First, of course, replacing the anode resistor with the "Current Regulator Diode" (or its equivalent circuit). What are the benefits?
With 100k & 60V +B the tubes anode current is ephermal (~250uA / 0.25mA), anode impedance is high (> 20kOhm), overload is early (only 35V across the tube) and the tube operates in a very curved part of the plate curves.
A CRD is actually a J-Fet (plus resistor in some cases) and it tends to have a near infinite or even negative impedance.
The E352 is a CRD in a classic wire ended glass case rated at 100V maximum Voltage and 3500uA (3.5mA) now in production with Semitech and normally stocked by Mouser.
The E352 with 30V across it actually passes around 3mA and has around 30kOhm NEGATIVE impedance, meaning as the voltage across the E352 increases the current drops (and visa versa) acting as positive feedback to plate, improving linearity and gain over a pure CCS.
That said, the effect is quite mild but present, so ideally an E352 CRD is used and not a BJT CCS.
With 1.2V bias & 3mA the EF95/6AK5 et al biases at around 60V and has a relatively low anode impedance (~5kOhm) a gain (Mu) of around 27 with 240R cathode resistor giving an effective output Impedance at the Anode of appx. 12.5kOhm.
60V across the 6AK5 and 30V across the CRD require 90V +B.
Our maximum signal swing on the grid will be 2V PP (~0.7V RMS) with appx. 55V PP available at the anode and 5.5V PP (~2V RMS) maximum output, much larger than the original design, despite the original using a 6:1 stepdown instead of 10:1.
There is no reliable DCR rata on NTE10/3, let's presume 1K on the "10" side, so the output impedance becomes around 150...200 Ohm and the gain from Capsule to Output around a factor of 2.7, load dependent.
The 240R cathode resistor is not sufficient to bias the tube, this means that initially the tube will draw grid current through the Microphone Capsule's capacitance, which will store the charge from the grid current until grid current stops, at around -0.5V (0.72V across the cathode resistor).
It is possible to short the 240R resistor out and/or lower it for lower impedance, without some resistance there is a possibility that bias will not start up correctly and the 6AK5 ends up in saturation acting as diode.
You didn't use this in the “final” circuit, but I've seen something similar before. Why do you do this, what are the advantages of this trick?
It creates a bass boost, which may be desirable or not. Without reliable detained data on the NTE 10/3 I felt it better to avoid this.
thor.zmt said:
Further trick, use the heater voltage to bias the Cathode, perhaps via a divider.
The same here. This is also known from some other microphone circuits. But what is the concrete advantage of biasing the tube in this way? It increases the demands on the quality of the heater voltage, but this can be easily managed if there are advantages elsewhere.
We want 1.2V bias and have (say) 180mA heater current.
This means a 6.8R resistor will bias the tube correctly without introducing degeneration.
It means we make our heater voltage 6.3+1.2V = 7.5V and make sure it's very quiet.
In my experience, tubes sound somewhat better when a little more current flows, but tubes in microphone headamps are very often operated with rather low currents (<1mA), as one would probably like to avoid current noise at this delicate point. Are you not afraid that the more than 3.5x current flow will be detrimental to the noise floor of the circuit?
Current is less relevant to noise than voltage.
The Lomo 19A-9 Mic used the soviet 6AK5 copy with 1.5mA+ current, 20k anode load and 90V+B.
Two sources for actual noise (not hum or microphonics) in tubes.
(1) There is inherent random noise from the flow (modulated by space charge) of discrete electrons.
The textbooks tell us that the equivalent noise resistance (a fictional resistor in series with the grid) decreases with transconductance:
RN = 2.5/gm, approximately, for a triode, in the audio range.
(2) There is also "excess noise" due to strange effects on the cathode surface, which are not so predictable, but the spectral density goes as 1/f.
It is also called "flicker noise" or "pink noise".
At higher frequencies, there is also induced grid current noise, due to impulsive effects from the electrons speeding past the grid wires.
There are possible defects in individual tubes, due to construction or deposition of unwanted material in annoying locations, over and above the inherent noise.
Noise (2) scales with cathode emission. Lowering heater voltage reduces this type of noise, as long as we do not exploit the tube near rated cathode current.
Noise (1) scales with Transconductance, which is lower at high voltage/low current operation and higher at higher current/low voltage.
A slightly more exhaustive and more math heavy discussion of tube noise plus actual data may be observed here:
Principles of Electron Tubes - Chapter 13 - Noise
There is also noise in gassy tubes, all older tubes that have been on the shelf a few years or decades should be baked to reactivate the gettering and capture the various molecules that leaked in over the years, do that before ever applying power to the tube.
Taking tubes out of the box after decades of storage and turning them on without first thermal treatment can permanently damage the cathode coating and result in noisy tubes.
I'm curious to see what difference this makes in reality.
Let's try. It will be easy to swap the CRD for a 100K resistor, 240R to 510R and drop the +B to 60V for
@Emmathom .
PS: If you are wondering why your mods/design generates so little resonance here, that's easy to explain. Important keywords are missing in the thread title. Try Neumann, Neumann, Neumann and AKG and invent a completely implausible background story with maximum WWII and "Braunbuch" references, then there will be more interest here.
I am not doing this as a popularity contest.
And yes, I know actually a lot about all this. In former East Germany we still had System Eckmiller Coaxial Speakers in studio service in the 1980's, that originally had been built by Konskie & Krueger in the same factory that made the Enigma Cypher Machines for Wehrmacht, Luftwaffe and Kriegsmarine...
From the mist, a shape, a ship, is taking form
And the silence of the sea is about to drift into a storm
Sign of power, show of force
Raise the anchor, battleship's plotting its course
Pride of a nation, a beast made of steel
Bismarck in motion, king of the ocean
He was made to rule the waves across the seven seas
To lead the war machine
To rule the waves and lead the Kriegsmarine
The terror of the seas
The Bismarck and the Kriegsmarine
Plus we had Microtech Gefell (Neumann Ost) where they made microphones as it was still 1943. And yes, we had STGW 44 in reserves for the Paramilitary and I got to train with it and like it better than AK-47.
But that's like so much from the ancient days of 2019 BC (before covid) are lost in the mist of times.
Enough namedropping?
Thor
God damn, that's really bad! LOL. Is that the Swedish Ramstein?
