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Old 30th May 2013, 02:43 PM   #21
TheGimp is online now TheGimp  United States
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Read the early tube manuals such as RC12 and look at the design examples in the back. For example the 6C6 / 57 is shown triode strapped as a mic preamp (design 17 p. 149 as well as the input stage to amplifiers)

In addition some of the early datasheets provide triode strapped curves although most of these are for power Pentodes .
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Old 30th May 2013, 05:37 PM   #22
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Here is an imaginative use of pentodes . He doesn't say how he got on . Having built similar I think it will work very well . What might surprise people is the so called pentode distortion is a bit of a myth. One gets it if one wants it . By cunning design it isn't a given . Some will say is it any different to a triode if equaling a triode ? Sometimes there is a free lunch in terms of gain .

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Last edited by nigel pearson; 30th May 2013 at 06:06 PM.
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Old 30th May 2013, 05:49 PM   #23
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Quote:
Originally Posted by FoMoCo View Post
This might border on being a stupid question... But... Does the extra feedback make up for the loss of linearity.
It depends on the tube- certainly, a remote cutoff pentode will be a lousy choice, even with the extra feedback. But a good sharp-cutoff pentode will start with reasonable linearity, and the extra gain will take it the rest of the way home. As a bonus, you'll drop the output impedance of the amp more with the higher feedback (i.e., better damping).
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Old 30th May 2013, 05:51 PM   #24
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I didn't want to leave my statement hanging in the air . Here are the graphs of my similar amp . I made it for a friend who perhaps would like to keep the circuit to himself .

All pentode with UL feedback to output . No loop feedback . The intention was to meet the old DIN 45500 standard . For much of the time it had a transistor current source and sink . In the end even they went . Distortion at 1 watt is 1 % all second harmonic and so it goes right down to mW . With a transistor current source distortion was 0.1% at 1 watt . The THD is perhaps 1.5 % at 8 watts . I didn't bother to calculate it . 8.4 watts at clipping . The 8 watts spectrum is not bad seeing how close it is to clipping . It sounds like a very good triode ( 300 B ) . In fact the friend was testing 300B's when he first listened to it . I feel we were closer to the expensive ones ( WE ) . These valves are more often used in guitar amps . The 1% is if you like the line I didn't want to cross , these are my notes so forgive .

Click the image to open in full size.

Last edited by nigel pearson; 30th May 2013 at 06:03 PM.
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Old 30th May 2013, 07:52 PM   #25
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Originally Posted by nigel pearson View Post
......The intention was to meet the old DIN 45500 standard....
This standard assumes that the output power is 10 W minimum with max. 0,5 % THD and IMD. With any 5 W amplifier this standard can not be fulfilled.
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Old 30th May 2013, 08:29 PM   #26
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Intention . Here is the 12 watts , 10 would be a bit nicer than 12 . Same everything except g2 settings this version and my preferred 5 watt version as you call it . Please forgive my shorthand for UL taps . The amplifier is a genuine 8 watts . It has 5 excellent watts .

Look at the 8 watts from the amp that more easily gives 8 watts ( 8 of 12 ) . Not my cup of tea . 8 of 8.4 is not bad .

I did get a DIN45500 version to work . It had loop feedback ( 9.5 db ) . I forget what file name it had ( there are 10's maybe 50+ files of this amp ) . The friend wouldn't have wanted a loop feedback amp nor transistor CCS . The transistor CCS was remarkable . I used 3 x 1N4148 as that seems to be better when setting reference voltage than anything else tried . Unlike most valve people I used a MJE 350 . It was excellent . Also LM 317 replaced by BD139 as a sink . It took days to better the LM317 . The one thing the transistors gave was a clue as to what should be possible . None of the graphs shown are with transistors .

One thing I did try was a signal activated bias idea . Worked a treat and got some excellent power . No obvious problems . Again my friend wouldn't have wanted it .

I hope everyone can see how pentode can be made to work . The all triode version was about 4 % THD and very insensitive ( similar spectrum ) . It had no wow factor to say it as worth the trade off over pentode . My PP amp is all triode .

Click the image to open in full size.

Last edited by nigel pearson; 30th May 2013 at 08:52 PM.
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Old 31st May 2013, 11:02 AM   #27
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Originally Posted by Rundmaus View Post
OK,

just looked up the 6M11 datasheet and what I said does not apply here. In this nice compactron, each electrode of all three systems is accessible separately. Looks very useful!

Rundmaus
while i do not have the 6M11, i do have the 6BH11 and is using them intwo current power amp builds...also cheap at a dollar a pop at Rogalskie's...

this tube makes implementation of the mullard 5-20 topology a single tube thing...
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Old 31st May 2013, 11:38 AM   #28
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If it wasn't for the fact that all compactrons and sockets have to be imported from the USA I would use just about nothing else.

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Old 31st May 2013, 12:18 PM   #29
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Originally Posted by nigel pearson View Post
Here is an imaginative use of pentodes . He doesn't say how he got on . Having built similar I think it will work very well . What might surprise people is the so called pentode distortion is a bit of a myth. One gets it if one wants it . By cunning design it isn't a given . Some will say is it any different to a triode if equaling a triode ? Sometimes there is a free lunch in terms of gain .

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This is called Schade feedback and was described in a different implementation but exactluy the same principle by the inventor of the Beam tetrode (BPT), specifically the 6L6 and derivatives. There was a thread discussing it at length about a year ago.
This is usable for all sorts of 'pentode curve' amplification devices, includin silicon - and, to a lesser extent to triodes. Essentially it implements externally what triodes implement by design, but with a different trade-off: the input capacitance stays pentode-like (very low) but input impedance drops drastically. The bonus part is that the external local feedback is variable through the choice of the plate voltage divider ratio, so you can essentially build a whole family of triodes out of a single type pentode or BPT (or MOSFET, or JFET, or BJT...)
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Old 31st May 2013, 01:25 PM   #30
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If it wasn't for the fact that all compactrons and sockets have to be imported from the USA
????? All of mine came from China. There has to be a path from China directly, if not there should be.
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