gringopig's guitars
We all have daft ideas and sudden impulses but as long as they don't involve driving into a wall at 60 mph or saying how you really feel to a work colleague, then all is generally positive.
My impulse was to buy a valve tester!
Now, I have valves. Rather a lot thanks to my shortage-fuelled toilet paper moment when I bought loads of valves after reading about the supposed valve crisis recently. This turned out to be utter crap but prices have indeed risen and quality has perhaps taken a nose dive so I thought given that all the supplier here in the UK have their own standards of measure and don't seem to own a copy of any standard valve test data manual, I would standardise all mine with one good old Avo MKIII valve characteristic meter. The proper tester which can group and characterise valves based on transfer curves and bias level with gain.
The prices however are extortionate!
I thought to start with a simple emissions tester first. One popped up on ebay and I paid the £200 the seller was asking.
I knew this device was merely a go-no go tester; an indicator of gross failure rather than some scientific measurement unit but it was OK for the price and the seller told me it was his grandfather's and assured me it was 240V.
Of course, it wasn't and I had to buy a step down transformer to run it at the US voltage and I had Time Travel Audio in Edinburgh put a grounded US 3 prong plug on. It looks quite cool.
Note the black box at the back. That's the step down transformer required to supply 110-120V AC to the unit. It's a Bronson++ and Time Travel Audio warned me to let it settle for a few sconds as it had an inrush current from off which resulted in a field wobble which put a much higher voltage on the secondary side. It's also non isolating, so a grounded cable was required.
The unit powered up fine and illuminated as expected and I set about determining whether it was anyway near functional. You can see the knob at the front left called 'Line Adjust'. This is a simple rheostat type voltage dropper to allow the unit to be set with the meter needle at centre with 6.3V being set on the heater of the octal socket: the 6V6/6L6 one to the right of the meter to the top side.
I probed pins 2 and 7 of the socket with a multimeter and found that setting 6.3V on those pins and a tweak of Line Adjust resulted in the meter needle being spot on in the middle, so it was calibrated!
Or rather, as much as this unit can be. Quite good. Maybe Time Travel Audio tweaked the internal potentiometer calibration for me when they had it or maybe Grandad did it?
It's a simple device. The circuit is below and all you can do is test for short circuits between elements of the valve structure that should be isolated, test for a gassy valve and to give an indication of good or bad in terms of emissions. You set the circuit type, the output load, heater voltage and look at the valve chart to set the switch positions appropriate for the valve under test and move switches in a specific order to test for shorts and leaks and then hold the value switch in the down position to get a reading of emissions: good or bad.
I made 2 videos to demonstrate the operation of this tester and you can watch them below. I am going to detail the much more complex Avo MKIII soon but I need to figure out how to use it first!
I tested some 6L6GC and 12AX7 aka ECC83 in the videos and although you can't really 'grade' them using this, it certainly tells you whether the valve is a good one or not.
