Tube matching/burn-in rig help required

[tubesinside]

I’m an amp tech and want to be able to burn-in and match sets of tubes before putting them into amps that are in for service…or my own ‘new builds’. I’m building a rig that will take 4 power tubes simultaneously and show me cathode current on 4 analog meters. While looking at designs I’m going to try measuring Gm (or a version of it) by injecting 1k sine wave at 1 v RMS and measuring the rectified anode output voltage on the same meters. I don’t need an accurate measure but I do need to be able to compare new/old tubes against a reference. I decided to build a modular unit that can also serve as a standalone PSU for testing my amp designs before committing to a full build.
Power Supply:
Variable HT voltage by using a 2A Variac on the HT AC output. This is then rectified to give a variable DC for Anode and Screen Grid. (I have a spare variac)
Filament voltage is fixed. Grid voltage is variable with ‘coarse’ switch and ‘fine’ pot to cater for KT88 to EL84 tubes.
Tube circuit:
Anode and Screen grid supplies are protected by fast acting ‘resettable’ PCT fuses and overload is indicated by LED (full circuit design not yet completed). Cathode current is measured on 100mV meter across 1R and protected by germanium diode.
Gm circuit: This is where I need help…
If I inject 1k 1V rms into the grid I should be able to measure voltage gain at the anode of the tubes under test with the same meter and the circuit shown. Question…what value of anode resistor should I use? Will I have to switch in different values to take account of KT88/6550/EL34/6L6/6V6 ?
I propose to use a different resistor for the EL84.
Please excuse the quick sketches and any mistakes that I’ve made so far…the final design will have voltmeters for HT and bias and I will probably have an attenuated audio output from the anode capacitor to be able to measure microphonics and noise on an external meter.
Any help appreciated.

[JazzGuitarGimp]

What is the voltage rating of your variac? If it is less than 350V, I wonder if it will work without overheating. Just thinking out loud here. I know variacs are also referred to as variable auto-transformers - and I also know that if you push a power transformer beyond its rated input voltage, the core will saturate and overheat. And I think it only takes a 25% or so increase in input voltage before this happens. This might imply that a 240V variac would start getting hot at about 300V. Is there any reason you wouldn't use the variac in the primary of the transformer (understanding you'll need a separate transformer for the filaments)?

[tubesinside]

Yes Lou...I had thought of that. The one that I have has a 250V AC rating...but I'll try it into a bulb before I commit to this design. I have also considered using a modified VVR circuit with paralleled MOSFET devices but I want to keep it simple if possible. Like you suggest I could also use it on the primary and use an additional traffo for the filaments and bias. Thanks so far.