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In Reply to: RE: Using both section of a dual triode for the driver tube posted by DAK on December 08, 2016 at 08:38:59
Double the transconductance, half the plate resistance, mu stays the same
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A free lunch!
I still don't relate to that term well. I know the definition is Gm = change Iout / change Vin. But when you say that value doubles when paralleling the sections of the 6sn7. In real world terms i don't know what that means.
Or, if Gm doubles while mu stays the same and Rp is halved. I understand what will happen in terms of the math but, in terms of a circuit how does one use that info? thanks for explaining.
Edits: 12/08/16
A tube with high transconductance will have a large change in plate current with a small change in grid voltage. Conversely, a tube with lower transconductance will have a smaller change in plate current for the same change in grid voltage.
This means that a tube with high transconductance will reach saturation and cutoff (clipping) much more easily (with smaller grid signals) than one with low transconductance. The graph of grid-voltage vs. plate current (Transconductance curve) may be rather sharply curved, and as a result a signal amplified by a high-gain tube may have more distortion even when it is not driven into clipping.
Tubes with lower transconductance will tend to have a broader curve to their graph, and for small signals may be more linear and have less distortion, depending on where the tube is biased. High transconductance tubes have less ability to drive the next stage miller capacitance.
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Some people say there is a smearing of detail but I think that the extra current available makes up for any chance of the speed of sound being able to out pace electricity.
I found in the case of 6SN7s that the halves are usually quite well matched and there is no gross imbalance between sections. That's not necessarily the case with a tube like a 6BX7 which can have a greater difference between sections.
Bottom line, it depends on your requirement and who you let make up your mind.
Thank you for replying. In What kind of amp did you use the paralleled 6sn7?
Both sides of a 6SN7GT in each channel of a 300B SET power amp.
Hi Frihed, how did you use the 6sn7 in your amp? What were the parameters?
I don't know. It's an Audio Note (UK) Conqueror Silver amp. I've never asked, or looked. It could be an SRPP in front of a coupling cap.
I think it's 6SN7 DC cascade then cap coupled to the 300B. A 3 stage amp.
This was the reply I got when I posted the circuit from the schematic on the web.
"Looks like DC coupled, cascaded, common cathode 6SN7 sections.
Eli D."
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There's a schematic of the Conqueror on the web. I'm not sure how accurate it is.
Edits: 12/09/16
I have used paralleled 6SN7s in SE 2A3 and 300B amps. The other thing, I usually load them with a CCS and LED bias them. Good power supply isolation on top and full mu combined with no cap bypass on the cathode. Heresy for some but the results sound good and you run the tube with a horizontal load line. I use two red LEDs for about -3.5V bias.
With the CCS, a 6SN7 has enough gain for a two stage amp if you use a preamp. If you're using a passive or straight out of a DAC, you should look at the voltage gain available.
So. you used a single 6sn7 to a 2A3.
Did you make your own CCS?
I mostly use a preamp so sensitivity for the input is not a big issue.
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