Just a quick one...
In a guitar amp, the lower the cathode resistor on a 12AX7 preamp stage, the greater the current that gets drawn through the tube, which raises the bias point...
That being the case, am I right in thinking that something like an 820 ohm cathode resistor would mean that a 12AX7 stage would have less headroom for the signal than if you used a resistor with a higher value - 1k for example? (Assuming that the plate resistor is 100k and that the 'balanced' bias point is with a cathode resistor of about 1.5k...)
Thanks in advance!
In a guitar amp, the lower the cathode resistor on a 12AX7 preamp stage, the greater the current that gets drawn through the tube, which raises the bias point...
That being the case, am I right in thinking that something like an 820 ohm cathode resistor would mean that a 12AX7 stage would have less headroom for the signal than if you used a resistor with a higher value - 1k for example? (Assuming that the plate resistor is 100k and that the 'balanced' bias point is with a cathode resistor of about 1.5k...)
Thanks in advance!
I do not like the word "headroom". Unless we are building a house, it has WAY too many meanings.
The Fender-standard 12AX7 with 100K and 1.5K biases the plate near 70% of supply. A *wider* peak-to-peak swing is possible with a smaller cathode resistor, setting plate voltage down to 60% or 50% of supply. Gain will go up a trace. Input overload will be higher.
But why bench-race this question?? This isn't a Mars-rocket, which should be perfect before you light the fuse, because a near-miss is a major loss. Resistors are 12 cents apiece. If you are gonna experiment, you need a 200-resistor assortment pack, now they are 2 cents each. Try one, play, try another, play.
The Fender-standard 12AX7 with 100K and 1.5K biases the plate near 70% of supply. A *wider* peak-to-peak swing is possible with a smaller cathode resistor, setting plate voltage down to 60% or 50% of supply. Gain will go up a trace. Input overload will be higher.
But why bench-race this question?? This isn't a Mars-rocket, which should be perfect before you light the fuse, because a near-miss is a major loss. Resistors are 12 cents apiece. If you are gonna experiment, you need a 200-resistor assortment pack, now they are 2 cents each. Try one, play, try another, play.
There are three things which can limit headroom in a grounded cathode stage:
1. valve cutoff - when the grid goes so negative that the valve stops conducting so the anode voltage peaks
2. anode saturation - when the grid goes sufficiently 'positive' (i.e. non negative) that the anode voltage goes about as low as it can
3. grid current - when the grid goes sufficiently 'positive' that the grid starts taking non-negligible current
Note that 2 and 3 are in a race to see which one can hit the limit first - but how much 3 limits the signal depends on the source impedance.
With a 12AX7 there is only a small window between cutoff and grid current, so you need to set your quiescent bias point near the middle of this window. Grid current can occur if the grid gets more positive than about -1V with respect to the cathode - although this varies from sample to sample. If you have a low impedance source then you don't need to worry about grid current.
1. valve cutoff - when the grid goes so negative that the valve stops conducting so the anode voltage peaks
2. anode saturation - when the grid goes sufficiently 'positive' (i.e. non negative) that the anode voltage goes about as low as it can
3. grid current - when the grid goes sufficiently 'positive' that the grid starts taking non-negligible current
Note that 2 and 3 are in a race to see which one can hit the limit first - but how much 3 limits the signal depends on the source impedance.
With a 12AX7 there is only a small window between cutoff and grid current, so you need to set your quiescent bias point near the middle of this window. Grid current can occur if the grid gets more positive than about -1V with respect to the cathode - although this varies from sample to sample. If you have a low impedance source then you don't need to worry about grid current.
Dave, may I add to your comment, that some tubes start grid current in the negative voltage?. Also, all high µ triodes and most high gm pentodes are critical in which biasing respect. Not only the 12AX7 , as µ is the ratio between δEb/δEg, there is a small range of values in which it is effectively biased at class A amplifier, as a difference with medium or low µ.
Or audio grade valves are far away from this rule?
Best regards.
(Look at this page extracted from Langford-Smith "Radiotron's" book, chapter 2, page 19).
Or audio grade valves are far away from this rule?
Best regards.
(Look at this page extracted from Langford-Smith "Radiotron's" book, chapter 2, page 19).
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In s guitar amp you can actually stick with the typical 100k load resiistor value (with 12ax7s), and just tune the cathode resistors. I've done this a lot in the past, so i know it works absolutely fine.
As I mentioned before, you will find you get maximum clean swing when the anode sits close to around 2/3 of the positive rail voltage.
BTW I often like to have one stage which soft limits on the down swing, for a nice thick crunchy distortion. I achieve that by setng the anode to idle close to half of the tail voltage.
Sent from my phone with Tapatalk. Please excuse any typpos.
As I mentioned before, you will find you get maximum clean swing when the anode sits close to around 2/3 of the positive rail voltage.
BTW I often like to have one stage which soft limits on the down swing, for a nice thick crunchy distortion. I achieve that by setng the anode to idle close to half of the tail voltage.
Sent from my phone with Tapatalk. Please excuse any typpos.
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