There is still plenty to be done on the software side and there is also a need to put the Valve Analyser through its paces with some higher-powered devices but, in theory, the hardware platform is in a pretty good state. However, one element of the design is still a concern – using a high voltage opamp to create grid voltages up to -66V works very well but it’s a specialist (and expensive) part. Unless you’re going surface mount you’re pretty much restricted to the OPA445 – these are over £12 each from Mouser.
A bit of research turned up the Nat Semi AN272 which details a number of different booster circuits for opamps. One of the designs described in the Application Note is a voltage booster which simply cascades a discrete inverting amplifier (capable of delivering the requisite voltage swing) after a low-voltage opamp. Accordingly, I gave this a try with a TL072 running off the +/- 7V rails, and with the booster amp powered from the +7V/-70V rails. It didn’t work.
I scratched my head a little bit more and it then occurred to me that the number of transistors in the booster circuit (8 in total) was pretty close to that required for a simple discrete opamp (9 transistors). Before committing this design to a PCB, this time I tried it out on some breadboard and it worked a treat (once I noticed that I’d put one of the output transistors in backwards). The entire amplifier is built out of 2N5551s and 2N5401s (in the schematic below I’ve used a BD139/BD140 output pair but the grid drive shouldn’t need any appreciable current drive) – these are more than capable of handling the voltage demands and are as cheap as, er, chips…
In practice, the emitter degeneration applied to the long-tailed pair, its current mirror and the output pair is probably unnecessary but can be included on the PCB or linked out as desired.
The end result of these investigations is (I hope) a complete design with components that are all easy to source. I will post photos in due course but my intention is to create a finished design with an enclosure made from laser-cut MDF and an acrylic top cover