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Old 13th April 2005, 09:11 AM   #11
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It is most certainly not new
Certainly isn't... i'm slowly scanning interesting articles out of my 1940s radio-Crafts and see it fairly often... an example attached.

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Old 7th September 2012, 09:47 PM   #12
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Quote:
Originally Posted by planet10 View Post
Certainly isn't... i'm slowly scanning interesting articles out of my 1940s radio-Crafts and see it fairly often... an example attached.

dave
That layout looks hot hot hot, I guess windings for heaters and plate voltages back then were a little relaxed
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Old 7th September 2012, 10:40 PM   #13
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Another difference is the coupling of the output tube to the load. In triode mode the speakers' impedance is directly coupled to the triode plate so the triodes gain goes up as the speakers impedance goes up. You get a little more output voltage gain to compensate for loss of efficiency in the speakers. In UL the feedback is tied to the G2 through the tap so the UL tap has dominant control over the output tube minimizing this effect.
I will listen to triode mode when I want to do serious listening and for the parties, when volume is more important than quality to be heard over the din, go UL.
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Old 8th September 2012, 01:43 AM   #14
TV7 is offline TV7  Australia
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Originally Posted by roline View Post
Another difference is the coupling of the output tube to the load. In triode mode the speakers' impedance is directly coupled to the triode plate so the triodes gain goes up as the speakers impedance goes up. You get a little more output voltage gain to compensate for loss of efficiency in the speakers. In UL the feedback is tied to the G2 through the tap so the UL tap has dominant control over the output tube minimizing this effect.
I will listen to triode mode when I want to do serious listening and for the parties, when volume is more important than quality to be heard over the din, go UL.
That has to be one of the most insightful explanations I have heard yet. Never expected it. Thank you.
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Old 10th December 2012, 04:37 PM   #15
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Originally Posted by roline View Post
Another difference is the coupling of the output tube to the load. In triode mode the speakers' impedance is directly coupled to the triode plate so the triodes gain goes up as the speakers impedance goes up. You get a little more output voltage gain to compensate for loss of efficiency in the speakers. In UL the feedback is tied to the G2 through the tap so the UL tap has dominant control over the output tube minimizing this effect.
This is probably what's responsible for most of the triode sound.
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Old 10th December 2012, 04:49 PM   #16
DF96 is offline DF96  England
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Quote:
Originally Posted by roline
Another difference is the coupling of the output tube to the load. In triode mode the speakers' impedance is directly coupled to the triode plate so the triodes gain goes up as the speakers impedance goes up. You get a little more output voltage gain to compensate for loss of efficiency in the speakers. In UL the feedback is tied to the G2 through the tap so the UL tap has dominant control over the output tube minimizing this effect.
Actually, the opposite is true! In triode mode there is little increase in gain if the speaker increases its impedance. UL gives some increase in gain. Pentode mode gives a huge increase in gain. It is the lack of an increase in gain which is the advantage of the triode mode. However, the global NFB often used with pentode or UL mode can negate this advantage so the net effect may simply be that triode mode gives smaller peak output power.

Triode mode is acceptable if you want to avoid global NFB and don't mind a poor damping factor (so presumably either have suitable speakers or prefer one-note bass).
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Old 10th December 2012, 06:50 PM   #17
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I recently completed a 1625 (12V version of the 807) SE amplifier - running in pentode mode with regulated screens. It even has a 12J7 pentode driver and loop feedback from the secondary. It's sort of an "anti-300B" design with no special tricks, just old fashion RC coupling and cathode biasing. End result? I really like it, especially considering the sub-$600 price point.

I've built SE 2A3, SE 300B, SEUL 6550, and PP 6B4G amplifiers, but the pentode SE design easily runs with these previous designs. It just sounds different in its own way.
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Old 10th December 2012, 06:57 PM   #18
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Originally Posted by kevinkuang View Post
besides the low distortion/low power vs high distortion/high power,
Actually, ultra linear mode gives higher power but *lower* distortion than triode mode. That's why its called ultra linear.

This is not quite the same as ordinary distributed loading which works on the same principle but doesn't tap the screens at the right percentage OT primary to get the lowest possible distortion.

Last edited by Merlinb; 10th December 2012 at 07:05 PM.
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Old 11th December 2012, 12:46 AM   #19
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What would happen in a blind test switching an old VTL Monoblock from mode to mode?
This came up in another forum, about how the poster cold hear the difference at an audio show.
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Old 11th December 2012, 10:45 AM   #20
DF96 is offline DF96  England
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Naive mode switching will cause gain changes too, so almost impossible to compare like with like. It would be a bit like swapping engines in a motorbike, but forgetting about the change in weight and centre of gravity and torque profile.
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