Constant Current Sources in Tube Amps

[runeight]

The regulator thread generated this post. Once again, I realize that most of the folks on this list understand what CCSs do for tube circuits (at least some of them). So, I thought that if you all would indulge me once again, we might talk about it some.

Grounded Cathode

To start we will use the basic grounded cathode amplifier. This diagram shows two identical gain stages except that the one on the right is loaded with a perfect constant current source. I'm going to look at the gain, distortion and frequency response of both of these and compare them. Once again, I will use PSpice for the simulator and the trusty old 6DJ8 for the tube.



First, we can look at the gain. The tubes are driven by a 0.1v 1KHz sine wave and Ca=Cb=10u. Here is the output from points A (green) and B (red):



Notice that the CCS loaded device has more gain, about 25 for A and 30 for B. This is helpful, but not that important.

To look at the distortion, we'll take the Fourier transform and look at the second harmonic at 2KHz. Here it is, blown up:



Notice that the 2KHz amplitude for the non-CCS amplifier is about 5mV, but for the CCS loaded amp it is 2.5mV. This reduction in distortion can be of great benefit. When compared with the increase in gain the relative distortions are: 0.2% for A and 0.08% for B. Small numbers to be sure, but in the world of audio perfection, well . . . . .

Why is this so? Without dragging out the plate curves, this change simply comes from the fact that in the CCS loaded amp the load line is completely horizontal (minimizing distorion), whereas in the non-CCS stage, the load line has a slope. I know that you all know this anyway.

Perhaps, more important, is the frequency response curves. Here we will sweep the input from 10Hz to 1MHz. Again, Ca=Cb=10u. Plots are in DB of gain. Here is the comparison:



Notice that the CCS loaded amp is very flat all the way to 10Hz. Why is this so? Because, as all the better circuit guys are screaming to me, if the current is constant through the tube, then the bypass capacitor is essentially out of the circuit and, therefore, does not limit the low frequency response. In fact, if the CCS device really were perfect, we could eliminate Ck altogether.

The only way to approach this response for the non-CCS stage is to put C=1000u. Still, you won't get there and you've got another big ugly electrolytic in the circuit.

However, the goodness of this result also depends on the response curves of the actual CCS device. Since it will be made of real components, it won't have a perfect response and, hence, any real circuit will deviate from this result. How much, you may ask? Well, that's the subject of the next post.

[runeight]

Tube CCS

Will a tube CCS really be fast (or slow) enough? Here's a fairly typical CCS tube load for our previous grounded cathode amp.



This circuit draws the same idle current as above. I am setting Ck=10u and Cc=470n. The gain of this configuration is nearly identical to the perfect CCS above. Here is the three-way distortion comparson:



The new blue curve is the Tube CCS stage. Its distorion is identical (at 2KHz) to the perfect CCS stage.

What about the response? Here is the three way comparison:



Notice that the Tube CCS blue curve is very close to the perfect response curve. This graph shows just the perfect and Tube CCS response comparison:



The Tube CCS stage is only 0.2db down at 10Hz (with a 10u Ck). I would hope that this would be unoticeable to all but the most discriminating ears.

So, if you want to go tubes all the way, you can load your grounded cathode amp with its mating tube ccs and you should get very good results. Of course, the price for this is an additional 100V or so on the PS (from 200 to 300 volts).

I've looked at CCS loaded cathode follwers (at least for a few configurations) and there seems to be no significant effect from the CCS. But, real circuits may be different.

A further question is, does a CCS help a circuit that has no bypass cap? For example, does it help a common cathode amplifier? Next post.

[runeight]

Say, I made an error on the CCS effect on CF stages. I'll post that too.

[Sch3mat1c]

Thanks for the review. Also try load capability drawing output from the top tube's cathode, and using a pentode instead.

Speaking of pentodes, I wonder what output would look like with a pentode as the amp! Hmmm...

Tim

[Bas Horneman]

Hi Runeight,

Love your postings!!!

From an amateur's point of view I am somewhat suprised however by the performance of the triode CCS...

Alan Kimmel says that a triode cannot be considered a true CCS since "because it hasn't enough gain to respond adequately to the small voltage changes which occur across Rk2"

http://home.zonnet.nl/horneman/ml/akimmel.htm

Regards,
Bas

[Bas Horneman]

Another thing I would be interested in is how could the shunt capacitance be lowered without a cascoded CCS? Is that something you can easily measure/simulate.

From Gary Pimm's pages...
http://home.pacifier.com/~gpimm/Acti...t_control.html

Another poor performance aspect of single mosfet CCS’s is high shunt capacitance. Mosfets look like great parts until you take into account their high capacitance issues. I suspect that the shunt capacitance is the source of most of the sound coloration’s that make CCS’s sound different.

The use of a cascode circuit will greatly reduce the shunt capacitance and improve the DC performance of a CCS.

As a diy'er I was thinking...how about just using a 1k resistor or so? Would that "hide" the shunt capacitance of the mosfet CCS??? In the same way that you can hide capacitance from a tube rectifier with a choke or resistor???

Cheers,
Bas

[EC8010]

Hello Runeight, audio runs on dBs. It produces slopes of 6dB/octave etc, and humans respond to sound logarithmically. We just can't get enough dBs...

Although constant input voltage is fine when looking for changes to gain, if you want to look at how a CCS changes distortion, it's best to hold the output voltage constant.

[SY]

And, of course, you have to look at loading from the next stage CCS circuits are particularly vulnerable to loading effects.

[Sch3mat1c]

...Hence why you'd want to look at the output from the top tube's cathode.

Tim

[fdegrove]

Hi,

Hence why you'd want to look at the output from the top tube's cathode.

And...surprise, surprise...I'd like to see a penthode as the top tube, not just to make pentahodos Tim happy.

Cheers,