More Tube Learning (for me) Cathode followers

[athos56]

Ok todays lesson was tube buffers. I tried to work out a simple Cathode Follower and a White Cathode Follower. Here are my results. I still haven't figured out what the cathode bypass cap would be on the WCF.... The B+ and rk and tube choice for the regular cathode follower were lifted from one tubecad page and the idea to replace the cathode resistor with a choke was lifted from another. I selected a choke with a dcr equal to the recommended rk. So I believe the CF draws 10ma, not sure about the WCF. My hope would be that one would be worth building to as a buffer for a CDP and later as a active volume control for a tube amp, with the addition of a 1m dual ganged audio pot replacing the 1m resistor at the input. As usual I'm avoiding CCS and the like, I worked out a low noise PSU as well. So here they are, is one of them usable?

[athos56]

Here it is...

[kenpeter]

The plate resistor of 140R might have been correct, if Mu was 84.
Since your 5687's Mu is only 16, value 3K9 may be closer to right.

The top plate will multiply the input by Mu 16.
The 3K9 plate and 1M grid resistor divide by 256 (or 16^2).
The lower plate multiples again by 16, thus following the input.

I am not sure, the cathode bypass cap to GND might be large
enough to temp one to abuse an electrolytic for that purpose...

I'd be tempted to divide the output by 16 and regeneratively
(positive feedback) to that cathode, creating a virtual bypass.
The cap needed to bypass this way would be smaller by factor
of 15. The bypass (feedback) resistor needed would be equal
cathode bias resistor *15 or 4K3.

Since the plate resistor was wrong (possibly scaled for 12AX7?)
I'd doublecheck that your cathode bias resistor isn't similarly off.
If it changes, so does the value needed for bypass.

[kenpeter]

Roughly like this. Though I still havn't done anything to check
if the 287R suggested cathode bias was optimum for 5687.

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Occours to me that degeneration of the cathode resistor
reduces the lower triode's effective Mu by 1, would be
easy to compensate for that small difference in the upper
plate resistor value, and use no lower cathode bypass at
all. Less parts for the same result is almost always better.

And occours the me the upper Triode's plate is also Mu 15,
and the missing "16th Mu" appearing at the cathode as
the output of the follower. So my 3K9 was wrong anyway.

4K46 (4K7 might be close enough) at the upper plate and
eliminate the lower bypass altogether. Do retain the bias!

[athos56]

Your right about the values, I chopped this out of a circuit that had a gain stage in front of it using a 12AU7. I'm still in the process of learning how to calculate rp and rk for the WCF.

I had an easier time wrapping my head around the simple cathode follower, I just thought I'd try and learn the white cathode follower if I was going to use a twin triode.

I'll try and figure out the rest in the morning!

I see you posted an example while I first wrote this, Thanks! I'll take a look when I'm not falling asleep on the keyboard

[kenpeter]

From the Raytheon CK5687WA spec sheet, I am thinking that 287R
gives you about 16mA of cathode current and slightly more than 4V
of negative bias (assuming 120V per plate). But this is only half the
5687' recommended design center 36mA for that plate voltage.

Raytheon design center values seem to suggest maybe 55R5 for
36ma and cathode at 2V of negative bias. Maybe that doesn't
give us enough head room for our input signal? I dunno, its a
follower for cripesake...

Maybe next spin I fudge for 25ma idle current? And specs seem
to indicate slightly higher Mu, 16~18 depending on the loadline.
Like weevils in my Cheerios, Devil's always in the details...

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So I recalculate 120R for cathode and 3K1 for the plate, then
realize that (plus the cathode resistor) is gonna eat up more
than 80V! Not a problem that much drop, but does throw all
my earlier assumption about quiescent operating point back
into the drink. Up the power supply by another 80V, doesn't
seem necessary, and might hazard the maximum standoff of
the filament. I just need to keep playing with the resistors.

[kenpeter]

I am having difficulty getting idle current much above 15mA with
voltages these filaments will withstand. To run this tube full bore
would want higher power supply, and elevated filament voltage.

If you don't care running at only 1/4 of its true capability, then
15mA at 250V B+ with grounded filaments is certainly do-able...

I have to keep tellin myself, its only a follower. No huge swing...
Transconductance benefits of 5687 will apply to the output even
if I don't run both plates to the limits of dissipation.

[athos56]

I seem to remember reading somewhere that a CDP only need around 5v swing MAX, worst case. I'm still digesting your posts .

And I was also thinking that I could reference the heaters to a portion of the B+ if I needed so I put the parts on the spread sheet.

[hemgjord]

Just an observation. Raytheon datasheet states a max gird circuit resistance of 0.1M not 1M. I never really figured out if this is that crucial in CF application. Anyone?


/Olof

[kenpeter]

If it ain't broke, don't fix it. Why can't I leave well enough alone?
Oh the misery... Well, it don't look much like it started anymore.
I am hating the parts count, I am not real pleased with where
this circuit variant has gone way too far the path of complexity.

I unstacked the totem to get more plate voltage (and current).
And also to keep the filament stand-off voltage within tolerance.
I am burning both candles as hot as they will go, at 3.75W ea.

Then I stated noticing a bunch of goofy things as I mucked with
the plate and cathode resistor, finally concluding that an equal
split (cathodyne) was perhaps the best compromise.

Then I notice I can opt to get an inverted output by tapping the
other plate and cathode... Its a cathodyne now too, after all?

Then noticing that impedance at the cathodes are not equal the
impedance of the plates, and the coupling cap might be kinda big.
So I made em equal by adding a series resistor to the cathodes.
That also helped resolve an issue about cathode feedback that
was buggin me. It might have been better just use the big caps
and screw making the push and pull impedances be equal?

I think the impedance at either output sum is close to 1K?
And enough headroom to follow and/or split an input signal
maybe as large as 80V Peak to Peak?