Directly heated cathodes, but why? - diyAudio
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Old 6th June 2004, 05:55 PM   #1
Hesky is offline Hesky  United Kingdom
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Default Directly heated cathodes, but why?

Hi all,

I've been pondering over directly heated triodes, why are most triodes (2A3, 300B, 845 etc...) directly heated and beam tetrodes such as KT88, 6550s indirectly heated? I'm sure there’s a valid reason, but triodes such as the 6C33 with its indirectly heated cathode just confuse me.

Hesky
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Old 6th June 2004, 08:45 PM   #2
DougL is offline DougL  United States
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Quote:
why are most triodes (2A3, 300B, 845 etc...) directly heated
Most Triodes and Pentodes designed before about 1940 are directly heated.

Most designed after that date are indirectly heated triodes and pentodes. The EL34, 6550, KT88 etc. all came later.

The triodes you mentioned are from the 20s and 30's. Totally antiquated, except they have very low distortion.

With the general use of feedback, pentode output stages became popular, due to their ease of drive/ high gain and good dampening. It made a less expensive, more efficient amplifier.

The 6C33 and 6AS7 were designed for shunt regulator use much later. In this application triodes were useful.

Cheers;

Doug
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Old 6th June 2004, 09:09 PM   #3
Hesky is offline Hesky  United Kingdom
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In that case to me it just seems like it’s a simple case of 'if it works don’t change it'. These said vacuum tubes are antiques but perform well with v low distortion which is an audio requirement as you say. I think that if a company produced an indirectly heated 300B say, it just wouldn’t be accepted not traditional. Might be overall a better valve, but not to everyone’s taste.
Or I could be completely wrong

Hesky
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Old 6th June 2004, 09:19 PM   #4
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I would agree, from a beginners standpoint at least
I've thought about this for a while now, and I really can't think of any technical reason (for audio use at least) why direct heating should be better than indirect heating, though I haven't actively searched for such information yet.
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Old 6th June 2004, 10:07 PM   #5
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Hi,

Quote:
I've thought about this for a while now, and I really can't think of any technical reason (for audio use at least) why direct heating should be better than indirect heating, though I haven't actively searched for such information yet.
Surely I'd be very happy if someone, somehow, could come up with a technical reason as to why directly heated triodes/penthodes sound the way they do.

Then, one could say the same about mesh anodes and plenty of other enigmas in audio....

Maybe some day we'll know. In the meantime, enjoy.

Cheers,
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Old 6th June 2004, 10:42 PM   #6
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Hi Frank,
It would be nice to know though, I guess audio is always going to be a little subjective for science to always have the answer...
Steve
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Old 6th June 2004, 11:00 PM   #7
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One reason to use directly heated cathodes are that they are more efficient , (higher cathode saturation current for same heater power) Most high power transmitting tubes are directly heated due to this reason otherwise thay would consume even more heater power than they already do.

For a cylindric, elliptic or rektangular tube structure it is possible to show theoretically that linearity will be better the smaller physical surface the cathode has, a directly heated cathode have smaller surface area for same emission than an indirectly heated one , that is one reason why tubes like 2A3, 300B and similar are very linear. Theory regarding effects on linearity by different tube structures was well known before 1940.

Tubes with indirectly heated cathodes can be made with similar linearity by using other tube structures, (non-cylindric). So it would probably be possible to make a indirectly heated tube with similar linearity as a 300B.

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Old 6th June 2004, 11:06 PM   #8
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Hi,

Quote:
So it would probably be possible to make a indirectly heated tube with similar linearity as a 300B.
It certainly is possible and some triode connected penthodes have curves that show very similar linearity to DHTs.
Yet for some reason not yet clear to me they still don't sound as good.
Not even close.

Quote:
guess audio is always going to be a little subjective for science to always have the answer...
Audio is perfectly scientific....for as long as it's kept in a box.

Cheers,

P.S. Steve, what's that nice sportscar in you avatar?
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Old 7th June 2004, 08:32 AM   #9
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It's an Ultima GTR, a little random giant killer british kit car with a silly v8 in it. They raced it in British GT's for a while with mixed success, but I guess that was against the might of McLaren, Marcos, TVR, Porsche and Lister etc etc.

Quote:
It certainly is possible and some triode connected penthodes have curves that show very similar linearity to DHTs.
Even with this in mind, do you think it might be worth it to add a switch to my amp (PP Williamson based KT88) for switching between triode and UL mode?
Thanks,
Steve
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Old 7th June 2004, 09:23 AM   #10
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Can't say I have a SEDHT amp but I suspect a lot of it is money and psychoacoustics... you know, how cables 'work'... For instance, "Frank hates pentodes period, thus loves real triodes." (Obligatory ) Works for me given there's no electrical reason for it.

Filament cathodes are used with unipotential coatings (like all your common DHTubes, 2A3, 5U4, etc.) for portables because they use the bare minimum cathode area possible to save on A battery. Transmitter tubes use thoriated tungsten filaments SOLEY BECAUSE the high voltages (over 3kV) cause high energy ions which would destroy a unipotential cathode in no time. "Lower voltage" transmitting tubes such as the 4CX250R use unipotential cathodes.

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