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Can someone explain the purpose of having a cathode follower in an amp, and more specifically, why its always right in front of the tone stack?

--Andy

SECONDandBOWERY wrote:Can someone explain the purpose of having a cathode follower in an amp, and more specifically, why its always right in front of the tone stack?

--Andy

My understanding is that it lowers the impedance without adding gain. The goal is to prevent the tone stack from loading down the circuit.

In a BF Fender the tone stack is NOT cathode follower driven. Lower all the tones to 0 and you have no output. On a Tweed Fender the tonestack is cathode follower driven and it doesn't have the same problem.

Yes, the CF is an impedance buffer. It's often used to isolate the preceding gain stage from the slugging effect of a TMB tone stack, so that the gain stage can achieve it's maximum gain potential.

Also, in a Marshall type amp a CF is biased to saturate, which adds some thickness to the overdrive tone.

So if most Marshalls use a cathode resistor of 100k for the CF, then what would a non-saturating values be? Isn't it more that the preceding gain stage is set for higher output with an 820R cathode resistor?

I believe it's both. IIRC it's the 56k cathode resistor that saturates best..

Really low 'starved' cathode resistor values (e.g., 10k) are used for trainwreck-style heavy saturation.

The 100K follower (the classic 'plexi' value) doesn't really saturate the same way, and I thought were strictly acting as a buffer..

I guess 56k might end up being a 'half way there' compromise?

--mark h

A cathode follower has a high input impedance, relatively low output impedance, and is non-inverting. I think Ken Bran used it because Leo Fender did in the Bassman that they copied, and I think Leo did it mostly because he had an extra half of a 12AX7/7025. It probably helps drive the load presented by the TMB tone stack.

markh wrote:The 100K follower (the classic 'plexi' value) doesn't really saturate the same way, and I thought were strictly acting as a buffer.

Even with a 100k load you get a similar effect, although the reasons are different than for a common-cathode gain stage. See Sections 3.3 and 3.4 in this paper - http://www.coastin.com/AESPaper.pdf

zaphod_phil wrote:

markh wrote:The 100K follower (the classic 'plexi' value) doesn't really saturate the same way, and I thought were strictly acting as a buffer.

Even with a 100k load you get a similar effect, although the reasons are different than for a common-cathode gain stage. See Sections 3.3 and 3.4 in this paper - http://www.coastin.com/AESPaper.pdf

I did say "[it] doesn't saturate the same way."

Cool paper, btw -- it sounds like the 100k is more about soft compression and less about hard/asymmetric clipping, though.

--mark h

very interesting article.. One of the things that is different in the 18 TMB or 36,and some of the other amps.
Most copy the stock marshall fender values in amps that use EL84s.

When i was first messing with the first TMB,i used a sub box,and listened to the differences between using the 100k and other values.
Since the vox used EL84s..it also used a 56k cathode resistor that the signal came off of..with a 100k plate resistor. The 100k did not sound as good to me as the 56k in this position..or seemed to add more gain or harshness. I guess backward thinking in a CF. I had also heard people say to use the same for plate and cathode resistor. But the vox AC30 didn't do this,and also used EL84 tubes. So thats why i stuck with it in an amp with EL84s. I guess you could do the math and find out the differences,but Vox used it and it worked for them..lol
Richie

markh wrote:..it sounds like the 100k is more about soft compression and less about hard/asymmetric clipping, though.

Which is also what you get with a warm-biased gain stage as it gets into the saturation region. You get a warmer, fatter distrotion tone that way than with hard clipping.

Good God, Phil, WHERE do you find this stuff ??

Igbit wrote:Good God, Phil, WHERE do you find this stuff ??

School of hard knocks, I'd imagine...

Just a quick note. As I recall a cathode follower, in addition to what is noted above, is a current gain stage as opposed to a voltage gain. Thus it can drive things such as a tone stack.

Gene

True - it also makes it handy for driving line outputs. However, one other fact that I think many people miss is that a CF has a very high impedance on the input side. So if you place a CF between a gain stage and a low impedance (eg tone stack), it makes for minimal loading on the output of the gain stage, so that it's capable of achieving maximum gain.

The Cathode Follower: a silent hero in audio amplification. This humble circuit delivers pristine signal reproduction, faithfully preserving the integrity of your audio source. With low output impedance and high input impedance, it acts as a loyal buffer, enhancing clarity and ensuring your music is experienced in its purest form. Timeless brilliance. top follow