[AMRadio] Mic opinion (Jim Candela)

[Jim Candela]

    On Tuesday, July 27, 2021, 11:34:19 AM CDT, Donald Chester <k4kyv at hotmail.com> wrote:  

In mine, I circumvent that problem by interconnecting each of my audio units with 500/600 balanced line: the mic preamp, line amplifier, compressor/limiter and transmitter.  Levels are adjusted using Daven attenuators appropriately inserted in the lines between units. The first stage of the mic pre-amp is designed so that the microphone output feeds it at optimum level, and the "mic gain" control is the attenuator between the mic pre-amp and the following line amplifier that also contains the selectable low-pass filter.

The  signal in a multi-stage amplifier should never reach peaks high enough to drive the next stage control grid(s) into grid current, but the level at each stage should be high enough to render hum and noise negligible.

Another factor to consider with triode amplifiers is the Miller Effect.  Essentially, the tube amplifies the effect of the grid-plate capacitance and effectively shunts this phantom capacitance across the input, thereby attenuating the high frequencies. It is less a problem with tetrode and pentode amplifiers because the grid-plate capacitance is minuscule in the first place. 

There is scant mention of this in the amateur radio handbooks, but it is given more attention in guitar amplifier and audiophile circles.  (Remember, ham radio audio is "supposed" to sound scratchy and distorted, with limited high and low frequency response.)

https://www.aikenamps.com/index.php/what-is-miller-capacitance
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Don, that is a good description that I was alluding to.
I have seen several vintage circuits that use a high mu triode with the cathode grounded, and a grid resistor around 4.7 meg ohms. This configuration provides some level of contact bias, and a low level of grid current flows all the time, and the amount will vary moment to moment due to the audio swing. Examples of this is the Gonset G76 2nd audio stage, and the Mosley CM1 receiver audio driver stage. In both cases, there is a loading issue on the prior stage that is influenced by the volume control position. A simple switch to cathode bias might be helpful. I've some cases using just a red LED unbypassed as a cathode bias source to provide about 1.2v bias. In the CM1, I replaced the plate load resistor with a FET based CCS set to 500ua, and the output signal swing is more even around the center +/-...I still want to try the LED bias.
JimWd5JKO