biased grounded cathode with electrolytic capacitor bypassed cathode resistor.
2. Two stage resistor biased grounded cathode with unbypassed cathode resistor.
Basically, between an extra tube stage, and an electrolytic capacitor in the signal path, which is the worse of 2 evils? And what if, if we pick a good "electrolytic" like Oscon and bypass it with a film capacitor?
And of course, LED bias is an option too. But there are already quite a bit of discussion on this out there.
Thanks.
1. Single stage resistor biased grounded cathode with electrolytic capacitor bypassed cathode resistor.
2. Two stage resistor biased grounded cathode with unbypassed cathode resistor.
Basically, between an extra tube stage, and an electrolytic capacitor in the signal path, which is the worse of 2 evils? And what if, if we pick a good "electrolytic" like Oscon and bypass it with a film capacitor?
And of course, LED bias is an option too. But there are already quite a bit of discussion on this out there.
Thanks.
Other things being equal, two stages instead of one may shift the balance between low order and higher order products slightly towards more higher order products (because you get distortion of distortion).
Other things being equal, cathode degeneration may produce a similar shift. Both shifts will be small unless the final stage is nearing overloading. The correct value of electrolytic will not do much harm; too small or too large may end up with signal voltage across it and then some scope for a small amount of extra distortion.
If you really feel that triode gain stages or electrolytic caps are 'evil' then you should stick to listening to live acoustic (i.e. unplugged) music only. I assume you don't really mean this, but are just ensuring some 'street cred' on an audio site.
Other things being equal, cathode degeneration may produce a similar shift. Both shifts will be small unless the final stage is nearing overloading. The correct value of electrolytic will not do much harm; too small or too large may end up with signal voltage across it and then some scope for a small amount of extra distortion.
If you really feel that triode gain stages or electrolytic caps are 'evil' then you should stick to listening to live acoustic (i.e. unplugged) music only. I assume you don't really mean this, but are just ensuring some 'street cred' on an audio site.
Triode stage without cathode bypass has local feedback and thus lower distortion than same stage with cathode capacitor.
Electrolytic capacitor is not needed between two triode stages. Any 220nF...680 nF capacitor is sufficient. LED as a biasing component does not give any advantage (in performance) compared to fixed bias done with resistors.
Two stage amplifier has douple gain compared to single stage. To manage with this extra gain is the challenge.
Electrolytic capacitor is not needed between two triode stages. Any 220nF...680 nF capacitor is sufficient. LED as a biasing component does not give any advantage (in performance) compared to fixed bias done with resistors.
Two stage amplifier has douple gain compared to single stage. To manage with this extra gain is the challenge.
This is typically done in output stages to have an adjustable negative voltage to the control grid. This is not very practical for voltage amplifying stage.
The problem with LED biasing is that it is really fixed. The minimum THD of the amplifying stage can be obtaineded by adjusting the bias voltage or cathode resistor.
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Correct. This can be avoided but must be taken into account when designing an amplifier.
True, but the advantage is small unless the anode load has lowish resistance. Having said that, many people do design with too little HT/B+ so anode resistors have to be too small so cathode degeneration will help.artosalo said:
The electrolytic was for cathode decoupling, not interstage coupling. The choice is between almost fixed bias with LED or cathode bias with resistor, not fixed bias with resistor. Few preamps have a negative supply for fixed bias, although I suppose one could be provided from the heater supply.artosalo said:
Two stages only double gain when you use dB, otherwise gains multiply not add.
This was the original question:
This is not very clearly stated, but I undertood that the electrolytic was between stages.
Context is everything.
Reading through the entire original post, I find the problem well stated with concrete examples followed by discussion around the reasoning.
What is the point in taking one sentence out of context?
Yes, is that not obvious?
Who uses electrolytics as coupling caps in tube circuits?
The context of this was well stated in the first post. A couple of arguments that have taken this thread off track seem to relate to a basic misunderstanding of the context. What's up with that?
not me, i'd rather dc couple if i can get away with it....
LED's or diodes in the cathode biasing is "fixed biasing"....
I've had great results with the TL431 programmable shunt regulator for adjustable self-bias. Super low dynamic impedance and no cap needed.
As for the OP, assuming 6H30 at 14 mA, 80V plate-cathode, and -4V g-k, we may use 10K plate R dropping 140V, Rk would be about 300 ohms. Bypassed the gain would be ~14, unbypassed -4 dB or about 8. Output Z with a 10K resistor load and 300 ohm Rk unbypassed would be around 4K as a rough estimate.
Using a CCS and high impedance load the gain would be about 16, even with unbypassed Rk bias. With the unbypassed Rk and CCS plate load the plate resistance would be about 5K ohms so would still drive something like a 2A3 miller capacitance easily.
Even with the unbypassed Rk and 10K resistor loads, 2 stages would give you a voltage gain of ~60. It doesn't compare gain-wise with the bypassed Rk or even CCS plate load. But if it provides the gain you want it may be a useful approach. Distortion even with unbypassed Rk may not be super low and won't cancel perfectly between 2 stages.
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