[quote author="PRR"]I think the slower more-considered pace of theory/design threads would justify having a section just for them.
But we had that discussion already, and the consensus was against it. [/quote]
I have mixed feelings. On the one hand, a separate forum for theory and original designs would certainly enhance my enjoyment of this place. But at the same time, it could make it altogether too easy for unscrupulous manufacturers or other ripoff artists to harvest other people's designs that were intended for noncommercial use by hobbyists. Of course, something as mundane as having to sort through a few pages of postings isn't going to be enough to dissuade a suitably motivated pirate. I guess I can see all sides of the issue, and therefore I am not qualified to run for President :wink:
12AV7... Rp under 10K
I figured 16K, but whatever.
No, you were right, I was wrong. I said "10K or less" based on a reckless assumption made after glancing at data on a "typical operation" table. But when I go to my loadlines and draw the tangent to the curve where the Q-point sits, I come up with 16.6k.
Granted my 500K would be awful marginal.
But still usable if you're careful with layout. The criteria I used was simply that the frequency where the reactance of Cin equalled the resistance of Rg should be above the audio band. That would ensure that the rolloff would be unobjectionable regardless of variations in the plate R of the tube, the setting of the pot, etc.
Simple passive circuits are one of the few instances where I'm willing to trust a simulator instead of using paper and a calculator or physical breadboarding. Here's how I draw the equivalent circuit of V1A coupled to the input of V1B:
Note that I did not include the fixed grid resistor in this analysis. That's because it's assumed to be either sufficiently large with regard to the pot, or simply not used at all. The pot is at 50% resistance to give the "worst case" source Z.
Here's the frequency response with a 100k pot:
-3dB at 100kHz relative to 8.5dB loss
with a 250k pot:
-3dB at 42kHz relative to 7.9dB loss
with a 500k pot:
-3dB at 24kHz relative to 7.6dB loss.
So a 500k pot is usable if you do need that extra dB and can tolerate a tiny amount of droop at the very top of the audio band. 100k gives the widest bandwidth with little loading of the tube, and 250k is a good compromise if you want even less loading but still insist on keeping the lowpass corner frequency well above audio.
drawing and interpreting load lines is tedious
Which is why I wasn't going to do it...... and was not-right.
I hope you don't think my aim was to prove you wrong. I like it when you and other sharp people criticize my designs; that causes me to consider my own reasoning more closely, keeps my analytical skills (such as they are) sharp, and ultimately leads to better circuits!
My next bad guess: bias V1A to put the plate voltage down around 60-70V. Gain may rise and noise (if all else is clean) may drop. Might need to go that way for ribbon mikes.
I understand this to mean increase the standing plate current and in turn increase the drop across the plate resistor. Or do you mean using a larger Rp and keeping all else the same?