CF vs plate fed tone stack

[iknowjohnny]

Would you say a plate fed tone stack is less compressed and more dynamic usually? I've gone back and fourth with my amp (similar to a jcm) since i first tried it w/o a CF. I like it both ways but it seems to me like the tone is a bit more natural and less compressed when i bypass the CF. Makes me wonder why they use a CF in the first place. I've heard the lossless argument, but why is it i see no loss of volume when i bypass the CF? Anyways, instead of doing this every few months when i start wondering if i still like it best however it is at that moment, i think i'm going to put a switch in to bypass it.

[RobBozic]

One of the new Mesa Boogie amps has a switch to go between plate fed & CF.

I don't know the name of the amp but if you google mesa boogie schematics & look through the current amps you'll see the way they implement the switch.

Rob

[Cliff Schecht]

The plate-driven stage that was being buffered by the CF is now being loaded down by the tonestack. Before the plate was seeing a high impedance from the CF input which it likes a lot. Now that it's feeding a much lower impedance, it has a harder time staying linear throughout it's full swing because of the lower load. This may account for the increased dynamics, you are getting slightly less headroom and clip easier than before. The CF also adds it's own almost completely even-order distortion which has a different sound than an unbuffered TS.

The other thing that happens when you remove the CF is the TS impedance is going to shift down. This is because instead of being driven by a more ideal low impedance source (the CF), it's now being driven by a much higher output resistance which shifts the filters overall response down a bit making for a typically darker amp.

[iknowjohnny]

It doesn't sound like it clips more tho. In fact it seems less. But I think theres a bit wider range of clean to distortion with the guitar volume than b4.

[iknowjohnny]

One of the new Mesa Boogie amps has a switch to go between plate fed & CF.

I don't know the name of the amp but if you google mesa boogie schematics & look through the current amps you'll see the way they implement the switch.

Rob

Just found it...the maverick. Surprised to find they did it exactly the way i did. I figured there would be other switched changed to accommodate the different circuit.

[iknowjohnny]

I have a question. First of all, i was wrong when i said i did it the same as the boogie maverick. They remove the input of the tone stack from the cathode of the CF stage along with the 100k cathode resistor. I just removed the wire on the tube from the CF stage and connected it to the plate of the 3rd stage. So the difference between mine in the CF removed configuration that is unlike the boogie is i still have the 100k cathode resistor in place connected to the tone stack. It's no longer connected to the CF cathode, but still connected to the input of the tone stack giving it a 100k ground reference. So i tried lifting that as in the maverick schematic and the tone is lousy like that. Any thoughts on this as to whether i should just leave it like that or change the value or what?

[guitarmike2107]

With the different impedance loading there will be greater loss as a result of having a plate driven tone stack, which will cause the rest of the amp after the tone stack to distort later on comparable settings of the volume knob.

The significant tonal difference comes from the cathode follower, which adds a smoothness and compression to the tone. You can still have a plate driven tone stack at the front of the amp and then have a cathode follower later in the amp to add this effect. It works well.

I suspect that if you used a cathode follower that was not DC coupled then you might not get the same compression effect. I have not tried it though and am not sure I would want to.

[iknowjohnny]

I find them both nice, but if i had to choose, right now i'm leaning towards no CF! I found that the circuit is optimized for the CF because i spent many hours perfecting this amp. But once i started trying different things i'm finding i can make the tone w/o the CF as good or better, tho with a very different feel and tone. Mostly feel. it really gives it a slight fendery quality in tone and even moreso in feel and i'm just loving it. I like it a lot and i fully intend to make it switchable. However, before i do i intend to try a number of changes to optimize the preamp for this new tone. I wouldn't really say the CF adds smoothness and compression. I guess you could say that in a way, but it's more like it changes those qualities to different versions of them more than more or less of them. As to a plate driven TS near the beginning, i wouldn't want that because i prefer tone shaping after the distortion stages.

The only "issue" for lack of a better word is the tone is a bit more high endy than with the CF. I intend to try a few simple things that i couldn't use with the CF but i think will work w/o it because the lows are tighter. (lake a fender 22uf on one of the cathodes) I may however end up needing a big 4P4T switch or such to add any changes while switching the CF in and out. But even just as is it's well worth making it switchable because both sounds are great yet very different, so it's the closest thing i have found to make a amp switchable in a way that feels like having to different amps at the flick of a switch, especially because of the different feel.

[guitarmike2107]

you may want to read this, Merlin explains the added compression well

http://www.freewebs.com/valvewizard1/dccf.htm

But yes without the CF the feel of the amp can be more “in your face” or raw or whatever words you want to use

[iknowjohnny]

I hear/feel it as more organic, more natural. Like it has more acoustic qualities which i guess speaks to your "raw" description. I'm sure after playing it a while i will probably prefer the CF and keep wanting to go back and fourth, hence the switching.

[jaysg]

what can I add...the signal flow usually seen with a CF is: input -> triode -> 1M volume -> triode + CF -> tone stack. In a blackface circuit, you've got: input -> triode -> tone stack -> 1M volume. As you crank both amps, the former design will overdrive the triode + CF. That can't happen with the blackface architecture unless you overdrive it with a pedal...so apples to oranges.

...so I'm saying, if your triode + CF can't be overdriven, it only alters the frequency response of the tone stack. If it can be overdriven, then the two options should sound different.

[guitarmike2107]

If you have an express type amp you can easily change the 3rd gain stage into a cathode follower to see how a cathode follower affects the overall tone without it driving the tone stack.

of course if you change a single stage in the pre amp from a cold biased stage to a hot biased stage and visa versa it will change the feel of the amp too.

non is better than the other, smooth and compressed might sound good but might not be what you want...

thats the fun of building these ourselves!

[iknowjohnny]

i don't see it as preferring one over the other guys. I see it like strat vs tele. I like both and want both, and in this case i can have both via a simple switch. Of course i will, inevitably complicate things but thats a whole other story.