I'm reasonably happy with the initial gain and driver stage Proof of Concept build on the testbench, after having played around with resistor values to tweak bias voltages and measuring outputs. The small problem I had was that my oscilloscope was going into clipping on the input before the signal went into clipping, so I couldn't measure the gain accurately.
I ordered a X100 probe to remedy this problem and with its arrival today I was able to make some accurate measurements on the 'scope.
So I was interested in two things: Overall Gain, and Frequency Response.
For testing the gain, I sent in a 200mV RMS signal and advanced my gain control all the way up (no attenuation) and measured the voltage at the outputs of the driver stage, loaded with 100K resistors to simulate the output valves.
At the outputs I am getting 148v RMS from a 200mv RMS input signal. This equates to around 742 X gain for the initial gain and driver stages, or put another way, or 57dB
The next measure is frequency response. This is typically measured up to 100kHz, but my (borrowed) function generator has a maximum frequency of 20kHz. Better than nothing.
To measure frequency response, I monitored input voltage (to ensure the signal generator is honest!) and voltage at the outputs of the 12AU7 into the 100K load.
A spreadsheet exercise resolves this into a graph, which wants to be as flat as possible:
Click for larger size
I am satisfied enough with this response.
Up next: I'm going to re-design the adjustable bias circuit so that it's fail-safe, rather than fail-unsafe as currently designed. If one of those pot wipers should lift, the bias voltage will disappear from the KT88, followed shortly thereafter by pyrotechnics as the valve red-plates. Instead I'll use a design with fixed resistors and trim pots where if the pot fails it gets more negative bias, not less.
Also I've ordered my volume control and input remote switcher today.
I have also decided these cheap Chinese valves I've been using in my testing will not do for the final project, they are too variable. I'll get some better ones, along with my output valves. Fortunately I have a local source for those.
The Chassis is going to be the next challenge...