a mono vacuum tube audio amplifier
In my ham shack i have a low power audio amplifier, built on a cheap cake pan, that I used for experimenting and just as a general purpose audio amplifier. I thought it might be useful to have a similar device here at my second house, and also take an opportunity to experiment with some different construction techniques for vacuum tubes.
It won’t need to be much of an amplifier - it will join the kitchen table lineup at my lake house and provide some tube audio sound for the planned second generation of RF stuff, while also providing a low current, tube voltage, power supply if / when I might need one through an octal socket on the rear panel.
I have decided to try to build it on a 3D printed chassis instead of a cake pan, and already have a chassis printed and working power supply constructed.
Here is a temporary schematic. It is dirt simple at this point
The input is a triode - 6AB4 is 1/2 of a 12AT7, and 6C4 is one half of a 12AU7. The ideal load for a triode is a current source, and an IXYS 10M45 is used as a current source here. The trim pot sets the cathode voltage to about 1.25 volts.
The CCS IC is somewhat expensive, and a conventional resistor load can be substituted if desired. 10K - 15K is a good value for the 6AB4 at the lowish voltages here, and would probably be OK as a start point for 6C4. If more gain is desired, 6AV6 is 1/2 of a 12AX7; I would probably start around 36K as a plate load for that tube. Tie the diodes to the cathode.
The power tube is wired as a triode for simplicity. The cathode resistor needs to be set to the correct bias voltage depending on the tube and B+ voltage. The grid resistor on the power tube is low at 240K, but seems to be OK here.
At some point, I will try some different input and power tubes, and different configurations with feedback. The whole point of this besides a utility audio power amplifier / tube voltage
power supply, is just to play around and experiment with.
One objective was to determine whether or not a 3D printed chassis is suitable for vacuum tube projects. The answer so far is, yes, at least for low power stuff like this. Another objective, perhaps more unique just to me, was to determine if the Red Pitaya was as useful for audio measurement as it is for RF. Again, at least for low power stuff like this, the answer is yes.
Interestingly, the measured results of the 6X5 6AB4 6BQ5 combination is surprisingly close to the results predicted by SE Cad.
Win W5JAG