Good enough CFs

[mashaffer]

As I understand it two major features determine the performance of a particular tube in CF circuits.

1. gm: High gm leads to low Zout (about 1/gm if I recall correctly).

2. current capacity: High current capability helps to drive capacitive load and any input current requirements of the following stage.

Is that approximately correct?

Ideally one wants the lowest Zout and highest possible drive current but the question is how do we determine what is really good enough for the given application.

For example a tube with a gm of 1200 umhos running at 1mA would be considered, I believe, a very poor choice in a full range preamp that would be driving an unknown power amp.

However the Zout would be less than 1kohms which would generally be enough to deliver most of its signal voltage to even a worst case SS power amp with 10 to 20k input impedance. The problem would seem to be the low current capability which would most likely cause significant high frequency losses.

But suppose that the application is a subwoofer output where nothing below 120Hz or so would be needed. In this case is it not possible that the given tube would provide adequate performance?

So the overall question is how do we calculate the necessary current and transconductance capabilities for adequate performance in a given application.

So you ask, why bother... just use a better tube? The reason is efficient use of available materials and intellectual satisfaction.

mike

[Miles Prower]

Originally posted by mashaffer
As I understand it two major features determine the performance of a particular tube in CF circuits.

1. gm: High gm leads to low Zout (about 1/gm if I recall correctly).

2. current capacity: High current capability helps to drive capacitive load and any input current requirements of the following stage.

Is that approximately correct?

Pretty much. rk= (Rp + rp)/(1 + u) (Where Rp= 0 for the most part with CFs)

Ideally one wants the lowest Zout and highest possible drive current but the question is how do we determine what is really good enough for the given application.

For example a tube with a gm of 1200 umhos running at 1mA would be considered, I believe, a very poor choice in a full range preamp that would be driving an unknown power amp.

However the Zout would be less than 1kohms which would generally be enough to deliver most of its signal voltage to even a worst case SS power amp with 10 to 20k input impedance. The problem would seem to be the low current capability which would most likely cause significant high frequency losses.

The cathode follower is always at its best when you need a Lo-Z source impedance for a Hi-Z load. If it's as you describe, gm= 1.2mA/V and Ipq= 1.0mA, it'll promptly roll over and die for any significant load impedance. When using a follower driver, after determining the load current for whatever purpose, then apply the "Rule of Five" from SS practice (States that the collector current of the driving stage should be five or more times the base current of the driven stage.) and make Ipq=> 5 X Il. As for max current from the CF, it's approximately: Imax~= Vpp / rp. This serves as a rough guesstimate as to what you can expect from it.

As for what tubes make the best CFs, these will usually be the medium-u triodes (6J5, 6SN7, 6C4, 12AU7, 12BH7) or possibly some of the higher-u types with decent current handling (12AT7, 6BQ7A, 6BK7). The low current types like the 6SF5, 12AX7, 6AV6, 6SL7 seldom are good choices. If you need higher voltage gain, then small signal pentodes, run as pentodes, make excellent followers even if they are a bit more complex.

[SY]

With all due modesty, harrumph, harrumph, I detailed a lot about cathode followers and the necessary calculations that you're looking for here.

[Gordy]

But if you really, really wanted to experiment with a Cathode Follower driving a low impedance (30 Ohm) headphone where would you start?
6AS7?
Others?

[SY]

For what voltage swing? A 6AS7 will have a gm of more-or-less 6mA/V, which translates to an output Z of about 160 ohms. Gain will be about 0.6. Not promising.

If the load really is as low as 30 ohms, I'd be inclined to use an output transformer or a solid state output.

[mashaffer]

Thanks guys, I think we are on the right track.

SY;

If we assume a 2 volt input sensitivity (that’s RMS; peak will be about 2.8V) for our power amp, the signal current needed to drive the 1000pF load capacitance to the full voltage while maintaining the 90kHz bandwidth is about i = 1.6mA.

This is precisely what I want to calculate. Lets say the subwoofer amplifier is going to have a 3V peak maximum signal at 10k input impedance with normal input and cable capacitance. How do we calculate drive current for 3kHz bandwidth (just to give us a little wiggle room)? Do we have to go back to the definition of capacitance and calculate charging time at particular currents?

mike

[SY]

That's exactly it. But... you'll have to have an ENORMOUS capacitance and/or a pathological source impedance to have a problem with that sort of limited bandwidth. Do it as an exercise anyway, but you'll find that with any reasonable choice of tube and interconnect, your bandwidth will be grossly better than you need.

[mashaffer]

Thanks SY. BTW I look forward to seeing your website at full strength. It looks interesting.

From Slew Rate Explained

an equation is developed for the current requirements to avoid slew rate limitating. If I understand this all correctly their equation should provide what we need to know.

I = C * 2 * pi * F * Vpeak

They got this taking into account the definition of slew rate and capacitor charging current.

Putting in our numbers of .001uf input capacitance, 3kHz max frequency and 3V peak input signal I get 56.5uA. Does that look about right? If so it seem that just about any small signal triode should do fine regardless of reputation for this specific application. Is that right?

mike

[SY]

From a bandwidth (or slew rate) basis, yes, almost anything will work. Now the next question to attend to is distortion- even if (say) a 12AX7 can slew fast enough, a 10k load will be a killer.

[mashaffer]

I used the 10k example as a worst case estimate. In any case I have picked up a bunch of NOS and used tubes so maybe there might be something in there.

6DT6 pentode
6BZ6 pentode
6GM6 pentode
6JC6 sharp-cutoff pentode
6EW6 pentode

6CB6A pentode/triode
6CG8A pentode/triode
6GH8A pentode/triode
6EB8 pentode/triode
6BL8 pentode/triode
6U8 pentode/triode

I weeded out the obviously inappropriate ones from the list like damper diodes.

I plan on using a pair of 6N1Ps for the 60Hz to 200Hz bandpass channels. If I end up buying something for the sub out filter would you suggest one of the same, 12AT7, 6922 or something else?

To determine the distortion would you use the 10k load line on the standard plate curves (like you were designing a CC gain stage)?

mike