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Importance of Screen Voltage of Pentode???
Importance of Screen Voltage of Pentode???
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Old 15th September 2020, 04:38 PM   #41
20to20 is offline 20to20  United States
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... Why mess up a good thing? If it's good going into the amp, it needs to come out the same way only bigger. But if you want a distorted signal then don't use any feedback and that is true for any tubes, not just ...sweet...tubes. They all have it. You either accept it or use distortion elimination circuits to get rid of it. Your choice.
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Old 15th September 2020, 05:01 PM   #42
operater is offline operater
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"Going into the amp and needs to come out the same way only bigger." This is not the first time I hear this philosophy around here. Okay, but.
It depends on what you want from tubes. Push pull layouts are beyond the point as second order harmonics cancel out, can't argue with that. But with class A so many people apprecaite the even order harmonic distortion created by class A operation with 0 feedback. That small amount of even harmonics(which is double the frequency and each time exactly an octave higher) is what is found to be so pleasing to the ear. By applying NFB to class A circuits (which is sacrilege IMHO) all that air, sweetness and musicality get eliminated.
For matters to get even worse simple class A designs don't benefit that much from internal kinds of feedback and external ones even make matters worse and invite stability issues. Just take a look at the analysis by that tubecad guy. Okay you do lack power with a single noval power tube but on high efficiency speakers and not too high volumes it's marvelous. However, what I managed to achieve with 3XEL83s in parallel is so much power to drive a pair of my speakers even to unpleasant levels of volume with no audible distortion whatsoever. Even with bass heavy tracks on top of vocals for instance.

What we're really messing up is this thread so I'd like to get back to the point.

I am reading an insanely interesting article on screen grids here: Tubes

They quote that around 3% screen grid voltage swings should be tolerable for pentode operation. Also, given the fact that class A doesn't suffer from noticeable plate voltage and current swings I would like to understand why would it be so bad to just supply the screen from B+ over a suitable value precision resistor with say 10V drop and maybe two filter caps in between.

cheers.

oprtr

Last edited by operater; 15th September 2020 at 05:20 PM.
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Old 15th September 2020, 05:28 PM   #43
20to20 is offline 20to20  United States
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.... I would like to understand why would it be so bad to just supply the screen from B+ over a suitable value precision resistor with say 10V drop and maybe two filter caps in between. ....


It works great. Most good (simple) amps do it that way. Hundreds of them. It's a common method.
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Old 15th September 2020, 05:37 PM   #44
rdf is offline rdf  Canada
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What does the screen voltage look like on an oscilloscope? Particularly with strong bass content.
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Old 15th September 2020, 06:12 PM   #45
PCL200 is offline PCL200  Netherlands
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Quote:
Originally Posted by 20to20 View Post
Keep it simple and ditch the EL83's. They aren't worth much and they were designed for TV service.
The Marconi datasheet for the N309/PL83 seems to contain data for use as an audio power tube. Note that Va max. and Vg2 max. of the N309/PL83 are 250 V, while they are 300 V for the EL83.
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File Type: pdf N309 - PL83 Marconi.pdf (496.7 KB, 20 views)
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Old 15th September 2020, 07:25 PM   #46
operater is offline operater
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Thats really nice news that macroni datasheet. It's interesting but both Mazda-Belvu and Philips datasheets as well as one tesla sheet all indicate limiting values of 300V for EL83 too, and ofcourse 250V as nominal just like the EL84. It is a one amazing audio tube I'm telling you <3
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Old 16th September 2020, 01:48 AM   #47
PRR is offline PRR  United States
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Importance of Screen Voltage of Pentode???
Quote:
Originally Posted by operater View Post
...........Could I just attach the G2 to the B+ with a high enough resistor to lower the voltage just enough below the plate? ...........
Why do you think G2 has to be lower than plate? This is not a True Tetrode.

The '83 is optimized for gain, the '84 for power. But either one will do audio.
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Old 16th September 2020, 08:32 PM   #48
operater is offline operater
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Didn't actually want to say it has to be lower. My idea of using a resistor was to create a voltage drop in an effort to dissipate some more ripple and maybe use a cap to filter G2 supply a bit more.
I still don't see why are voltage fluctuations such a concern when with class A operation they are almost inexistent. Just trying to wrap my amateur head around it.

We really don't see many pentode builds explored, yet, I don't see why we couldn't do it in class A without a dedicated supply given that the B+ one is beefy enough. Say for instance I have an overdesigned and very stiff chokeless solid state power supply and virtually no voltage swings even at highest amounts of volume.

Best.

oprtr

Last edited by operater; 16th September 2020 at 08:39 PM.
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Old 16th September 2020, 11:31 PM   #49
rdf is offline rdf  Canada
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The screen current draw rises dramatically when the plate nears clipping, which will be often with a low power amp. An RC screen screen supply's source impedance also rises dramatically as frequency declines, essentially approximating the value of the screen dropper resistor at DC. Not a great combination.
The 6LU8 pentode SE on my bench has a highly stable ~110 vdc screen supply. To my ear improvements in screen stability were very and unexpectedly audible, especially at both the low end and under clipping.
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Old 17th September 2020, 07:14 AM   #50
6A3sUMMER is offline 6A3sUMMER  United States
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My first rule for Hi Fi playback of recorded sound:
If it is clipping, it does not sound good, turn it down.

My second rule:
Refer to rule number one.

Just my opinion.

But yes, a change in the design of a screen circuit that makes clipping come at a higher amplitude, will allow you to turn it back up.
And the distortion before clipping will likely be improved too.

Last edited by 6A3sUMMER; 17th September 2020 at 07:17 AM.
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