300b lovers

Hi @lynn_olson ​​​​@atmasphere ,

Thank you a lot for detailed answers!

I have another question:

In balanced push-pull self bias output tubes is not a big issue because the cathode resistor and capacitor are out of the main signal path. But what to do in my case with a 300B SET. 

Now I use self bias with 50uF AN Kaisei NP bypassed by 4750uF Nichicon and two Ohmite Gold 10W resistors in parallel.

So the question is what is the best solution for SET? Fixed bias? Is fixed-bias reliable and safe? What is important to know for building a negative power supply for fixed bias?

To complete the current loop around the 300B (or any other power tube), there has to be a low-impedance path from the cathode to the ground side of the filter cap of the B+ supply.

This can be (A) fixed-bias operation with the cathode at ground, or using something like a 10-ohm current-sense resistor. This requires an adjustable low-noise -80 volt supply, and a meter across the current-sense resistor. Adjust the minus supply until you have 70 to 80 mA flowing through the 300B. The plate voltage also has to be reduced by 80 volts or so, since you’re not dropping 80 volts across a cathode resistor.

A potential source of instability for the fixed-bias circuit is having a regulated minus supply while the main B+ supply is unregulated. Small variations in AC power voltage can result in large changes in bias point. The solution is either have a tracking feature for the minus supply, or regulate the main B+ supply, which is not cheap. In other words, when both ends of the tube are controlled by plus and minus supplies, they should track each other, or be fully regulated.

The (B), self-bias option is dropping 80 volts across a 20-watt cathode resistor, and bypassing it with 200 uF of film capacitor. B+ will be around 425 to 480 volts at the top of the output transformer primary. Sonics will be strongly affected by the quality of the film capacitor since it is in the main current loop of the 300B. Adjustment of current flow is not necessary since the cathode resistor provides a degree of negative feedback for the DC current flow through the tube.

I continued tuning my amplifier for the Hashimoto A-107 interstage transformer (IT).

I changed the load resistor from 57K to 39K Ohm. The difference in square wave overshoot is very small, but frequency response peak is just +0.5dB instead of +1.3dB.

https://photos.app.goo.gl/tdDuNM15cziP4o5g9

But the most important - the rough upper mid coloration was gone with 39K.

I also increased 6sn7 idle current from 7.5mA and 9mA. When it was RC coupling before the idle current was 6.2mA.

In general, sound became more refined and organic with much better low level details. Now in addition to advantages that IT gives versus RC that I talked about before (like faster transients, better more detailed rhythm reproduction and bass control). I got more low level details, much more realistic drums kick/punch reproduction, piano is more real, as well as timbales, better instrument separation (less congestion on complex music), and more real musicians presentation in my room. 

Next step moving from an old amplifier box + power supply external maquette to two box solution (stereo amplifier + power supply).

https://youtu.be/Hro6eYBvb68

I changed the load resistor from 57K to 39K Ohm. The difference in square wave overshoot is very small, but frequency response peak is just +0.5dB instead of +1.3dB.

You can also see that bandwidth has opened up a bit. Something to keep in mind: when you load down that interstage transformer, it changes the load presented to the tube driving it. You might be able to further optimize the circuit by making changes in the driver tube circuit. Also, if I  were you I'd be try to sort out why the FR drops off so dramatically at the bottom end (is it the transformer or something else).