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Old 29th May 2013, 02:20 PM   #1
M Gregg is offline M Gregg  United Kingdom
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Default Why use a pentode?

Why,

did early tube amplifiers use pentodes in the first stage?


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Old 29th May 2013, 02:31 PM   #2
TheGimp is offline TheGimp  United States
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Gain and economy.

Multiple triode gain stages cost more than a single pentode gain stage.

Distortion levels were low enough to be acceptable.
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Old 29th May 2013, 02:39 PM   #3
SY is offline SY  United States
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Lots of open loop gain mean more distortion reduction closed loop. Great voltage swing as well. Low input capacitance.

A pentode is ideal for the input of a power amp.
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Old 29th May 2013, 03:07 PM   #4
M Gregg is offline M Gregg  United Kingdom
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I was looking at this old text..

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



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Old 29th May 2013, 06:55 PM   #5
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Quote:
Originally Posted by M Gregg View Post
...The EF86 is not so popular these days...
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.
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Old 29th May 2013, 07:04 PM   #6
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5879 is a good, low-noise pentode. It was used in many microphone preamps.
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Old 29th May 2013, 07:16 PM   #7
SY is offline SY  United States
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You don't need super low noise at the input of a power amp, though it's certainly not a disadvantage! I'd look at higher gm pentodes. C3g, D3a are superb. I'm playing with some compactrons like 6M11 which seem to be inexpensive and high performance.
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Old 29th May 2013, 07:26 PM   #8
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Quote:
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I'm playing with some compactrons like 6M11 which seem to be inexpensive and high performance.
How much of the compactron is really usable/used then, do you leave many sections 'idle'? I think I remember reading something about the limited possibilities with compactrons due to the often shared cathode...

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Old 29th May 2013, 07:33 PM   #9
SY is offline SY  United States
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You could leave sections idle, but why? If you're doing something along the lines of a 5-20 topology (which is what I'm doing), the extra two triodes are perfect for the long tail phase splitter. If you're doing a Dyna style circuit, they can be paralleled.
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Old 29th May 2013, 07:44 PM   #10
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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!

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