CCS, how it actually works?

[timf]

Hi All,

Great forum you have here!! Just learning all things tube audio, so discovering your place here is quite timely.

Now, I did some extensive searching on Constant current source information on this site. A great wealth of info came up concerning different design ideas etc. etc.

However, I am looking for some information that will break it right down and tell me why you would use one, and most importantly, how exactly it works, the effects on tube operation, benefits and drawbacks etc.

Any inod out there would be very much appreciated. Thank you :0)

[Shoog]

Others will correct me if I am wrong, but here is my understanding of why a CCS benefits a circuit.
In normal operation the current through a valve varies with the voltage out. This means that you have two shifting variables. By applying a CCS you fix at least one of those variables (the Current), which allows the valves voltage amplification to be more predictable- which equates to been more linear. This also means that the two halves of the output waveform have more equal current rather than one side having more current than the other. This would be especially noticable as large bass passages losing definition and sounding "flabby"- one of the main faults of SE designs.
All in all this adds up to the CCS loaded valve behaving more linearly and with a much reduced distortion. A Constant Current load is the best load in all situations !!!

Hope that helps.

Shoog

[Shoog]

Having written that it occurs to me that the whole situation is entirely more complex.
When we say that a CCS delivers constant current, this is only true for DC. This is good because DC sets the Bias point, which effectively gives us fixed bias. However at AC the CCS delivers a variable current, which complements the variable current of the loaded tube (they effectively enter a PP arrangement at AC). This then effectively delivers a more Constant current to the load. Since the loaded valve is the dominent contributer in terms of voltage output, and since it is operating at a fixed bias, the whole arrangement is very linear. The secondary role of the CCS is to give current to the load as needed, it has very little effect on the output voltage.

With a resistive load, as the current through the resistor varies, its voltage drop varies, which shifts the valves operating point, which in turn varies the current through the valve. This tends to intoduce none linearatites (ie distortion). For most situations this will be marginal but say we have a low value for the load resistor it will be more pronounced.

Shoog

[SY]

However at AC the CCS delivers a variable current

I've understood it the opposite way, i.e., the current is as constant as possible over as wide a frequency range as possible. That way. the tube is operating with essentially a horizontal load line. When you look at triode characteristics, it's evident that a horizontal load line avoids the portion where the curves start bunching up, so the distortion minimizes.

Another way to look at it is to consider the triode as being a Thevenin source with a voltage generator equal to mu times Vgk and a source resistance equal to rp. Now, rp varies with current much more so than mu, so running a CCS minimizes changes in rp. And because the CCS has such a high source impedance, the voltage divider formed by rp and the plate load resistor (in this case, close to infinite) reduces to unity.

In a differential amp tail, the AC constant current characteristic ensures balance, since AC current must be equal and opposite in the two plate load resistors.

[Shoog]

SY you are correct about AC current been constant. Forgive my confusion, i'am trying to think this through in my head. Essentally my main point is that the two halves of the waveform have equal current delivery, which is different to the standard SE behaviour where current varies over the whole wave cycle. In standard SE as the valve approaches cut off (max input voltage) current falls to zero. In the CCS situation the current to the load remains constant at cut off because the CCS picks up the slack.

I had one more though.
The reason that the CCS contibutes little to the output voltage waveform is because it functions as a cathode follower with 100% NFB and so it follows the output valves voltage waveform very closely.

Its a complex concept in its details, but essentially its essence is that it makes the output behave more linearly.

A output pentode cannot be loaded with a CCS because it would be making a CCS load a CCS. Any slight inbalance will send it into a meltdown with big instability issues.

Shoog

[janneman]

Originally posted by Shoog
[snip]Its a complex concept in its details, [snip]Shoog

Really?

Originally posted by Shoog
[snip]A output pentode cannot be loaded with a CCS because it would be making a CCS load a CCS. Any slight inbalance will send it into a meltdown with big instability issues.

Shoog

So, how would that work then? Since a CCS is, well, a constant current source, how can two of those cause a meltdown?? This is all very new to me.

Jan Didden

[Shoog]

Not a litural meltdown, but an unstable situation which at the best will take a long while to settle down, and at the worst will never settle down.

Shoog

[janneman]

But what kind of stability issues? You mean high-frequency oscillations? Motorboating? I've never heard about something like that.

Jan Didden

[Sch3mat1c]

Which is why there exists feedback.

Y'all need to sit down with solid state sometime.. variation of 5x in gain in the same part (depending on current and temperature) oughta learn 'ya!

If you fix the plate voltage (as with a voltage follower-biased CCS), you can get gain in the 2000 range from a "Mu-stage" style pentode stage. Heh, even more if you used a negative resistance load (to cancel the pentode's plate resistance: since gain (mu) = Rp * Gm).

Tim

[EC8010]

A CCS as a pentode load works - I used one in a simple voltage regulator. I was expecting minuscule ripple (< 1mV) as a result of the high gain, but the regulator wasn't all that spectacular. I expect I had some leakage somewhere that was causing the 2mV of hum.