Cathode follower cathode resistor question

[Mark Buckingham]

Can anyone tell me what different values for the cathode resistor on a cathode follower do?

Basically, I'm looking at the tone stack driver for a Marshall, and the cathode resistor there is 100k... The same as the plate resistor on the preceding stage.

On a vox top boost or ac-50, it's 56k, but the plate resistor on the preceding stage is still 100k.

What difference does this make in the tone?

Thanks.

[Joe N]

there was a long discussion about it at the ax84 forum not too long ago. you should jump over there and do a search. I think you will find what you are looking for.
Joe N.

[Ray Ivers]

Mark,

In the Marshall direct-coupled CF circuit, the follower's cathode resistor (along with the CF grid voltage) sets the idle current of the CF stage, determines the output voltage swing (it has far more effect on the negative swing than the positive), and - most importantly IMO - sets the point at which the CF stage will draw grid current.

If the Marshall circuit's CF is biased hotter with a smaller-value Rk, its grid will conduct earlier, loading down the preceding gain stage and causing it to distort sooner, and in a more symmetrical manner. To my ears, this results in a much better quality of distortion than a conventional gain stage clipping by itself; "all the grind, without the fizz" - whatever that means.

Ray

[Mark Buckingham]

Originally Posted by Ray Ivers

Mark,

In the Marshall direct-coupled CF circuit, the follower's cathode resistor (along with the CF grid voltage) sets the idle current of the CF stage, determines the output voltage swing (it has far more effect on the negative swing than the positive), and - most importantly IMO - sets the point at which the CF stage will draw grid current.

If the Marshall circuit's CF is biased hotter with a smaller-value Rk, its grid will conduct earlier, loading down the preceding gain stage and causing it to distort sooner, and in a more symmetrical manner. To my ears, this results in a much better quality of distortion than a conventional gain stage clipping by itself; "all the grind, without the fizz" - whatever that means.

Ray

Interesting... So, if I'm after distortion, the 56k value is better?

[Ray Ivers]

Mark,

Interesting... So, if I'm after distortion, the 56k value is better?

Yes, all else being equal - and if you're looking for max gain/distortion, you could also increase the plate resistor value of the gain stage as well (the CF's light loading allows very high Rp values and a higher GS Zout, which in turn will be affected more by the CF's grid conduction). It's all kind of a balancing act between the gain stage's Rp and (unbypassed) Rk, the CF's Rk, the B+ voltage, the signal level hitting the GS grid, the load on the CF output, etc.

Ray

[TDS]

This mechanism is very interesting. I wonder how it applies to the use of a MOSFET source follower used in the same way. I have used a source follower (RG Keen style) in a SLO100 type amp and never gave a thought to the fact that the GS plate resistor was 220K and the SF source resistor was 100K. As a drop in replacement for the standard CF driving the tone stack it has worked transparently, except for the fact that I mistakingly used a 220K gate stopper resistor instead of a 220R resistor. When I changed it to 220R I came to the conclusion that the sound had been a little smoother with the 220K gate stopper and decided to leave it that way. There does not appear to have been any detrimental effects in spite of the fact that the amp has been gigged hard.

I would like an explanation as to the implications as in truth, I dont fully understand what is actually happening. How does the gate in the SF behave compared to the grid of a CF?

Any insight welcome,

TDS

[Arthur B.]

Originally Posted by Mark Buckingham

Can anyone tell me what different values for the cathode resistor on a cathode follower do?

Basically, I'm looking at the tone stack driver for a Marshall, and the cathode resistor there is 100k... The same as the plate resistor on the preceding stage.

On a vox top boost or ac-50, it's 56k, but the plate resistor on the preceding stage is still 100k.

What difference does this make in the tone?

Thanks.

To put it succintly, the cathode resistor in a cathode follower (grounded plate) is a load resistor just like the plate resistor in a typical grounded cathode aimplifier. It will behave in much the same way.

[woodyc]

Originally Posted by Ray Ivers

If the Marshall circuit's CF is biased hotter with a smaller-value Rk, its grid will conduct earlier, loading down the preceding gain stage and causing it to distort sooner, and in a more symmetrical manner.

In the direct-coupled CF, both triodes have the same B+ and the same operating voltage. So don't they clip at the same level, regardless of the value of the CF cathode resistor? The cathode resistor mainly affects the average current in the CF. So it would seem the choice of Rk is largely about how much signal you get thru the stack vs how hot you want to run the CF.

[Ray Ivers]

woodyc,

In the direct-coupled CF, both triodes have the same B+ and the same operating voltage. So don't they clip at the same level, regardless of the value of the CF cathode resistor?

The operating and B+ voltages are only two factors that affect clipping behavior; the type of stage in use is far more important. Common-cathode, cathode-follower, and grounded-grid stages all will have radically different overload characteristics, even if voltages across the tube and B+ voltages are identical for each.

Assuming a 12AX7 GS/CF @ 350V B+, 100K Rp/1K Rk (GS), 100K Rk (CF):

The GS will be drawing @ 1.5mA (out of 2.5mA max), and will have @ 200V across the tube.

The CF will be drawing @ 2mA, and will have @ 150V across the tube.

IMO these are quite different operating conditions, even leaving out the apples/oranges nature of a conventional GS vs. a CF. With these quiescent conditions, at some point during the positive GS swing the CF will start to draw grid current, and the loading on the GS plate will change from very light to much heavier, and the positive swing will take on a rounded character (this is by no means what I'd call 'hard' clipping, but it's quite apparent to both the ear and the 'scope IME).

The cathode resistor mainly affects the average current in the CF.

Yes - which in turn affects a number of other things as well. It also sets the signal swing and gain, although to a far lesser extent than a change in Rp value affects these things in a GS, obviously. And it also determines the point at which the CF grid conducts and loads down the GS plate. NOTE: In this circuit under these operating conditions, a 12AX7 CF may over-current with an Rk value below 82K.

Ray

[woodyc]

I didn't realize how close to saturation the bias ends up in those CFs. I was just plotting up the construction on p.391 of RHD4 for a 12AX7, and with Rk=100K the available + swing is only about 40V. When Rk goes smaller the bias moves even closer to saturation (as you said.) But the distortion doesn't get more symmetrical, it gets less symmetrical. And I would describe it as pretty hard clipping. I've got a 2204 preamp on my bench and I added a switchable Rk to the CF. I could post a scope trace if anybodys interested.

Its a sticky and interesting design problem since with a stack load you want max current in the CF but with the forced bias thats available you actually get more headroom with less current. I guess the way Fender/Marshall wired it up isn't a bad compromise. You could do better by sacrificing a little gain (not a problem in the 2204) and splitting the previous stage plate resistor so you could get more dc volts to the CF grid. But then I guess you'd bump up against the 12AX7 heater-to-cathode rating. I'll stop thinking out loud now.