Long tails and current sources

[dgta]

I seem to recall seeing some proposed circuits where a long tail pair had 2 (equal) current sources, one in each anode.

I can't quite understand how/why this would perform better than a single current source in the (common) cathode. Can someone explain it to me?

[nigelwright7557]

Because the currents would be the same through each valve.

If the valves have slightly different mu's then a single current source would allow different currents through each valve.

[smoking-amp]

Using two separate CCS's in the plate circuits (only) would no longer allow any differential effect between the two tubes. The constant current thru the tail resistor would give a fixed voltage on the tail, just like for grounded cathode stages. This prevents any swing of the tail voltage to re-distribute current between the two tubes.

If the tail also has a CCS, then differential effect is re-enabled, but then no stable plate voltages are possible unless the plate CCS's are changed to gyrator mode.

[dgta]

OK, so the part I'm not grasping is this: it's no longer a differential amp. So there isn't any point to tying the cathodes together. Is there? It's no different than 2 separate amps. You lose the benefits of diff operation. Correct?

[DF96]

No. You still have a differential amp with CCS in both anodes. You get exactly mu/2 gain from either grid to either anode, if the tail resistor is high enough or a CCS too.

If you have CCS in all three places then there is a risk that the anode voltage may settle to a poor quiescent value, unless the CCS are adjustable. This is because you are, in effect, trying to set one current through two CCS in series and this means the voltage at the junction is poorly defined as it depends on their impedances and current difference.

[smoking-amp]

Hmmm, last time I checked Ohm's Law, a constant current (sum of the two plate CCS's) thru a resistor was still a constant voltage. Maybe if the imperfect impedances of the plate CCS's (MegOhms?) are comparable to the tail resistance some differential effect would occur. A CCS tail with gyrator plate loads will work in differential.

[dgta]

Originally Posted by DF96

No. You still have a differential amp with CCS in both anodes.

I don't see how. Let's say each source is 2mA. The tail current is 4mA fixed. If the cathode resistor is 1k, both cathodes sit at a fixed 4V. Anything going on in one tube has no influence on the other.

[DF96]

OK, I see what you are saying. The solution to this puzzle is that the anode CCS set the quiescent current, but there is also the signal current in the applied load. The signal current, although nearly balanced, can generate a cathode signal voltage.

Next problem: what if there is no load (apart from a voltmeter/scope)? Then it all depends on the ratio between the CCS dynamic impedances. No CCS is perfect.

[kevinkr]

Morgan Jones has suggested in "Designing Valve Amplifiers" that LTP set up with 3 CCS should have a much higher performance CCS in the tail than in the anode loads, and seems also to be generally recommending not using CCS anode loads in LTP applications.

[dgta]

Originally Posted by DF96

The signal current, although nearly balanced, can generate a cathode signal voltage.

I don't think so. Ohm's law still applies.