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Old 16th October 2007, 08:11 PM   #1
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Default triode pentodes

Just a wild musing of mine on a Tuesday afternoon.

Why not use the triode side of a triode-pentode TV tube by itself for a preamp application? For example something like a 6gh8 or 6kt8? Of course it would take two tubes to replace a single dual triode, but heck, you can often find a 5 pack of oldies for dirt on ebay (or my box o' funky toobs).

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Old 16th October 2007, 08:28 PM   #2
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Or along the same lines, how about using the pentode side as a triode?

And 12GN7's in a push-pull power output configuration?
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Old 16th October 2007, 09:37 PM   #3
00940 is offline 00940  Belgium
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have you seen this ? Audiovalve RKV schematic (hybrid headphones amp)
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Old 16th October 2007, 10:39 PM   #4
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All tubes are excellent; why people forget about TV tubes is beyond me. Many have impressive transconductance and perveance!

The 6X8, a common-cathode monstrosity, is actually very similar to a 6J6 when the pentode is triode-strapped. This should come as no suprise, as the constructions are even the same. Even with the screen grid used as intended, it still makes an excellent LTP: an interesting alternative to a single triode or pentode input stage. The dissimilarity makes little difference, especially if you're only interested in one plate's output. Since you have two opposed grids, you can easily apply NFB, subtract signals or do even more interesting things, instead of using the low impedance cathode as NFB input, as is traditionally done.

The 6U8 (and many others like it) has independent triode and pentode sections, easily used in all the two-tube topologies: LTP, cascode, mu stage, SRPP and so on. The triode is roughly similar to a 12AT7.

Pay much attention to video pentodes. The 7KY6 is cheap and plentiful, yet has Gm ~ 20,000umhos! I've graphed its triode mode curves and they are excellent as well (I should do this again, gathering accurate data and actually plotting it for use). There are many tubes that combine a video pentode and a high perveance triode, something like a 6DJ8's triode, into one bottle. I could see this used as a high bandwidth driver and cathode follower for some insane driver stage.

And let's not forget those RF pentodes, either. The lowly 6AU6 was extremely popular in quality test equipment, where linearity is at least as stringent as the most discerning golden ear. Probably a quarter of the 6AU6's in my collection have Tektronix labels stuck on them. I also have a bunch of 6DK6s, probably for the same reason. All are high gain pentodes with excellent transconductance and, given the size, suprising current capacity. Don't like pentode? Triode strap them! Who needs a 6BX7 when you can push 20mA through one of those little bottles! Seven pin sockets are cheap, go nuts.

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Old 16th October 2007, 11:43 PM   #5
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Only thing might be microphony as compared to e.g. EF86 etc. But otherwise I share the amazement. I have for decades used ECF80, 6U8 etc. In a power amplifier I am fond of using those as first stage/concertina phase splitter. (Stabilisation of operating point can be enhanced by feeding G2 from next stage cathode.)

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Old 17th October 2007, 06:40 AM   #6
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'cept 1B3's, Tim.
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Old 17th October 2007, 06:47 AM   #7
coresta is offline coresta  France
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and the EF184 / 6EJ7 , one
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Old 17th October 2007, 07:46 AM   #8
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Big triode-pentodes have a good place too, ECL82 (6BM8) and ECL86 are great 1-watt-amp-in-a-bottle fellows.

Here's some 6X8 work Tim spoke of:
http://dogstar.dantimax.dk/tubestuf/paraph.htm


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Originally posted by coresta
and the EF184 / 6EJ7 , one
Spectacular tube, especially trioded


Here's some mini triode-pentode work and impressions I've done for guitar:
http://tonegeeks.sicomm.us/ToneGeeks/index.php?topic=56

Cheers!
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