Very rough estimates.
A 16H primary inductance output transformer made on a Magnetec 070 core connected to the anode of a triode connected EF183 prepreamp operating at 3.6 V anode and heater voltage gives a -3dB point at about 300Hz.
This is great for compensating for Fletcher-Munson and voice effort curves for example for MD21 at soft singing voice.
Example setup:
MD21 HN at Hi Z out - Pleiades V0 - Sony TCD5 Pro II - Sennheiser HD580
This gives unheard off low noise and clear voice reproduction
Higher inductance such as found on Altec Lansing 15095 red color octal transformer connected again as single ended with the 100μA anode current passing through it as in the above example gives a 3dB point at around 100Hz. This transformer is not designed to pass DC current.
This may be great for vocal use and any further bass reduction needed can be performed at the primary of the input transformer, by winding or taping suitable number of turns.
A Pleiades output transformer on a Magnetec 070 ring core of double the number of turns should give an inductance of 64 Henries (4 x 16Henries). It is under construction. What would the -3dB point be? It may be difficult to find out as there is still the 100μA further magnetising the core with an H field 2 times as strong? Is there risk of saturation even at such low anode current?
A military 29H output transformer made by Admiral designed for carrying DC current gave a -3dB point at about 100Hz when the output transformer was terminated with a low of the order 600Ω load impedance. This transformer is used on the Pleiades V0 prepreamp. The tube is the ECC82 operated by just 12V.
On all the above circuits very low voltage plate or anode operation is made possible by biasing with a high Megohm resistor from anode to grid. The grid is still negative but much less so by compensating internal nature electron cloud effects.
Specifically on the triode connected EF183 Rag=3.9ΜΩ whereas on the ECC82 Rag=10ΜΩ.
Cathode is connected to minus, ground.
The grid is coupled to external circuits by a coupling capacitor so that it's self assumed bias together with Rag and cathode temperature is not upset.
The heaters are normally underheated at lower temperatures than specified by the electron tube manufacturers for normal high tension operation. For example the EF183 heater is supplied by 3.6 volts instead of 6.3 volts.
The operation is ultra low noise.
One reason may the almost complete absence of any secondary electron emission effects.
The optimum point for maximum signal and lowest noise is found by playing with a variable series resistor to,the heaters thereby adjusting for optimum cathode temperature. Disconnecting the heater circuit while listening and watching the anode current on a micrometer is a quick way to find optimum conditions of operation. The other variable of course is the anode to grid resistor and the Vb. it resembles playing with analog computers.
References:
Operating features of the Audion - Edwin Armstrong
On preserving transconductance of an electron tube at anode poetical as low as 3.6 volts - euroelectron blogspot
No comments:
Post a Comment