The Dark Arts of Direct Coupled Cathode Follower Drivers!

[redelephant]

Yo mAmA!

[Merlinb]

Still, just not the answer we were looking for... The question was : why not run your cathode follower to source 10 ma instead of 2 ma ?

Because the output valves run class AB, so you only need a little negative swing to drive them to cutoff, but lots of positive swing if you want to drive the power grids positive (at which point you need plenty of current available from the CF). Hence the off-centre biasing of the CFs.

[priesty]

Because the output valves run class AB, so you only need a little negative swing to drive them to cutoff, but lots of positive swing if you want to drive the power grids positive (at which point you need plenty of current available from the CF). Hence the off-centre biasing of the CFs.

Merlin, you have hit the nail on the head there. It's kind of obvious when you read your post about three times!

[mooreamps]

It's because the driving impedance is significantly lower sourcing 10 ma verses 2 ma.

-g

[kg]

It's because the driving impedance is significantly lower sourcing 10 ma verses 2 ma.

-g

this is true.

however, this is also where it gets interesting.

the grid hanging off of the CF's cathode is VERY high impedance, most of the time... ie, when it's not positive.

even with a very tiny trickle of CF current, it is still more than beefy enough to drive those grids. orders of magnitude better than a plate loaded driver for example.

WHEN the grids draw current, and the cathode of the CF has to source it, then the CF is inherenly going to drop its cathode impedance. why? because it's passing more cathode current, and cathode current is proportional to gm, and gm is inversely proportional to cathode impedance.

so the more grid current flows, the lower impedance the followers become. the more you ask it to work, the harder it can work.

is cathode current going to become 10x what it was before? well.. it MIGHT! depends on your circuit values.

it's not often something works out so nicely...

[J M Fahey]

When I read these threads I feel so HAPPY for having stayed on the SS side of The Force !!
It's so *easy* to have whatever power you want !!! The Sky is the limit.
Anyway, to each his own.
May Yoda bless you.

EDIT: what I find *very* funny is that to get high power you inject *current* into your grids (as if they were Bipolar) , while I have resorted to drive with *voltage* my MosFets, asif they were tubes !!
Oh well, maybe it's a Cosmic joke or something.
I'm sure we'll meet in the middle someday.

[mooreamps]

I was thinking of driving tone stacks, not power tubes.. But if driving power tubes, it allows you to drop the input impedance on the front end of you power amp, and thus drop the noise floor of your PA.... :| Maybe something to think about.... Also, the input grid resistor should keep the grid current in check if and when you drive the grid positive, should it not ?
From what, I've seen values anywhere from 1.5 K to 50K series resistor on the input of a 6L6, is this not so ?




-g

[kg]

EDIT: what I find *very* funny is that to get high power you inject *current* into your grids (as if they were Bipolar) , while I have resorted to drive with *voltage* my MosFets, asif they were tubes !!

just don't walk across a carpeted floor on a dry day wearing rubber soled shoes and touch one of your gates!

[kg]

I was thinking of driving tone stacks, not power tubes.. But if driving power tubes, it allows you to drop the input impedance on the front end of you power amp, and thus drop the noise floor of your PA.... :| Maybe something to think about.... Also, the input grid resistor should keep the grid current in check if and when you drive the grid positive, should it not ?
From what, I've seen values anywhere from 1.5 K to 50K series resistor on the input of a 6L6, is this not so ?




-g

gary, in a tone stack config, or somewhere else that i wanted as low a driving Z as possibe (like a line output) i would absolutely run them hotter. they will perform better passing some good cathode current.

ken

[Merlinb]

Also, the input grid resistor should keep the grid current in check if and when you drive the grid positive, should it not ? From what, I've seen values anywhere from 1.5 K to 50K series resistor on the input of a 6L6, is this not so ?

Good point. If you look at the whole SVT power amp:
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif

It has big grid stoppers, so the grids aren't ever going to be driven very positive (and there is little point trying to run in class AB2 with pentodes anyway). The use of a CF driver stage is perhaps unecessary in this instance, and the low quiescent current may be to reduce the loading on the PA screen-grid power supply, since the CF is running off that.

[kg]

(and there is little point trying to run in class AB2 with pentodes anyway).

i've seen you say that a couple of times already, but i assure you i can get more power out of an amp with a grid current capable driver vs. one that cannot source it. pentodes/ultra linear/triode included.

[Merlinb]

i've seen you say that a couple of times already, but i assure you i can get more power out of an amp with a grid current capable driver vs. one that cannot source it. pentodes/ultra linear/triode included.

Triode yes, but with pentodes there is only a couple extra watts to be squeezed out, assuming the load impedance is 'normal'.

[ThermionicScott]

Triode yes, but with pentodes there is only a couple extra watts to be squeezed out, assuming the load impedance is 'normal'.

I think that last part is the critical one. Without changing the load, yeah, you can't go much further in AB2. It's when you reduce the load (pointing the loadline skyward) that you can realize a lot more power.

But you guys knew that already.

- Scott

[kg]

there's also the point of not having any bias shift/grid block. i haven't really played an off the shelf amp yet that didn't suffer from that at some point or another.

that alone is worth the price of admission for me! (and incidentally is an advantage of the SVT design even if those stoppers are huge...)

