Continuing from the previous build, this phase of the test build is focussed on V2 - the driver valve. This is a 12AU7 twin-triode, and each half of it will be amplifying one phase from the previous inverter stage to the approximately 50v needed to drive the KT88 output valves.
This is a simple grounded-cathode topology, the same in fact as the single-ended initial gain stage. These are identical and to encourage the valves to balance, I have tied the cathodes and they share a common cathode resistor.
This is the schematic for today's build – the previous build is grayed out:
Click to see larger size
The yellow highlights are measured voltages .... well almost. The vagaries of valve electronics means that the current draw from each half of the triode is not exactly the same, which results in differing voltages at the anodes. In this case I had two 12AU7s to hand, and on the first, the measured voltages at the anodes were 180V and 189V, the other 190V and 180V
My original design called for a 10K pot to provide an adjustment in dividing the HT+ voltage between the two halves, to ensure identical amplitude output, this was omitted in this test build (no 10K pots handy!) but will be included in the final build.
This difference is clearly visible in the oscilloscope traces, one trace per anode, from test points A and B in the circuit schematic. See below:
Note the differing peak-to-peak voltages of the signals.
I've changed the colours of the traces on my oscilloscope to match the identifying colours on the probes. Makes for less confusion with a powered-up circuit on the bench.
These different voltages will need to be balanced and if I switch to the different valve, the difference is reversed. The outputs from the phase inverter are much closer together.
This is how the test rig looks with the second valve wired in. It's not meant to be pretty it's just meant to work, please be gentle with your comments on my construction technique, the final build will be a thousand times better
All wired up with voltmeter and oscilloscope probes:
Astute readers will spot the series resistors needed to tame the B+ to usable voltages for the 12AU7
According to the oscilloscope I'm getting the signal amplitide I need (after a worrying half hour where the signal was clipping, only to find I'd left the probes on X1 not X10 – D'oh!)
Also according to the spectrum analyzer the THD in this section is clean, at around 0.08% at 50v drive. The readings are taken through 0.68µF capacitors loaded to earth thru 1M resistors.
Once again only informal measurements of freq response have been made from 20Hz to 20kHz, the amplitudes on the 'scope screen did not change. However more measurements will be made to be certain of the results.
Next steps - get a 10K Pot to try to balance the outputs from the 12AU7.
Final thought for the day
This shit is real. I'm also really hoping that'll come down a bit when the OPTs are connected and the output valves biased up. I think I might see if I can contrive a 200mA load to simulate the 4 KT88s at quiescent, and see what this does to my voltage. Perhaps a couple of 100W lightbulbs connected in series might do the job.