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Old 29th May 2013, 07:53 PM   #11
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Originally Posted by M Gregg View Post
I was looking at this old text..

However we don't seem to use pentode input...The EF86 is not so popular these days..

M. Gregg
Hi M. Gregg,

The evolution of the vacuum tube amplifier started with the Triode, with which we are very familiar. For high-frequency radio use, however, the Triode has the serious limitation of its Miller Capacitance which limits the upper frequency at which a Triode can be used. Not usually a problem at audio frequencies, but at RF it was a show-stopper!

The Tetrode was invented to reduce the Miller Capacitance by shielding the Anode from the Grid using a "screen-grid". It was easy to manufacture (in the good old days), but had some issues with linearity (the "tetrode-kink"), so a 5th element (the "suppressor-grid") was added to suppress the 2ndary emission which caused the kink.

The Pentode was born, and here we are. Other tubes (especially the Heptode) were developed as mixer-oscillator tubes but we don't seem to use them much in the audio world (unless you are making a Theremin, for which they are very well suited - but that is actually an RF device, another story...)

The higher gain of the pentode has teh cost of higher anode resistance, which makes the interstage coupling a bit twitchier but is not a show-stopper. A majority of designs we see use the triode, but this is likely an historical artifact - triodes were used in the original designs that are replicated today in many projects, they were (way back when) inexpensive and readily available.

As tube prices go through the roof in these days, many folks are looking at alternatives to the "classic" designs and are beginning to look at the less-commonly used pentodes. No reason to not consider them - you will just have to put on the designer hat if you go that route.

I suppose you could use many of the cheap pentodes wired as triodes for the input stage as well, and not have to completely re-invent the wheel. We do that a lot on the output stages - why not on the input as well?

My 2-cents on this.

-- Just an average Man of Bronze --
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Old 29th May 2013, 08:26 PM   #12
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M - here is a link to links with "older" amplifier designs using pentodes on the input. Articles Contents Page

The designs I looked at use EF86 pentodes, teh Mullard uses the EF86 in "pentode-mode" and some of the others have the EF86 wired in "triode-mode".

May provide some background information for your original question.

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Old 30th May 2013, 12:12 AM   #13
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I love pentodes - use 'em all the time for single-ended amp drivers - mostly for driving other pentode output tubes in native or ultralinear mode - loop feedback included. The EF86 is a nice little tube. I've also had good luck with the 6J7 family along with the 6SJ7. 6C6, 12J7, 12SJ7, EF37, etc. I've also used the EL84/SV83 as a driver for a 300B, ala Thorsten Loesch. One stage for all your gain needs is nice.

Last edited by kstagger; 30th May 2013 at 12:14 AM.
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Old 30th May 2013, 01:55 AM   #14
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SY mentioned the Mullard 5-20. Perhaps the most elegant implementation of Mullard style circuitry is the H/K Cit. 5. For very good reason, Stu Hegeman loved high gm. The "Catch 22" here is the dwindling 12BY7 supply that is (IMO) best left to keep existing H/K Cit. 2 and Cit. 5 amps running. Fortunately, similar high gm/high bandwidth pentode types are available to the DIYer interested in building a "1 Off". The Cit. 5 can be improved by installing a 10M45S CCS in the LTP's tail.

BTW, Octal enthusiasts have the 6AC7 for something with HIGH gm.
Eli D.
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Old 30th May 2013, 02:12 AM   #15
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Originally Posted by artosalo View Post
It is strange today that many DIYers think that EF86 is the only choice if pentode voltage amplifier should be built.

I try to balance the situation so that I never use EF86, but all the rest.
I can't tell for the rest, but EF86 (6J32P actually) I used only as a teenager in my first power amp back in 1974. In my latest amps I used pentode parts of 6F12P, ECF200, and 6F1P
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Old 30th May 2013, 03:04 AM   #16
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The "Catch 22" here is the dwindling 12BY7 supply that is (IMO) best left to keep existing H/K Cit. 2 and Cit. 5 amps running.
There are probably just as many, if not more, ham radio transceivers out there eating up 12BY7's than there are Citations. The 12BY7 is the RF driver tube that feeds a 6146A or a sweep tube final RF power amp in dozens of tube and hybrid radios.

Fortunately, similar high gm/high bandwidth pentode types are available to the DIYer
There are a few pin compatible tubes that can be plugged into a 12BY7 socket with a little bias tweak. The 12GN7 and 12HG7 come to mind....there are others that are not as common.
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Old 30th May 2013, 07:44 AM   #17
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Almost none of the compactrons have shared grids or cathodes.

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Old 30th May 2013, 12:37 PM   #18
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I find the old Mullard EF 86 to be very open and slightly warm sounding in a typical 1950's preamp ( Leak ) . Replacements sound very different ( thin ) . Where forced to do this I use the Mullard in the earlier stages . A remastering of the 1940's Bing Crosby Radio Show on virgin acetates were transcribed using a mixture of 86's and won an award using a mildly improved Leak Varislope . I imagine things have improved as there is a market for good pentodes . This might be why the interest in pentodes died ? Those who loved them remembered the past . Some say a cascode is the better replacement .

Do not assume pentode to have more distortion or even distortion of an unpleasant sort . Sometimes in the right situation they do a better job . You have to get the analyzer out to know , or be good at reading the curves . The strangest part is the gain almost comes as a free lunch sometimes .

I am told the EF184 is a bit special as it can be like a ECC81 as a triode and be a pentode if preferred . Having 10 mA current it should drive things very well . I have never tried it with a 300 B . I imagine it would be OK ?

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Old 30th May 2013, 01:00 PM   #19
FoMoCo is offline FoMoCo  United States
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Originally Posted by SY View Post
Lots of open loop gain mean more distortion reduction closed loop.
This might border on being a stupid question... But... Does the extra feedback make up for the loss of linearity.

In other words: If I had a a triode with slight feedback to produce a certain gain and compared it to a pentode with more feedback to get the same gain, which one will have better linearity?
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Old 30th May 2013, 01:08 PM   #20
rongon is offline rongon  United States
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An example of recent design power amp with pentode input tubes = Bandersnatch's E-Linear amp.

Looking to build a 25-30 WPC Push Pull amp - Need Help
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