On Saturday, January 16, 2016 at 10:18:09 PM UTC-5, Phil Allison wrote:
John Heath wrote:



I do not see feedback from output tubes to split phase tube as can be seen on the diagram.

** Feedback comes from the 8ohm tap on the output transformer to the cathode of V3A. The phase splitter cannot accept feedback since it uses the "concertina" circuit rather than the more common cathode coupled pair.

Tube matching is normally refers to nominally identical tubes that show the same plate current under class A conditions. For pentode and beam power tubes, actual plate and screen voltages are critical so the best test conditions are those found in the amp itself.


... Phil

Yes , now I see it. It is marked with a big sign saying FEEDBACK -- , ha. So who cares about matching tubes if it has feedback. The feedback should compensate for that yes / no ? As long as the output idling current is in a reasonable range and not increasing over time it should be okay. There is an obligation in service to look after the customer's best interest. Does he need German imported tubes with color coded matched specification or will a china cheap knockoff do just as well. I suspect the latter is in the costumer's interest.

And I would add that the rumor that China products are of less quality is just sour grapes from an industry that can not compete. 90 percent of all desktop , laptop , flat TVs and cell phones come from China. These are not the easy but the hardest electronic products to make and they are all coming from the shores of China. The winds of change are in the air. I can see I am rambling off topic. Why is it when we get older there is a need to talk all the time? It is as if life as filled the brain with stuff so you need to talk to vent some of it off.