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Old 28th July 2012, 09:52 AM   #1
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Default Electrolytic capacitor vs extra triode stage

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.
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Old 28th July 2012, 10:06 AM   #2
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for my taste single stage 6H30 with a 2V Cell battery as bias.. and a CCS as load
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Old 28th July 2012, 10:22 AM   #3
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Both of the listed evils are unnecessary. Led bias is so much better. And simpler. And cheaper.
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Old 28th July 2012, 10:42 AM   #4
DF96 is offline DF96  England
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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.
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Old 28th July 2012, 12:30 PM   #5
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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.
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Old 28th July 2012, 12:33 PM   #6
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Quote:
Originally Posted by artosalo View Post
LED as a biasing component does not give any advantage (in performance) compared to fixed bias done with resistors.


So, how exactly is fixed bias done with resistors?
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Old 28th July 2012, 02:05 PM   #7
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by DF96 View Post
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).
Two stages will give you mire IMD too, for the same reason.
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Old 28th July 2012, 02:33 PM   #8
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Quote:
Originally Posted by analog_sa View Post
So, how exactly is fixed bias done with resistors?
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.

Last edited by artosalo; 28th July 2012 at 02:36 PM.
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Old 28th July 2012, 02:40 PM   #9
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Local degeneration, given by an unbypassed cathode resistor, will increase the output impedance of said stage. So, due to Miller capacitance of the following triode stage, high end bandwith will be reduced.

Cheers!
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Old 28th July 2012, 02:48 PM   #10
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Quote:
Originally Posted by Kay Pirinha View Post
Local degeneration, given by an unbypassed cathode resistor, will increase the output impedance of said stage. So, due to Miller capacitance of the following triode stage, high end bandwith will be reduced.
Correct. This can be avoided but must be taken into account when designing an amplifier.
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