(this blog entry is an excerpt and the story is in "mid swing". See the full five page article here: DynaMutt: The ST-70 That Should Have Been)


DynaMutt was the end result of a rebuild of a Dynaco ST-70.

The story began with the need for a Dynaco ST-70 to develop new and unique driver boards for them. A few emails to friends located a ST-70 in the basement of Planet10-HiFi and I was free to use it for R&D purposes on the condition that I get the unit working.

I was able to pick up the Dynaco on a spring visit to Planet10.





Here is the supply as used in my version:




The original Dynaco can cap was specified at 30uF/20uF/20uF/20uf @ 525V. The capacitor combination in the circuit above gives us about 47uF/22uF/22uF/22uF @ 700V. The 47uF...

MooseFET is a low gain, power MOSFET based line stage that fills the gap between a passive preamp and a standard line stage.

The design goals were:

- Lower than usual linestage gain.
- Be simple.
- Be reliable.
- Be rugged.
- Be easily portable.
- Not sound "solid state".
- Not need a complex power supply, typically required of single ended MOSFET stages.
- Be inexpensive to build.




** NOTE: The forum resizes the images. Please click on them to see fullsize.

Let's take a quick look at MooseFET's circuit:



The first thing you might say is, "Isn't that a BoZ?". The answer is not quite, but it was definately inspired by the magic of Nelson Pass (who contributed to this circuit with insights and suggestions)....

Anyone who has used the mu follower topology and likes it, must also understand one of the frustrations with the circuit is the heater supply for the top tube.

Why is this a problem in the first place? Well, I can think of two reasons... hum suppression of the heater supply and heater to cathode breakdown potential.

One of the tricks to suppressing hum on the heater I like to use is to lift the heater to some positive potential above the cathode potential (Figure 1). That is no real problem where the cathode is near ground, but when one cathode of a dual section tube is floating higher up in the sky, raising the heater voltage to where the top heater is above the cathode may inavertently cause heater cathode breakdown of the bottom tube.

** NOTE: The forum resizes the images. Please click on them to see fullsize.



With a lower high voltage power supply,...