What's a cathode follower?

[Mike M.]

Just trying to expand my understanding of things here. I have a Marshall DSL 50 which has 4 preamp tubes. Of what I gather, the function of the V3 preamp tube is for the "cathode follower" and the signal then goes into the tone stack.

I've been doing a lot of experimenting with the V1 and V2 preamp tubes but have yet to change the tube in V3. Can anyone explain what the cathode follower is and what effect a lower gained tube would have as well as the effect on the tone stack?

Thanks.

 

[timorousme]

Re: What's a cathode follower?

It's the thing that follows the cathode... Duh...

I'd try googling it. I actually did try googling it, but amp jargon goes WAY over my head. Maybe you'll have better luck.

 

[Mike M.]

Re: What's a cathode follower?

It's the thing that follows the cathode... Duh...

I'd try googling it. I actually did try googling it, but amp jargon goes WAY over my head. Maybe you'll have better luck.

Thanks. I've asked this at a few sites and can't get an answer from anybody! I'm no tech head either. As they say, "I only know what it sounds like."

 

[timorousme]

Re: What's a cathode follower?

Everything that I've found talks about old radios. The one amp thing that I found just briefly mentioned it saying that it's rare that an amp uses one. Maybe that explains why nobody has any idea what the hell it is.

 

[MattPete]

Re: What's a cathode follower?

I don't completely understand cathode followers, but it has something to do with lowering (or increasing?) the impedence of the signal before it gets to the tone stack, and prevents the tone stack from interfering with the previous gain stage somehow.

 

[Curly]

Re: What's a cathode follower?

from the Trainwreck pages:

"the cathode follower is basically used to convert the high impedance signal coming from the previous three gain stages to a low impedance signal to drive the tone control circuitry. A cathode follower has no gain at all."

 

[Rich_S]

Re: What's a cathode follower?

Right, it's a unity-gain stage placed between the preamp stages and the tone stage to buffer them from each other. It provides the ability to source extra current to drive the tone stack, which is a veryu loss section of any amp.

Look at this Tweed Bassman schematic. The cathode follower is the right half of the leftmost 12AX7. Note how the wire leading to the tonestack (dead center of the page) comes off the cathode on the bottom of the tube, rather than of the plate (top) like the earlier gain stages. (Signal flow is left-to-right.)

Marshalls also have them - look at the right side of V2 on this 50 Watt JMP. Later Fenders (Blackface and beyond) got away from cathode followers.

 

[Mike M.]

Re: What's a cathode follower?

Very informative. Thank you very much.

 

[boonhogganbeck]

Re: What's a cathode follower?

I've heard it said that the cathode follower is a big part of the crunchy sound of tweed Fenders (Bassman) and Marshalls when you crank them up, as opposed to blackface fenders that overdrive but don't really crunch. Any truth in this?

 

[Lake Placid Blues]

Re: What's a cathode follower?

Well Hiwatts don't use a cathode follower setup, and niether does the Marshall Jubilee. The Jubilee used a very similar tone stack to a Hiwatt as well. These amps certianly don't have a problem with getting great crunch.

It's interesting to note that some of the amps that are noted for their wonderful cleans, when run clean, such as Hiwatt, and black faces, are not using a cathode follower setup.

I wonder if this has something to do (in the case of Hiwatt and the Jubilee) about having a post tone stack master volume or not?

 

[Fresh_Start]

Re: What's a cathode follower?

I've heard it said that the cathode follower is a big part of the crunchy sound of tweed Fenders (Bassman) and Marshalls when you crank them up, as opposed to blackface fenders that overdrive but don't really crunch. Any truth in this?

Don't think so. The cathode follower does what Curly & Rich S. said - in non-technical terms it's a buffer that actually has some loss of gain.

"Crunch" comes from a lot of preamp design details. A lot of it has to do with selecting specific frequency ranges to focus the gain on.

Chip