Driving EQ from kathode resistor

[b_force]

I was looking at some guitar amp schematics and I discovered that the EQ before the phase splitter is always connected with the kathode resistor.

For example:
http://www.schematicheaven.com/marshallamps/jcm800_lead_mstvol_50w_2204.pdf

Do you look at some audio amplifier schematics than you see that the anode resistor is used.

Are there reasons to use the kathode resistor?

 

[leadbelly]

If you search on the correct terms you will find a tremendous amount of information.

In the schematic you linked to, the tone stack is being driven by a cathode follower.

In amp designs where the tone stack is driven off of the plate of the triode, this stage is called common cathode or grounded cathode.

Basically, the cathode follower is a sort of current booster, to better isolate the previous stage from the signal loss of the tone stack.

 

[b_force]

Ok, that is very clear.

But when you look at this schematic:
http://www.schematicheaven.com/newamps/laney_chrome-o-zone.pdf

You see that the first channel (after V3A) is driven by the anode.

Can I conclude that a cathode follower (or common cathode) is used when there are more stages (like a drive/gain channel) before the stage for the EQ?

 

[leadbelly]

Not exactly, but you can at least conclude that a cathode follower is inappropriate as the very first stage of a guitar amp.

A cathode follower has less than unity gain [gain = mu/(1+mu)], and gain is a very important ingredient in a guitar amp. Gain is very necessary in the first stage to get the signal higher for a good signal to noise ratio (SNR).

 

[b_force]

Thank you

I've searched with google and I found a good website about those thinks:

http://www.freewebs.com/valvewizard/index.html

A book is also available!

 

[jjman]

Here's a guitar amp that uses both. The only "rule" that I surmise is that the cathode follower type tone stack driver won't ever be the 1st stage.

http://www.schematicheaven.com/fenderamps/bassman_6g6b_schem.pdf

 

[mashaffer]

In the first stage(s) of a guitar amp you want voltage gain to get the signal "stronger" and (very important) to induce distortion. In the stage immediately before the tone stack you are trying to drive a lot of passive components and are likely to need a fair amount of current drive. A common cathode stage gives good voltage gain but not the best current supply unless extreme tubes are used. The Cathode follower provides no voltage gain (actually a slight loss) but much better current.

So horses for courses. You use the voltage amp when you want voltage gain and the current amp when you want current gain.

mike

 

[b_force]

No, because you will have some GAIN in the first stage I think

The other reason to use a cathode follower, is to make also a DC Coupled Cathode Follower at the same time. This is very often used by guitar amplifiers to introduce even more harmonics.

 

[b_force]

I think it would be also possible to drive a SE EL34/6L6 with a cathode follower?
Lots of current and just an simple voltage follower

 

[leadbelly]

I think it would be also possible to drive a SE EL34/6L6 with a cathode follower?
Lots of current and just an simple voltage follower

Yes, but what's your point? Also, EL34 and 6L6 have significantly different drive requirements.

 

[b_force]

Yes, but what's your point? Also, EL34 and 6L6 have significantly different drive requirements.

Not really a point, only a thought