600 ohm grid input resistor?

[KC9LKE]

I did this some time back and all seems to be well, just wanted to run it by the board to see if I was on the right track.

 I have a speech amp mod driver similar to the 1957 handbook design, crystal mike input, two pentode gain stages and a triode driver. I wanted to drive it with a 0 dBm input signal so I tried a voltage divider on the mike input, sure it worked. Then I thought why take a high level signal, attenuate it, just to run it through a high gain stage. I remember reading “design the speech amp with a little more gain than is needed to fully modulate the carrier”. Sure, copious amounts of unneeded gain just increases the noise level, so why use it.

So on to the next gain stage (pentode). Still to much gain. After some calculations I decided to keep the stock pentode but reduce it’s gain by configuring it as a triode. I Checked the DC operating point of the tube and it looked reasonable for class A. So I applied .700 Vrms at 1 kHz from the signal generator with a coupling cap to the grid, and bingo 95 % modulation of the carrier! 

So the .700 Vrms input gives some headroom for a 0 dBm input signal. I checked the symmetry of the test signal, input vs. output, its OK up to about 1.0 Vrms in, and the amp’s -3 db points, are around 100 Hz to 15 kHz.

 I would rather not use a transformer for input isolation so I ended up with a 10 uF coupling cap and then a 600 ohm resistor from the grid to ground. The signal generator and speech processor drive it OK fine and it sounds clean.

Effectively I just matched the tube to the previous stage, a source of .775 Vrms with an internal resistance of 600 ohms. Are there any issues, as far as the cathode biased tube is concerned, with using this low of a resistance value across the grid to ground? Just never seen it before. Comments, suggestions welcome.

Thanks Again

Ted
KC9LKE

[w4bfs]

hello Ted ...sounds like good work there .... no problem in a single ended voltage amp taking the grid closer to ground impedance except that it takes something with lower output Z to drive it ....also makes it less susceptible to external hum and noise .... if you use your 600 ohm driver to drive several rigs all loaded with 600 ohm input resistors then low signal and distortion could result .... I tend to make my 600 ohm balanced audio stuff load with 10k or so input Z ..... lets you drive several at once and no worries ...73 ...John 

[KC9LKE]

.... no problem in a single ended voltage amp taking the grid closer to ground impedance except that it takes something with lower output Z to drive it ....also makes it less susceptible to external hum and noise ...73 ...John 

John:
And this was also an absolute must due to the layout. Not mine.

73's
Ted

[The Slab Bacon]

Just my $.02 worth.....................

If you need a low impedance inpoot and dont want to use a transformer,
Why not make the input stage of the first amp cathode driven?? Especially if you have output to spare from the audio processing gear. That would give you a good low impedance feedpoint and a reduction in gain, as well as a greater resistance to hum and stray RF pickup.

I have seen this done on some older transmitters for the phone patch audio input,
so why not do it for the main audio input as well? Especially if you dont need mic level sensitivity.

Just use a cathode resistor somewhere in the impedance range that you want the input to be and no bypass cap wrapped around it.

[WA1GFZ]

after taking years of abuse over my poor audio quality I made some changes this past winter. I had a high Z input to my V2 but wanted to load it to 600 ohms for the processor. I just put a 620 ohm resistor across the mic connector contacts inside the connector so the load is removed when the cable is disconnected from the rig, if I want to go back to stock audio.

[KC9LKE]

OK:

Thanks all for the input!  When I have some time I’ll try the cathode driven input and see how it pans out. Sounds like there are several good ways to accomplish the task.

Thanks again!

Ted / KC9LKE