Friday, 16 September 2016

Proof-of-concept test rig

So after a few trips to Jaycar, and after the components that I'd ordered from RS had arrived, I had what I needed to start building the test rig:D

Watch as I light up the initial gain stage, then read on for some measurements and discussion


The point of the test rig is just to get the initial gain, inverter and driver stages modelled and to check the bias parameters for the valves with resistor substitution, ahead of the main build. That way, I'll need to do as little component swapping as possible when doing the final build.

So this is the circuit I built today. This uses a 12AX7 (ECC83) as an initial gain and DC-coupled Cathodyne/Concertina Phase Inverter.


The highlighted voltages are actual measured values. 

The letters are signal test points for the oscilloscope traces (to follow).

Calculating the bias, we have a voltage drop of 195V across R3, which for 390K suggests a bias current of 0.5mA which is right where I wanted it to be.

On the inverter we have a voltage drop of 115V across R8 which at 100K suggests a bias current of 1.15mA, a little higher than I was aiming for but a potentially a consequence of DC coupling.

This is what the build looks like. Those of a sensitive disposition my wish to look away at this point, it is not pretty (hey, it is only a proof-of-concept/test build, it only has to work, not look good as well!)

Red and Black wires for power, yellow wires for heaters, white wires for signal

100K volume control to allow easy adjustment and duplicate the setup in the final build

And yeah, I know that 150K resistor feeding the supply capacitor is too low-power. Tell me something I don't know! But it works and hasn't gone up in smoke yet. Obviously the final build will have a resistor with a more appropriate power rating for durability.

Below is the power supply. Those of a sensitive disposition PLEASE look away now. This is rude but it works

Unloaded supply voltage is 580V DC... extreme caution needed! 

YES the chassis is very carefully earthed!

Yes I do work with one hand in my pocket when the circuit is energized!


The transformer is 400-0-400V centre-tapped. the diodes I'm using (1N5408) have a peak inverse repetitive voltage of 1000v which according to PSUD2 this design will exceed, admittedly only by a couple of volts, so I've doubled the diodes and paralleled them with 220nF ceramic capacitors.

Also note the electrolytics - these are 470 µF 450V capacitors, but the power supply's open-circuit voltage is running at around 580V so I've connected them in SERIES so that each cap has around 260V across it. It does mean that the body of the second capacitor is at around 260V potential to the chassis, so this design will need special considerations in the final build. Note also the 100K bleed resistors across the caps, after power is removed it takes around one minute for the voltage to come down to around 30 volts.

This design is not anything you can listen to; without output valves and an output transformer there is no chance of driving a speaker with this setup, but that's not the idea. The point of this is simply to inject test tones from a function generator and look at the outputs on the oscilloscope and spectrum analyzer.

So to the oscilloscope traces: Firstly, the gain stage, measurements at point A and B in the circuit. (A is the blue trace, B is the yellow trace)


The initial gain stage is inverting the phase as expected, and will give up to 10v peak-to-peak before harmonic distortion starts being visible on the spectrum analyzer.

THD at this amplification is low at 0.05% and the spectrum analyzer looks pleasingly clean.


The next step is to use the other half of the 12AX7 to perform the phase inversion necessary to drive a push-pull output stage. The topology here is to use the other triode as a cathodyne/concertina/split-load with equal resistors on the anode and the cathode. This ensures an equal amplitude output at both ends, and an overall gain of around 0.95.

The potential difficulties here are getting the bias level correct; the grid needs to be at a lower potential than the cathode, but DC coupling from the previous stage means that the grid is at the same potential as the anode of the preceding stage.

This can be resolved by using a high value resistor for the anode - it will be noticed this design relies on a 390K resistor.

Measurements taken while the circuit is running indicate the valve to be self-biasing, to an extent. The voltages on the diagram above are taken from actual measurements while the circuit was running. This seems to indicate the idea of a DC-coupled cathodyne with a 12AX7 is technically feasible, at least on paper. This is the same idea as the Williamson design, except that uses a 6SN7 in this topology.

My idea in this test rig was to see if it was even possible to use a 12AX7 in this topology. The oscilloscope measurements seem to indicate it will work.

Below is the output trace from the Anode (Point C in the circuit, Yellow trace) and Cathode (Point D in the circuit, Blue trace):


This looks the way it should - I tried the test with frequencies from 20Hz to 20kHz and the results were consistent. 

The spectrum Analyzer shows no significant presence of harmonics:


Based on these initial measurements, I am optimistic this will be a viable initial stage for my amplifier. I have yet to make measurements of frequency response, so far the level of attenuation at higher frequencies appears to be minimal, but I'd rather have some numbers rather than just "minimal" so these will be my next measurements.

The next stage of building will be to add the second valve, the 12AU7 driver, to boost each phase of the signal from 10v p-p to approximately 50v p-p.

More measurements to follow...

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