cathode follower

[wsaraceni]

if you were modding an amp and someone told you to add a cathode follower between the last stage and the resistor that goes to the PI input, how would that be drawn on a schematic?

[bluesky636]

Cathode followers are used to drive high impedance loads, like a tone stack. Why do you want to use one to drive the PI? I really don't think it is necessary.

[wsaraceni]

I don't. I was looking at an amp that has it in the design. Wondering what it meant

[bluesky636]

What amp? Like I said, you usually use a cathode follower to drive a high impedance, lossy load like a tone stack that doesn't have a gain makeup stage following it. I don't think that would be a problem with a PI.

[wsaraceni]

. i dont have a schematic. but thats what i am reading about it.

[bluesky636]

I see. Sorry. I don't even want to try and guess why a cathode follower would be used to drive the PI. I don't see anything on the Komet website that mentions a cathode follower driven PI.

[romberg]

I see. Sorry. I don't even want to try and guess why a cathode follower would be used to drive the PI. I don't see anything on the Komet website that mentions a cathode follower driven PI.

A lot of guitar preamps use a cathode follower to softly clip the signal instead of driving a load like a tone stack. You overdrive the cathode follower and get a high gain sound without driving up the noise floor with more gain. Marshall uses them in many of their circuits.

Guitar amps are not exactly like HiFi equipment. Many "rules" are broken or flat out ignored.

Mike

[bluesky636]

I see. Sorry. I don't even want to try and guess why a cathode follower would be used to drive the PI. I don't see anything on the Komet website that mentions a cathode follower driven PI.

A lot of guitar preamps use a cathode follower to softly clip the signal instead of driving a load like a tone stack. You overdrive the cathode follower and get a high gain sound without driving up the noise floor with more gain. Marshall uses them in many of their circuits.

Guitar amps are not exactly like HiFi equipment. Many "rules" are broken or flat out ignored.

Mike

Please read my first and second posts again. I stated that a cathode follower is commonly used to drive a high impedance, lossy load like a tone stack. Leo Fender did it first in the 5F6/5F6A Bassman and again in the 5F8A high-powered twin. Marshal simply copied that design. It should also be noted that a cathode follower has slightly less than unity gain. It is not a high gain circuit. What it has is a very low output impedance (a couple hundred ohms) which allows it to easily drive a high impedance load.

The OP asked about using a cathode follower to drive the PI, not a tone stack. He says that is what the Komet Concorde does. I have no information on that.

[romberg]

Yes. In Merlin Blencow's book he mentions that when master volumes were added to bassman amps that folks found they sounded "cool". The cathode follower Leo had never intended to be overdriven was now being hit with more signal than he ever thought it would see. It could no longer follow the signal and clipped it. People (guitar players) thought it sounded good.

Some marshalls follow a 12ax7 gain stage with a cathod follower that is driven past it's limits and clips. Marshall uses this pairing up to 4 times in some amps.

So, putting one later in the preamp is done to overdrive the cathode follower. It has nothing at all to do with impedance matching.

Mike

[bluesky636]

What does this have to do with the OPs question about the Komet Concorde using a cathode follower to drive the PI? There is no master volume in the Concorde.

[JazzGuitarGimp]

A cathode follower will do a much better job at driving a low-impedance load than a common-cathode gain stage. The reason tone stacks tend to be lossy is because they present a low-impedance load to the preceding stage, not a high impedance load. To say "Cathode followers are used to drive high impedance loads, like a tone stack." is erroneous.

[bluesky636]

A cathode follower will do a much better job at driving a low-impedance load than a common-cathode gain stage. The reason tone stacks tend to be lossy is because they present a low-impedance load to the preceding stage, not a high impedance load. To say "Cathode followers are used to drive high impedance loads, like a tone stack." is erroneous.

Sorry. I got my high and low impedances mixed up. Otherwise, we are in agreement.

Nobody that has posted has yet to address using a cathode follower to drive a PI which is the OP's question. I would be interested in the thoughts behind that myself.

[JazzGuitarGimp]

I could be wrong, but I think it is more common to see cathode followers after the PI, so that you have more drive current into the grids of the output tubes. I am not familiar with the Concorde, so I don't know what they are doing. But I agree that, at least in my limited experience, it doesn't make a lot of sense to use a CF stage before the PI unless, as romberg pointed out, it's being used as a clipping stage. Someone please correct me if I am wrong.

[bluesky636]

I am not familiar with any amp that uses a cathode follower after the PI. Not with a long tailed pair PI, anyway. Perhaps you are thinking about the split load PI (aka cathodyne) used in the Fender 5E3 and other similar amps which takes opposite phase outputs from both the plate and cathode to drive the push-pull pair.

http://ampwares.com/schematics/deluxe_5e3.pdf

[cbass]

The concorde has a CF then a 220k resistor then the PI