Anode Loading Vs. Cathode Loading ?

[valveitude]

Ok, a generic question here, as I get deeper into the "paralysis thru analysis” syndrome.

Based on the experience of the group, what are the general sonic differences between a plate loaded SE and a cathode loaded SE ? I’ve only seen a few cathode loaded designs in spite of how good they look on paper. I’ve also read a review by Lynn Olsen (I think) on a cathode loaded SE a while ago, and he seemed to indicate that it kept the SE magic while not having some of there shortcomings.

Anyhoo…I would really be interested in what you guys think.

Casey

[ilimzn]

It seems they look good on paper until you start designing the driver - which needs to swing the full plate loaded drive plus the plate loaded output voltage... fun, fun, fun

[valveitude]

It seems they look good on paper until you start designing the driver - which needs to swing the full plate loaded drive plus the plate loaded output voltage... fun, fun, fun

Good point..assuming the driver issues are resolved, is there any "sonic" downside with the "follower" output stage ? There seems to be some potential advantages, on paper anyway.

[PRR]

Read CF biasing and load ???s

[valveitude]

Read http://www.diyaudio.com/forums/show...&threadid=60328

Thanx PRR..a very good technical discussion.
In that thread you said...

Yeah, long on good impressions and short on hard numbers. He's happy, I'm happy for him. Can I be happy? Hard to know without more info.

...in reference to the Lizzy CF Power Amp. You sort of hit on my dilemma here. The potential seems to be great for a CF power amp, I was hoping to hear from some builders that have done ‘em, and how they defined the sonic “signature”. It “seems” that if you can overcome the difficulties of driving them (no small task) it would be worth the effort.

Casey

[Sch3mat1c]

Originally posted by valveitude
Ok, a generic question here, as I get deeper into the "paralysis thru analysis” syndrome.

Based on the experience of the group, what are the general sonic differences between a plate loaded SE and a cathode loaded SE ?

Y'mean with transformer drive so they get the same grid-cathode signal, or anode load vs. cathode *follower*?

In the first case, there is exactly NO difference (note any grid current is kept in the transformer loop and doesn't enter the plate-cathode loop). In the second, you need an absolutely massive voltage drive, preventing it from being used by all but the most absurd designers.

Besides, a cathode follower has such low gain due to local negative feedback. You can apply exactly as much NFB, *or more* if you want, with amplifiers of much smaller signal capacity = lower distortion, with loop NFB. It's not quite the same because you have delay in each stage but a good designer knows how to handle this.

Tim

[cerrem]

I don't see a problem with CF amp....
The CF is a current amplifier with close to unity voltage gain...
SO your doing all your voltage gain at the driver stage ...but the CF input grid is not a tough load to drive.... just make sure you get the proper voltage swing....

CHris

[alexistheo]

there is an excellent discussion of the difficulties of cathode-follower output stages over at tubecad

[ilimzn]

And the gain of a CF in this application, power tubes having a low mu, is actually substantially lower than 1 for a triode connection. The problem being, your drive requirements increase by it's reciprocal - which means tensof volts for every .1 off of the theoretical gain of 1. You could, however, use a pentode strapped as a CF, that would indeed bring you very close to 1, at the expense of a bootstrapped or independent screen supply.

[PRR]

> gain of a CF..., power tubes having a low mu, is actually substantially lower than 1 for a triode connection.

You can't derive gain from Mu alone (unless you make some assumption about Rl/Rp.

You can derive it from Rp/Mu, but that's the same thing as 1/Gm. So Gm is the simplest way to look at it.

For a given size cathode and grid precision, Mu and Rp vary all over, in the same direction, but Gm does not vary much.

So for a given load impedance, gain is a function of Gm which is a function of tube general size/class, but for "reasonable" tubes it will not vary much.

Also most of the promised benefits of CF operation go-away very quickly as gain goes down (as load becomes not-large compared to 1/Gm).

It can be convenient to define Rk as 1/Gm.

> use a pentode strapped as a CF, that would indeed bring you very close to 1

Pentode strapped as triode is just a triode. Possibly a better triode, because the main thrust of power-tube work was with pentodes; possibly not so good because a pentode can give low drop with medium G1-G2 Mu and a triode must have low Mu for low drop.

Pentode strapped as pentode does not work a lot better, Gm is actually lower, and driving the G2 to track the cathode can be VERY hard work.

> make sure you get the proper voltage swing....

In practice this problem is MUCH larger than it should be.

Take a triode CF with 300V plate supply. Best power is around Rl=2*Rp. Cathode swing will be about 200V peak. THD at large power will be around 5% divided by the feedback factor. Gain will be about Rl/(Rk+Rl). This will come out about 0.9 for many cases. Distortion of the CF alone is about 5%*0.1= 0.5%, nice. But grid swing will be about 1.1*200Vpk or 220V peak. A resistance-coupled voltage amp sees a light load looking into the CF, but still will make up to 5% THD when swinging peak voltages of 20% of the supply voltage. Or: the supply voltage should be 5 times the peak signal voltage for 5% THD.

So given 220V peak to the CF grid, we want 5*220V= 1,100V driver supply! We get ~0.5% THD in the CF, but ~5% THD in the driver. The "sonic signature" will depend very-much on the driver. We have just shifted our troubles from the output stage to the driver.

> You can apply exactly as much NFB, *or more* if you want, with amplifiers of much smaller signal capacity = lower distortion, with loop NFB.

That's the way I see it. The CF configuration only works for ~100% NFB, and for power stages forces an earlier stage to ABSURD signal levels. With plate-loading, you can choose 50%, 100%, 200% NFB, and the driver does not work significantly harder than the no-feedback amplifier.