[kg]

also, i found a copy of a post i made to the "old ampage" eons ago about the SVT circuit and my own thoughts on it:

i have a grand statement to make…

the ampeg svt, the mighty of mighties does NOT run in class ab2. in fact, i maintain that very little if ANY grid current flows in this design. it’s very simple if you look at the schematic:

the grid stoppers on the output tubes, for both the 6550 and 6146, are on the order of 47k.

that’s a mighty high value if you expect to pull current. for example, say you expect to run 5mA Ig1, that means the voltage drop across Rg1 will be .005 * 47000, or 235V. clearly, you are no longer pulling 5mA Ig1. you probably aren’t pulling any Ig1 at all.

now, from a first-hand experience i know the importance of isolating the respective g1 circuits in a parallel output stage. yesterday i rigged up one channel of the BAGA with small 80uH 2R DCR chokes in parallel with the existing 3K grid stoppers. the hope was to preserve isolation at RF, but lower the DCR which was counter productive to grid current flow. (that means MO’ POWA! it’s a mantra of mine.)

so after tweaking away, i fired it up and it worked for a bit, but then the amp broke in to fierce oscillation, and i dove for the off switch. upon opening the chassis (today) i realized that the small chokes had shorted out, no doubt from exceeding their max current rating. they had essentially turned into dead shorts, of 0.1R DCR and ~100nH of L. there was a small subtle bubble on the side of each burnt choke, and three of them were toasted. that’s what lead to the oscillations, essentially because when they shorted i then had two g1′s directly connected together. bad news. luckily tubes are tough, especially kt90′s, and i was fast on the off switch.

so out came the small chokes. i think the idea is sound, but the current rating of the little buggers is too low (it’s something like 20-30mA, iirc) and the consequences of their shorting out is very high.

the current value of grid stoppers, which were simply paralleled by the chokes, is 3k. i did some quick calculations… assuming a (not entirely unreasonable) grid current of 5mA, that STILL drops 15V, which is enough to knock it out of grid current entirely. (really, it never ENTERS grid current, since as soon as it starts to flow the positive excursion is limited by the series impedance of 3k.) if you want to get into Ig1 with that kind of series R, then you’ve got to increase input Vac, but more on that later.

next, i went back and perused “an approach to audio frequency amplifier design” published by GEC and distributed by audio amateur press. at the back of the book is a description of a PPP kt88 amp good for about 450W. to make a long story short, i was looking to see what value of stopper they used to isolate g1′s. GEC used 15k–that’s high enough to stop Ig1 too, just like the SVT. they were driving their outputs with a interstage tranny coupled cathode follower, so low drive Z was not an issue in terms of stabilizing the output stage; they still relied on the high (at least by my standards) value of grid stopper.

soooo, i will have to be content with the 3k stoppers, which seem a good compromise. the amp was stable before, so i know they work at isolating the tubes from one another. maybe at some later date i will pursue the small chokes–or maybe not.

what i then decided to do was what i had been wanting to do for a while: pentode-wiring the cathode followers. clearly, if the CF’s are triodes, then as the grid swings high the CF will run out of steam, since Vak decreases. (in fact, i actually pushed the CF’s into grid current THEMSELVES, by attempting to pull the cathodes too high. i could tell this was happening by the all-too-familiar blocking distortion that ensued.) pentode CF’s ain’t gonna do this, since the screens will soar just as high as the cathodes do, maintaining a fixed Vs. the actual plate voltage, as pentodes, doesn’t matter much at all. they’ve also got a higher gain (lower loss) of signal amplitude when compared to triode CF’s. so that’s what i did.

pentode connecting a CF is a bit of tricky business, since you’ve got to bypass the cathode to the screen. during power up and power down you can have some strange voltages, so a non-polar cap is best. the screen of a pentode is akin to the plate of a triode, and as such it has a low “plate” impedance. the quick’n'dirty method of determining the screen dynamic impedance is to take the screen mu (which in my case using sv83′s is approximately 25) and divide it by the gm of the tube (in this case 15,000uS, or .015S). the result is 1.6K. the cap from cathode to screen must be sized with this dynamic impedance in mind… in PARALLEL with the “feed” resistor which supplies the screen’s current. in my case, a 2uF 400VDC plastic cap did nicely, going down to about 50Hz. as the frequency drops below that the tube will gradually revert to triode operation.

while in there, i also paralleled each cap with a 200V zener diode, limiting the max Vs to 200V. this was done with an eye to the data sheets of the sv83. i suppose i could have gotten away with a lower voltage cap, but that’s what i had on hand in a quantity of four.

when testing the performance of the new driver stage, i monitored the output of the CF on the scope while gradually increasing signal. peaks and valleys started to flatten at around 250VACpk. that means for a nominal bias voltage of ~-80VDC, i can expect to pull the CATHODES about 45V positive. that’s enough to dump about 15mA of Ig1.

oh yeah, i guess the moral of the story is that if you really want to run grid current, do it WITHOUT paralleled output tubes. that way one could safely reduce/eliminate the stoppers and go crazy. i know there is SOME benefit to the DCR of a grid stopper (namely limiting grid dissipation, being easier on the tube), but i say crank the shit out of ‘em.

one thing i’d still like to do is reroute some grounds, and move towards more of a star scheme. (it can never be quiet enough–dynamic range is indeed a battle fought on both ends of the spectrum.)

after that, i gotta button it up, lug the bastard back out to the garage, and shake some shit up.

kg