until the desired break voltage was reached and the amplifier switched over to being a Class AB push-pull one. This time we will look into using a current source made out of discrete parts. The advantage this approach holds over the first is that high quality chokes are both hard to find and expensive, not to mention physically large.  A further advantage is that active current source can made to track the current flow in the other tube so as to ensure a better balanced idle current. Additionally, this arrangement works a little more easily with a mono-polar power supply, i.e. one without a negative voltage for biasing the output tubes.
   An Op-Amp based current source could be used and it would yield the tightest current balance, but at increased complexity and expense. The simplest circuit would be made out of a few resistors and capacitors and two MOSFETs.

Part 2 to last month's Ian's E-mail

Subject: SE / PUSH-PULL
   First of all kudos to you and to your fine magazine. At first, I only understood a third of Issue 1, but now that I just reread it, I understand about 90% of it. I don't know if I will ever hit 100%, but I'll try.
   So here is my question. I love the idea you have of converting a ST-70 into a para feed, SE amplifier using tubes instead of chokes. I have a pair of MK 3's that I would like to convert to style of amplifier. So my question is: Would it be possible to make the amplifier work like the Nelson Pass solid-state SE, the Aleph, which is SE up to a certain amount of wattage and then switches to push-pull Class A. amplification for the rest of output power? I know the solid-stage guys have a huge advantage in that they are willing to use IC's in the signal path, which we tube purist aren't, but at the cost of less design flexibility. So is this worth pursuing? Thanks in advance.
   Last month we covered a modification to the Dynaco MK 3 that resulted in an SE/AB amplifier. That modification required the addition of placing a choke in series with the cathode of one of the output tubes. The choke made a constant current source out of the tube, as its cathode would follow its grid's voltage movement so well that no variation grid-to-cathode voltage occurred,

The output stage of a Dynaco MK 3 converted into an SE / PUSH-PULL amplifier.   


pg. 18


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