Amps that sport more than one cathode follower stage

[daz]

Are there any? Schematic?

[MWJB]

Ampeg SVT, a cathode follower stage in the preamp, then a cathodyne inverter feeding cathode followers feeding each bank of power tubes.

There may be amps that have more than one cathode follower stage before the PI...but I doubt that there's many. A cathode follower stage in a preamp (like in a tweed Fender/classic Marshall) can often introduce a slightly "raspy" character...not a bad thing if you want tweedy crunch, but then again probably not a thing that you would want to overdo? You might still want the option of a more scooped mid tone, which might not be achievable with lots of CF stages. Plus, from a manufacturer's point of view, it might be seen as a waste of triode stages.

Fender used it to stop the tone controls from loading down the circuit (like they do in a BF amp), it also drops gain compared to a plate fed tone stack (compare to say a Blues Deville).

[Chuck H]

+1

A cathode follower does add a particular character (usually cutoff only clipping) but has no gain. Actually a small loss in gain. Fender (and subsequently Marshall) did use a CF to decrease losses through the tone stack. But IMHO for modern design it's better just to re-amplify POST tone stack. You ultimately end up with a higher gain than if you try to decrease losses PRE tonestack and you use the same number of triode stages. Further, many new MFG preamp tubes are weak in the cathode. I haven't had a problem yet but I read all the time where someone has problems with modern preamp tubes in vintage CF stages.

Soldano, and the many similar designs and clones, as well as TrainWreck use a cold biased stage to create cutoff only clipping. This method allows for some gain increase too without loss and avoids the problem of weak cathodes in modern tubes. And people seem to really like those amps so there must be something to a cutoff only clipping stage that sounds good. I just don't think that adding extra cathode followers is the way to do it. FWIW the Soldano design uses a cold biased stage AND a cathode follower so there are actually two cutoff only clipping stages. So you may be onto something with your idea, I just don't think adding cathode followers is the way to do it.

Chuck

[cbarrow7625]

Marshall 30th anniversary lead channel has 3 CF's after the 2nd, 3rd and 4th gain stages. Major crunchy hi gain monster.

[daz]

I was just wondering after reading something about it. Anyways, thanks for the answers.

By the way, this is totally unrelated but instead of starting another thread i'll just ask here. Do amps that are cathode biased do better with less filtering and those with fixed bias do better with a lot? At least in respect to the PA? I ask because i tried big filters in my main amp which was cathode biased and it became rather thin and sterile. I recently converted it to fixed and now when going to a bigger can for the screens and plates it gained a lot of very rich and nice harmonic content. That was only going from 32/32 to 50/50. So i'm wondering if 100/100 would be even better. By the way, big filters in the pre stages never seems to sound good either CB or fixed.

[MWJB]

Filtering isn't really relevant to whether an amp is cathode or fixed bias. Was there much difference in idle current? A good reservoir may well be helping in fixed, when large current demands are made (plate current won't rise so much in cathode bias). Screens should be fine with 50uf max, 100uf at the mains won't hurt.

[clyde1]

+1


Soldano, and the many similar designs and clones, as well as TrainWreck use a cold biased stage to create cutoff only clipping.

Chuck

By cold-biased, do you mean a larger cathode resistor? You're not referring to CF's here are you?

[daz]

Filtering isn't really relevant to whether an amp is cathode or fixed bias. Was there much difference in idle current? A good reservoir may well be helping in fixed, when large current demands are made (plate current won't rise so much in cathode bias). Screens should be fine with 50uf max, 100uf at the mains won't hurt.

Thanks. I didn't even realize that i should have checked bias with the cap change !
Last nite i found that as good as it sounded, when i cranked it up to about stage volume it got painfully bright in the small hard surfaced room it's in. Thats an exaggeration of what i find with most any high gain amp i've had when turned up especially in this room, but not to that degree. I put the 32/32 back in and that tamed it a lot. Seems odd, no? Well, i will further experiment tonite and try checking the bias before and after i install the 50/50. That would be great if that was the reason because i really loved the tone with that 50/50 in there.

[MWJB]

Sorry Daz, I mean't was ther much different in idle current between the old cathode bias & new fixed circuit.

Upping the filtering will often bump up the B+ slightly, which has a knock on effect on bias...but only by nominal amounts regarding the values you have tried.

[daz]

i don't remember. I set the cathode bias to whatever was considered proper given the voltages and once right didn't check or touch it since long ago. But i set fixed to 70% and cathode as i recall fairly close to max.