WOW! that was GREAT! I love the colored step by step signal path and conditioning example diagram you created for this video. Best I have ever seen. Thanks.
Yes, your feedback is through the taps on the output transformer that go back to the second grid and also through the 350pf caps to the input stage, this is an impedance balancing type which smooths out the response curve to crazy flat.
I sold my ST-70 a couple of years ago. I'm going to have to get another one :) There are plenty of clones and MANY mods that aren't too expensive, but the A70 transformers on the original are pretty sought after. I can't wait to see your performance review of this amp. Thanks!
Thank you for that, it was very informative. I'm trying to learn about tube function and I wanted to ask, is the output from pin 8 of the 7199 equivalent to that of the plate? I mean, I'm surprised that the cathode has an output associated with it at all. I didn't know that could happen.
The chassis is the same ground as DC power ground. However the chassis is not used to distribute the ground connections, it is connected once, for a noise shield. All other ground connections are connected with a common ground wire from the power ground. Excellent engineering.
Jerry Carriera I would try putting a little WD-40 on the shaft where it enters the control. One or two drops with a screwdriver, and then wait a day or two and see if it has loosened up. If it has been I would put one or two drops of WD-40 on the material that holds the carbon run. I would try this first.
Trying to understand circuits...question about the 7199 - I understand that it is being used as a driver and phase inverter - but what prevents DC from going to the grid at pin 9? In other words why is some sort of capacitance not needed between pin 2 and 9 ?
Don't know how I missed this question, maybe Google never told me about it. The bias on the triode is between the cathode pin 8 and control grid pin 9. The cathode has a cathode resistor of 47K. The grid is connected to pin 2 and it has a 270K, then I goes to DC power. So there is a rather large voltage drop across the 270K and not so much of a voltage drop across the 47K. So from pin 8 to pin 9 is the bias voltage for the triode and it also has the audio signal on it.
