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
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
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
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
Or I could be completely wrong
Hesky
Hi,
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,
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,
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.
Regards Hans
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.
Regards Hans
Hi,
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.
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?
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.
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?
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.
Thanks,
Steve
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
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.
Tim
) 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.
Tim
this is just an idea, might not be correct, but does anybody think that the filament is almost like another node in out tri/tetr/pentode???
the potential the filament conducts put's it at a different potential to say the cathode/grid/anode whatever... and althogh it's not directly in the electron flow, it would still have a little bit of an effect on the flow of electrons within the tube??? maybe this might have some effect on the tone of the tube, wether the in/direct heating was used for efficiency or not, it might still have some effect on tone mightn't it??? what do others think?
like say it's force was strong enough to aply a resistive force against the electrons accelerating from the cathode to anode, this would in turn reduce secondary emisions from the anode and probably also effect flow of electrons through grid and what not. wether the filament can slow it down much because the anode is generally at a potential of a couple of hundred volts at least, the filament is at lower values, such as 6.3V, so much lower, therefore, probably not much force relative to anode, although with the filament closer to the cathode than the anode, it's force might be stronger... who knows? i'm not too sure at least)
the other point is the fact, like stated was one of efficiency. this is because these tubes were designed for use with battery powered radios. because it's a batery source, there is a DC filament suply, without the ripple you get on an AC rectified DC, or just straight AC... so therefore noise was not a concern in these designs, whereas to put AC, or DC with ripple on the cathode, this would lead to increased hum.... i am sure of this point unlike my last one as i'm just quoting out of an RCA tube manual.)
cheers
the potential the filament conducts put's it at a different potential to say the cathode/grid/anode whatever... and althogh it's not directly in the electron flow, it would still have a little bit of an effect on the flow of electrons within the tube??? maybe this might have some effect on the tone of the tube, wether the in/direct heating was used for efficiency or not, it might still have some effect on tone mightn't it??? what do others think?
like say it's force was strong enough to aply a resistive force against the electrons accelerating from the cathode to anode, this would in turn reduce secondary emisions from the anode and probably also effect flow of electrons through grid and what not. wether the filament can slow it down much because the anode is generally at a potential of a couple of hundred volts at least, the filament is at lower values, such as 6.3V, so much lower, therefore, probably not much force relative to anode, although with the filament closer to the cathode than the anode, it's force might be stronger... who knows? i'm not too sure at least)
the other point is the fact, like stated was one of efficiency. this is because these tubes were designed for use with battery powered radios. because it's a batery source, there is a DC filament suply, without the ripple you get on an AC rectified DC, or just straight AC... so therefore noise was not a concern in these designs, whereas to put AC, or DC with ripple on the cathode, this would lead to increased hum.... i am sure of this point unlike my last one as i'm just quoting out of an RCA tube manual.)
cheers
Hi,
Benny,
As far as IDH valves go, the heater can emit but not for long and only under certain conditions so I wouldn't get too paranoid about that particular aspect.
Heater to cathode insulation is more important here as you don't want the heater sticking to the cathode by exceeded insulation voltages.
If you want it absolutely dead, set the heater and cathode at the same potential and nothing can happen.
Incidentally I stumbled across a page out of the WE book written by one of their senior engineers that could be of general interest to you:
VACUUM TUBE HEATERS.
There's some other interesting stuff on that site so check it out.
Cheers,
Benny,
As far as IDH valves go, the heater can emit but not for long and only under certain conditions so I wouldn't get too paranoid about that particular aspect.
Heater to cathode insulation is more important here as you don't want the heater sticking to the cathode by exceeded insulation voltages.
If you want it absolutely dead, set the heater and cathode at the same potential and nothing can happen.
Incidentally I stumbled across a page out of the WE book written by one of their senior engineers that could be of general interest to you:
VACUUM TUBE HEATERS.
There's some other interesting stuff on that site so check it out.
Cheers,
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