Examples:
Neumann U47 microphone. Heater about 20 Volts less?
AKG C12A microphone. Heater 5V instead of 6.3V.
Pleiades V6 prepreamplifier. Heater 3V instead of 6.3?
Even power amplifiers may benefit.
For example
Pleiades Electra 2, headphones amplifier with one 12K5 tube per channel and nothing else.
Heater is 8.9V? instead of 12.6V.
While developing it, it was amazing how better underheating sounded. This came up by chance as the 12V lead acid battery was discharging as music was listened to for more than an hour.
In general it seems underheating the cathode makes the sound much more easy to listen to. Natural. Unstressed.
Although it is possibly not known yet why, here are some thoughts.
1. As the output impedance of the tube increases the surrounding reactive components introduce gentle roll offs at the frequency extremes. The first mathematical derivative or slope of the curve equals to time delay. An abrupt slope has abrupt change of time delay with respect to frequency. So a gentle slope should preserve better the waveform especially the transients.
2. Gentle roll offs also compensate for Fletcher-Munson, voice effort effects.
3. Electron tube running at low temperature. Reduced secondary emission of electrons from high temperature, from photo emission etc. (The cathode glows less). Less noise.
4. Perhaps better overload characteristics as saturation takes place by having no more electrons to do the job. Like on magnetic tape saturation. Gradually running out of steam of electrons. More instantaneous peak limiting, more tube sound or natural sound.
(How would an electric guitar preamp or amp sound with tubes underheated?)
5. If there is increase of tube input impedance this changes the release time of U47 compressing from the diode rectification effect at large signals.
6. Lower power consumption. Especially important to battery low voltage operation,
Less heat generated by the tube. Important when the tube is inside an enclosed space such as U47, Pleiades V6 etc. On the V6 preamplifier it is important as the battery is inside too.
7. More tube life. VF14 tubes used on U47 still operate. If not? a way to revive or repair the VF14 (your own risk) is to reactivate the cathode by connecting it carefully (with ell safety precautions) to the rated heater voltage for a while. Perhaps excededing a bit. This has been tried successfully to get back to normal a noisy VF14. See relevant euroelectron post.
8. Less electron cloud biases less negative the grid. (The cathode is less positive). This can be observed by just connecting even only heater voltage and measuring the potential from grid to cathode with any voltmeter, no special voltmeter is needed. Even one with not so high input impedance will show the effect. For some reason it seems underheating brings more information extraction from the source.
Less hiss.
Apart from the great sound quality, one way to distinguish if an amplifier is operating using the underheating principle, is the slow turn on time, this can be observed on U47, C12A, Pleiades V6 etc.
If there is more information extraction, less noise... How would an RF front end amplifier operate? Possible uses, radio telescope, (seeing the past of the universe). Has this great U47 cathode underheating principle been applied to space communication or radio telescopes?
8. More gain. On V6 the gain increases if the heater voltage is further reduced from 3.9V to nearly 2.5V! (This again with an 8Megohm electron accelerating resistor from anode to grid and 4.9V anode supply.
Is the Pleiades V6 front end amplifier suitable for space as it is battery powered so ready to operate with small solar cells as it only consumes 3V at 150mA.
Electron tubes are very suitable for space as they are not influenced or destroyed by ionizing radiation.
Other vacuum electron tubes that have been found to operate great with low anode voltage and possibly underheating are. ECC82, 7025 tested successfully with low anode voltage and underheating. EF86, EF37A, 6SK7, 12SK7, all triode connected tested successfully with low anode voltage but not tested with underheating yet. Low voltage operation is possible with the Anode to Grid few Megohm resistor. It serves to cancel the internal negative bias of the grid or positive cathode due to electrons emitted to the space charge cloud. The bias is still negative after connecting the resistor but much less so making the operation of tubes in anode voltages as low as 3V.
More investigation is needed.
References:
Neumann U47 schematic
Pleiades V6 schematic
On preserving transconductance (at low anode potential) - euroelectron blogspot
Operating features of the Audion - E.H. Armstrong
The use of Multigrid tubes as electrometers, J.R. Prescott, Melbourn University
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