Cathode follower question

[Tobyk]

I want to try a cathode follower for some nice compression.
However, I’m aiming for two 6SL7’s for the preamp, and I’d like a cold clipper as one of those four stages.
Can the gain stage part of the cathode follower circuit be a cold clipper? Why is this stage often set to 820R?

[martin manning]

JCM800 Lead 2204 (using the High input) has:
Stg 1 - Conventional gain stage (100k Ra, 2k7//68n RkCk)
Stg 2 - Cold Clipper (100k Ra, 10k Rk)
Stg 3 - Hot biased gain stage (100k Ra, 820 Rk)
Stg 4 - Directly coupled cathode follower driving the tone stack

[Lynxtrap]

Can the gain stage part of the cathode follower circuit be a cold clipper? Why is this stage often set to 820R?

The tonestack still needs a fair bit of signal before it. A tradtitional cold clipper isn't the best option right before a tonestack because it can sound a little muffled and "lame".

[Tobyk]

I’m not using it to drive a tonestack though, only for generating compression. And I would have a normal gain stage as V1, before the cold clipper.

[Tobyk]

JCM800 Lead 2204 (using the High input) has:
Stg 1 - Conventional gain stage (100k Ra, 2k7//68n RkCk)
Stg 2 - Cold Clipper (100k Ra, 10k Rk)
Stg 3 - Hot biased gain stage (100k Ra, 820 Rk)
Stg 4 - Directly coupled cathode follower driving the tone stack

Thanks, yes I’m familiar with this.
My question is if one can get rid of ”Stg 3” in your example and use ”Stg 2” instead, as part of the CF circuit.

[Ten Over]

I want to try a cathode follower for some nice compression.
However, I’m aiming for two 6SL7’s for the preamp, and I’d like a cold clipper as one of those four stages.
Can the gain stage part of the cathode follower circuit be a cold clipper? Why is this stage often set to 820R?

The gain stage that is direct coupled to the cathode follower determines the idle point of the cathode follower. A cold clipper stage idles very cold which will idle the cathode follower very hot. So hot that grid current will occur at idle. You don't want that. An 820R cathode resistor on the gain stage will still idle the cathode follower hot, but it will take some signal to get into significant grid current territory. The grid current is what creates the compression.

[Tobyk]

I want to try a cathode follower for some nice compression.
However, I’m aiming for two 6SL7’s for the preamp, and I’d like a cold clipper as one of those four stages.
Can the gain stage part of the cathode follower circuit be a cold clipper? Why is this stage often set to 820R?

The gain stage that is direct coupled to the cathode follower determines the idle point of the cathode follower. A cold clipper stage idles very cold which will idle the cathode follower very hot. So hot that grid current will occur at idle. You don't want that. An 820R cathode resistor on the gain stage will still idle the cathode follower hot, but it will take some signal to get into significant grid current territory. The grid current is what creates the compression.

Thanks a lot, exactly what I wanted to know.

[Tobyk]

I want to try a cathode follower for some nice compression.
However, I’m aiming for two 6SL7’s for the preamp, and I’d like a cold clipper as one of those four stages.
Can the gain stage part of the cathode follower circuit be a cold clipper? Why is this stage often set to 820R?

The gain stage that is direct coupled to the cathode follower determines the idle point of the cathode follower. A cold clipper stage idles very cold which will idle the cathode follower very hot. So hot that grid current will occur at idle. You don't want that. An 820R cathode resistor on the gain stage will still idle the cathode follower hot, but it will take some signal to get into significant grid current territory. The grid current is what creates the compression.

A follow up question: since the CF is just a buffer and not a gain stage, is it correct to say that the DC coupled gain stage before it will be the first to distort, if the CF is placed last in the preamp? And if so, will it distort/behave like a normal gain stage would?
I’m asking because I’d like my last gain stage to be hot (~820R) and touch sensitive.

[Ten Over]

A follow up question: since the CF is just a buffer and not a gain stage, is it correct to say that the DC coupled gain stage before it will be the first to distort, if the CF is placed last in the preamp? And if so, will it distort/behave like a normal gain stage would?

The cathode follower causes its associated gain stage to distort when the gain stage itself wouldn't distort with a purely resistive load. The cathode follower has very low grid current under small signal conditions. But when the signal on the plate of the gain stage swings positive enough, grid current starts to flow from the cathode follower. Cathode follower grid current causes a reduction in the gain of the gain stage. More grid current causes more gain stage gain reduction, so as the signal becomes more positive, the gain drops more. This results in a rounded-off signal during the positive phase. The cathode follower is still faithfully reproducing the signal on its grid, but that signal has been altered by the cathode follower.

The negative portion of the signal on the grid of the gain stage produces the positive signal on the plate that gets altered by the cathode follower. The positive portion of the signal on the grid of the gain stage can cause distortion with no cathode follower involvement, so the gain stage can distort before the cathode follower caused distortion if the stages are configured properly.

[LOUDthud]

The 12AX7 cathode follower needs it's Plate Voltage to be at least 100V higher than the input. Otherwise, grid current will steal gain from the gain stage. You could drive the cathode follower with a cold clipper if you have a higher B+ node to hook to the cathode followers Plate. That could stress the follower's heater to cathode Voltage. Another possibility is to lower B+ for the cold clipper, maybe just a 100K or 220K from the cold clipper's plate to ground.

The 820 you see in some amps lowers the Plate Voltage on the gain stage to give the cathode follower more headroom and a little less compression. The 56K cathode resistor you see in Vox amps causes more compression and gain loss.

[Tobyk]

Thanks a lot both of you!

[Tobyk]

Follow up question: does a cold clipper and a cathode follower render similar effect, i e asymmerical soft clipping?

[wpaulvogel]

Follow up question: does a cold clipper and a cathode follower render similar effect, i e asymmerical soft clipping?

No, a cathode follower usually doesn’t operate anything like a cold clipper. The trick to making the cathode follower to compress is to add significant resistance to the grid input of the cathode follower. Something like 470k-1M. I solder a small Piher 1M pot between the plate of the driver stage and the grid of the CF and adjust to taste. The high resistance squishes the CF output and creates an asymmetrical waveform that is very soft and rounded at the peaks but the compression reduces gain/volume by as much as 1/3-1/2 and you have to have enough strength either before or after this compressor to get the power amp to overdrive. I’m not saying you need an additional gain stage to compensate but you have to be ready for the drop in output as you turn up the grid resistance. As long as the amp has sufficient gain before adding the resistor you’ll be okay but it quite noticeable when playing/adjusting the pot. Here’s a picture of the pot.

[Tobyk]

Follow up question: does a cold clipper and a cathode follower render similar effect, i e asymmerical soft clipping?

No, a cathode follower usually doesn’t operate anything like a cold clipper. The trick to making the cathode follower to compress is to add significant resistance to the grid input of the cathode follower. Something like 470k-1M. I solder a small Piher 1M pot between the plate of the driver stage and the grid of the CF and adjust to taste. The high resistance squishes the CF output and creates an asymmetrical waveform that is very soft and rounded at the peaks but the compression reduces gain/volume by as much as 1/3-1/2 and you have to have enough strength either before or after this compressor to get the power amp to overdrive. I’m not saying you need an additional gain stage to compensate but you have to be ready for the drop in output as you turn up the grid resistance. As long as the amp has sufficient gain before adding the resistor you’ll be okay but it quite noticeable when playing/adjusting the pot. Here’s a picture of the pot.

Thanks. I didn’t mean that cold clipper and CF operate the same, but whether the effect of them is the same. Btw I think lowering Rk on the CF would yield similar compression as your grid resistor.