Directly heated cathodes, but why? - Page 2 - diyAudio
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Old 7th June 2004, 03:52 PM   #11
benny is offline benny  Australia
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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.)

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Old 7th June 2004, 04:26 PM   #12
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Quote:
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)
I guess that might be another reason to float the potential of the heater up higher, as well as helping to preserve the heater/cathode insulation
Steve
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Old 8th June 2004, 12:18 AM   #13
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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,
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