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Old 15th May 2015, 05:31 PM   #1
Bigun is offline Bigun  Canada
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Default Triode vs Transistor

I know it's only spice but here's a comparison between a common cathode amplifier using 12AX7 and a common emitter amplifier using a typical BJT. The BJT has to biassed differently because it is not a depletion type device. It also has an unbypassed emitter resistor in order to match the gain of the two devices.

So - same B+, same signal, same gain and same bias current. From an FFT perspective the BJT beats out the triode.
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Last edited by Bigun; 15th May 2015 at 05:36 PM.
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Old 15th May 2015, 05:36 PM   #2
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Quote:
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So - same B+,
What is it? Shouldn't the tube be running a much higher (relative to the transistor) voltage to be linear?

jeff
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Old 15th May 2015, 05:38 PM   #3
Bigun is offline Bigun  Canada
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The voltage at the plate of the tube and collector of the BJT is just over 200V. hmmmm, that's actually too high for the BJT I've used.
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Old 15th May 2015, 05:55 PM   #4
Bigun is offline Bigun  Canada
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Using MPSA42 instead since it has a Vce max of 300V (not idea but good enough for the purpose).
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Old 15th May 2015, 07:02 PM   #5
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I think it's better to remove the input capacitor in the BJT circuit so that the two circuits are DC coupled. Otherwise the triode one will have better performance at low frequencies.
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Old 15th May 2015, 08:28 PM   #6
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Isn't the BJT going to have higher output impedance in this configuration ?

However more to the point, who is ever going to run the BJT in a way to achieve the slightly better performance ? I suggest no one.

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Old 15th May 2015, 08:40 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

A 175M bias resistor ? The bias is hopelessly inaccurate.
Bias should never depend on device hfe.

rgds, sreten.
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Old 15th May 2015, 08:46 PM   #8
Bigun is offline Bigun  Canada
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Here's the thing, I've always seen a triode as the mose linear option for a VAS, something about the 3/2 law and all that compared with nasty transistor curves.

It's also said that a triode is linear partly because it operates at high voltage and works best when the swing at the plate is no more than 20% of the h.t. supply. Transistors often operate at low voltage.

I was curious how the transistor would perform if given similar operating conditions - lots of headroom and degenerated to same gain. I was pleasantly surprised - perhaps a single BJT could be used as a 'wimpy driver' for a 2A3.
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Old 15th May 2015, 08:54 PM   #9
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Just to add an anecdote which is not specifically relevant but interesting to me. I built a SLCF with a jfet input stage followed by FET cathode follower. I used a 1K cathode resistor on a negative rail to linearise the jfet, far more than the unbypassed cathode resistor on the equivilant triode. Comparing it to my valve based versions it was rolled off and lacking somewhat. I thought the degenerative feedback would make them interchangeable to the usual triode - but I was wrong. In the end I had to add a bit of gNFB to smooth the response out.

Lots of other parameters weren't equal but in all of the transistor amps I have recently built using jfet input they have consistently missed that top end sparkle.

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Old 15th May 2015, 09:04 PM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Bigun View Post
So - same B+, same signal, same gain and same bias current. From an FFT perspective the BJT beats out the triode.
Nothing especially new or surprising there: for a long time now (at least 40yrs), transistor amps have soundly beaten tube ones on specs.
It is as difficult to find a solid state amp doing worst than 0.1% THD as it is finding a tube one doing better than that.
The reason is simple: transistors may be intrinsically more non-linear than tubes, but they also have a much higher transconductance, and can tolerate a much higher level of local degeneration for the same stage gain.
The result is a higher (apparent) linearity. To compare likes and likes, another configuration or tube type might be required.
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