All about Plate/cathode resistor values in the preamp

[yeahyeah]

Ok, pretty simple theory question. I can't find anything about it in words that i can understand. hopefully it will help some of the other guys who pretend to know what they are doing.

What happens to the gain and tone of a tube stage as the plate/cathode resistors are shifted up or down?

i keep hearing talk about how 100k/1.8k are the best for cleans...Why?

describe the difference between that and the 220k/3.3k.

Is it just reducing the gain? what is happening here?

Can someone also explain what these resistors do? Does it set the operating voltage of the tube in relation to the power supply rails? bias them? what?

for simplicity, let's stick to explaining this in relation to preamp sections for now.

Thanks!

[heisthl]

The correct ratio is 100k/1k5, what the higher plates do is covered here:
http://ampgarage.com/forum/viewtopic.php?t=3719&start=0

[llemtt]

What really happens in a standalone common cathode triode stage changing plate/cathode resistors (i.e. changing it's operating point) is a subject for a 50+ pages chapter of a good tube book, Radiotron, Reich,... just to name a few.

But what really happens when such a triode stage is one out of four or more stages inside such an amp it deserve at least a 200-300 pages book... Even the very good TUT series from Kevin O'Connor fails to explain everything, he still has lot of arguments under hand for TUT6, TUT7,...

Do you really wanna know? My suggestion: just study from basics, simulate, try,...

teo

[drz400]

The correct ratio is 100k/1k5, what the higher plates do is covered here:
http://ampgarage.com/forum/viewtopic.php?t=3719&start=0

I'm not undertanding

the Higher plate resistance = more bass

Maybe I'm not understanding something but....

Higher plate resistance raises the output impedance of the tube which will reduce bass if the coupling cap and load stay the same but even then it changes actually not by more than a few hz when using 100k compared to 330k, since the plate resistor is in parrallel with the rp (62,500) of the tube.

Load has a much greater effect on the low F-3db

[dogears]

Ah... But you leave out the effect of the cathode resistor being much larger for the higher plates. That can significantly effect the lows.

The correct ratio is 100k/1k5, what the higher plates do is covered here:
http://ampgarage.com/forum/viewtopic.php?t=3719&start=0

I'm not undertanding

the Higher plate resistance = more bass

Maybe I'm not understanding something but....

Higher plate resistance raises the output impedance of the tube which will reduce bass if the coupling cap and load stay the same but even then it changes actually not by more than a few hz when using 100k compared to 330k, since the plate resistor is in parrallel with the rp (62,500) of the tube.

Load has a much greater effect on the low F-3db

[drz400]

Ah... But you leave out the effect of the cathode resistor being much larger for the higher plates. That can significantly effect the lows.

IF you are talking about the RC of the cathode resistor/Cathode cap ?
You are talking about making a change in the cathode resistor along with the Plate resistor? Well then it is easy to change the cathode cap as well, it doesnt take much to balance them out
All I'm saying is that just changing the plate resistor does not change the low -3db point to any significant extent.

Lets say you use 12AX7 100k plate 1.5K cathode 18uf cathode cap,300V supply 500k load .047 coupling cap.
You get gain of 57 (35dB) Z out of 35.7k -3dB 6.3hz
Vmax out = -74/+87.5v

Then you do 220K plate Rk 3.32k everything else the same but cathode cap 15uf, You get gain of 71 (37dB) Z out 61.9k -3dB 6.22hz
Vmax -113/+76v only matters if you go there of course.

[dogears]

Your knee frequency looks like the knee of the .047 cap and not the bypass cap. The knee of the bypass cap should be considerably higher than 6hz. I'll run the numbers in a bit and post the results, but in general the 3.3k resistor will form a knee almost twice as low as the 1.5k, assuming same size bypass. As discussed here ad nauseum, Rkout (Cathode output impedence) is:
Rk//Rk'
Rk= Cathode resistor
Rk' = (Ra+ra)/(mu+1)

This results in a result that is higher than if you just use the RC formula.

In Dumbles which use 4.7uf on OD1 and OD2, in almost all examples, switching to the bigger plates yields much more bottom. That was all the OP was saying. A 220K stage has a 28hz knee and the 100K stage has a 47hz knee when both use 4.7uf caps.

Ah... But you leave out the effect of the cathode resistor being much larger for the higher plates. That can significantly effect the lows.

IF you are talking about the RC of the cathode resistor/Cathode cap ?
You are talking about making a change in the cathode resistor along with the Plate resistor? Well then it is easy to change the cathode cap as well, it doesnt take much to balance them out
All I'm saying is that just changing the plate resistor does not change the low -3db point to any significant extent.

Lets say you use 12AX7 100k plate 1.5K cathode 18uf cathode cap,300V supply 500k load .047 coupling cap.
You get gain of 57 (35dB) Z out of 35.7k -3dB 6.3hz
Vmax out = -74/+87.5v

Then you do 220K plate Rk 3.32k everything else the same but cathode cap 15uf, You get gain of 71 (37dB) Z out 61.9k -3dB 6.22hz
Vmax -113/+76v only matters if you go there of course.

[drz400]

Your knee frequency looks like the knee of the .047 cap and not the bypass cap. The knee of the bypass cap should be considerably higher than 6hz. I'll run the numbers in a bit and post the results, but in general the 3.3k resistor will form a knee almost twice as low as the 1.5k, assuming same size bypass. As discussed here ad nauseum, Rkout (Cathode output impedence) is:
Rk//Rk'
Rk= Cathode resistor
Rk' = (Ra+ra)/(mu+1)

This results in a result that is higher than if you just use the RC formula.

In Dumbles which use 4.7uf on OD1 and OD2, in almost all examples, switching to the bigger plates yields much more bottom. That was all the OP was saying. A 220K stage has a 28hz knee and the 100K stage has a 47hz knee when both use 4.7uf caps.

Yes of course my calculations in the cathode are for full band width as I spec'd the cathode cap as basically full bandwidth and was to show how little the plate value alone changes the -3db point of the hi pass filter meaning the coupling cap RC.

The boost circuit in the cathode is not a Hi pass filter but a selective boost, there is no cut off.
By changing only the plate resistor from 220k to 100k and not readjusting the Ck you are only moving this selective boost knee from 35hz to 44hz.

The cathode knees I spec'd were basic full bandwidth

I just didnt understand the reference to "220K" or "100K" plate resistor was referring to the whole circuit in Dumbleland so I apologize for not understanding it is a referral to a whole stage.

But naturally you can have the gain of the 100k section with the bass of the 220k stage by just changing the Ck, you dont need to change the plate resistor if you are not looking for gain. The 100k/15k/4.7 should be 44hz ? and the 220k/3k3/4.7 is 22hz .... 2k2 is 28hz all assuming there is no load if we are picking hairs