Wednesday, September 14, 2011

Tube Matcher and Magic Wand Update

Ever since I built the Jack Price Tube Matcher I've been thinking a precision adjustable feature might be very handy to have when calibration time rolls around or there's a need for more precision.

After some thought and research here is what I came up with.

It requires eight 10 ohm precision trim pots, which are the lowest resistance value commonly available. The specifications say that these have no more than 2 ohms residual resistance, so I figured that doubling them up would get me right in the range needed as well as doubling the wattage rating. I mounted the pots on a piece of perf board and I'm going to try it out soon.

I think that setting up the tester with a single tube and then adjusting each socket to read exactly the same on the multimeter will fulfill the calibration function, and this could also allow compensation for any drift in the other parts of the system.

I also added a picture of a neat little signal tracerish idea from the Hoffman Amps website. The idea is to take a small audio amplifier set up for clean sound-in my case a $25 Crate bass amp off of Craigslist. Then taking a multimeter probe and connecting it with shielded wire (old guitar cable) to a project box containing a 1 meg pot for padding and a .1/600v cap for DC blocking completes the gizmo.

Now, you set up the amp under test to your dummy load about which more anon. You have some compliant soul play a guitar through it and using your newly built signal tracer you follow the signal through the amp until it deteriorates, thus locating the source of your unwanted distortion.

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