I am building a preamp and want to know if a 12AX7 or 6922 would be better to drive a 250k volume pot.
Either would be ok. A 12AX7 would typically have a 100k plate resistor before a volume control like that.
The 6922 would have less than half that on the plate. Of course, you'd need higher supply current for the 6922.
6922 will generate less distortion when driving a 250K pot, than the 12AX7. Several very high end and very expensive preamps use the 6922. Use a CCS in place of the plate R for even lower distortion and much better power supply hum rejection.
You forgot to mention in what configuration. Cathode follower, common cathode or other?? Makes a big difference 😉
Yes. Application? Proposed circuit?
There are very few sensible questions to which "6922 or 12AX7" gives a useful hint about the answer. They are very different valves: different gain (3:1 ratio), different anode impedance (25:1 ratio?), different preferred anode current (10:1 ratio?).
A related question might be: in what situation would a 250k volume pot be appropriate? Off hand, I can't think of one.
There are very few sensible questions to which "6922 or 12AX7" gives a useful hint about the answer. They are very different valves: different gain (3:1 ratio), different anode impedance (25:1 ratio?), different preferred anode current (10:1 ratio?).
A related question might be: in what situation would a 250k volume pot be appropriate? Off hand, I can't think of one.
Is there a disadvantage in having a 250K pot on the input to a stage that can handle a grid leak of 250K? Reduces the chance of loading any preceding stage doesn't it?
It looks like the 6922 for instance can handle up to 1Mohm of grid-circuit resistance, according to the datasheet.
Noise and the creation of a variable LPF are the biggest I can think of. A common cathode stage with 200kohm (low volume setting) in series with its grid will produce a lot of hiss. And you'll create a low pass filter with the tube's miller capacitance. Whether this is audible depends on the tube and the actual resistance. For example, a 12ax7 (CC) plus some stray capacitances and 200kohm in series will get you a -3dB point of about 5.3kHz. That's one dull sounding amp! So it's not a 'gridleak' issue, but rather a 'gridstopper' one.
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That makes sense. For some reason I hadn't considered this before.. I'm specifically thinking about a volume pot on the front end of a preamp, and I suppose most modern audio sources have a pretty low output impedance, so it wouldn't likely be an issue to end up with a much lower input impedance on that preamp.
The maximum source resistance of a 250k pot (which happens at -6dB) is actually 62.5 k.
The smaller the volume pot is, the larger any blocking capacitors ahead of it have to be,
for a given LF bandwidth. Also, a tube phono stage may need such a large value of volume pot
to avoid excess loading.
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And that kinda takes us back to my previous question; what kind of stage would be driving the pot. It's a bad idea IMO to drive a 250k pot from a common cathode stage. Might be fine in guitar amps, but in hifi, no. Drive it from a low impedance source like a cathode follower and use a 10k to 50k pot instead. A 1uF cap preceding a 10k pot will give you a -3dB point of 15Hz which is more than fine.
Valve radio receivers often used 500k or 1M volume pots. This is because they did not worry too much about noise or frequency response. For hi-fi 50-100k is about the biggest value which should be considered for a volume pot. These are too low to be driven by a 12AX7 (except possibly as a cathode follower).
Of course, a hi-fi preamp is unlikely to use either of a 12AX7 or a 6922 as a grounded cathode amp as both will have far too much gain.
So we go back to the question: what exactly is the aim?
Of course, a hi-fi preamp is unlikely to use either of a 12AX7 or a 6922 as a grounded cathode amp as both will have far too much gain.
So we go back to the question: what exactly is the aim?
Not sure I understand this.. Are you saying that with a 250K pot, 62.5K would be the maximum value that would be in series with the grid of the stage it's connected to somehow?
Yes, as the grid "looks backward", the most resistance that it can see from a 250k pot's
wiper at any setting is 62.5k. This happens when the wiper is set at half the total resistance.
The two halves are seen by the grid as being in parallel. Then, 125k/2=62.5k. Other settings
are even lower in source resistance than that. This assumes a low source resistance before
the pot. Read up on "Thevenin equivalent circuit".
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Yes, that is what he is saying.
A 250k volume pot imposes a load on the previous stage from 250k down to 250k in parallel with the pot load.
A 250k pot provides a source impedance for its load of between 0 (at minimum volume) and 62.5k at -6dB (assuming negligible source impedance - actually [250k+Rsource]/4).
A 250k volume pot imposes a load on the previous stage from 250k down to 250k in parallel with the pot load.
A 250k pot provides a source impedance for its load of between 0 (at minimum volume) and 62.5k at -6dB (assuming negligible source impedance - actually [250k+Rsource]/4).
Okay, slight derail of this thread to my particular case.. My preamp has a Cm on the input stage of 37.4pF as it's configured, so a 250K pot (62.5K) would only limit my HF rolloff as low as 68Khz.
A few questions
- Why the 27k gridleak. Why not e.g. 470k with a much smaller input cap?
- Why the 5k6 cathode resistor. It will set a very odd operating point
- Why the 220k resistor connected to the pot's wiper, paralleled to part of the pot. It'll make the impedance seen by the preceding stage variable and reduce it even further.
- What do you need the high gain of a 12ax7 for?
- Why the 27k gridleak. Why not e.g. 470k with a much smaller input cap?
- Why the 5k6 cathode resistor. It will set a very odd operating point
- Why the 220k resistor connected to the pot's wiper, paralleled to part of the pot. It'll make the impedance seen by the preceding stage variable and reduce it even further.
- What do you need the high gain of a 12ax7 for?
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