Fun little toy that should be a handy addition to any hobby bench! Dialed it right in at around 100 ohm (which explains why 50k was a bit ... twitchy) and went with 200 ohm pots, one per tube. I hot glued a couple scraps of perf board to the back of the meters to hold the pots and give secure mounting for the wires. Wires that came up a bit short with all the variations of the circuit I tried - I'm tempted to rip the whole harness off and redo it so I can bundle the wires nicely, but ... must ... resist ...
Next came dialing them in ...
- Adjust the bias to 55mA at the test points with the VOM, and with the meters off
- Adjust each trim pot on the meters to 55mA
So far they're reading consistently. I do have to switch the meters off when I'm actually setting the bias at the test points as those read high due to the additional resistance with the meters in line, but that's where the center off SPDT switches are nice. I like being able to just give the meters a glance with a couple quick flips to make sure the tubes are all working correctly and that they're warmed up. I'll play around with tweaking the bias just using the mark one eyeball and verify with the VOM to see how close I can get.
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Speaking of tubes working correctly ... It's the little things ...
When I was done playing about with the meter circuit, somewhere between the basement shop and the audio rack something went horribly wrong. V6 started red plating on power up. Huh??
Figured I fat fingered something when I had it on the bench, so ...
- shuffled tubes - the problem stayed at V6
- checked voltages (all good)
- checked continuity to eliminate the chance of a stretched wire. I've seen stranger things happen.
- checked bias resistor (tested good, but I replaced it anyway)
- double checked all solder connections at V6 and the VTA board, and retouched a few here and there.
- gave it a good check for whiskers and wire cuttings that could be shorting stuff out.
Finally ... noticed the bias pot for V6 had a sticky spot ... kinda raspy and tested open. Replaced that, and there was once again joy and jubilation here in sKiZoLand ... as for the rest of the testing, hey ... I needed the practice anyway.
Moral of THAT story ... Last time I buy cheap Chinese junk pots of da bay. I plan to replace all four with some decent ones. Anybody know a good source? I found these roller pots over at All Parts for $6 a pop that look like they'll do nicely.
Should have gone with my first instincts. I went with some OEM Fender 50k pots that were a couple bucks less each, and come to find out, they're NOT a linear taper. Halfway up and they're only at around 10mA and only around 20mA at 2 o'clock. From there, it get's tight and another step or two puts me at 55mA, but it's a bit twitchy because the range tightens up so much. MUCH better pot though. Ah well ... I may order up the roller pots from All Parts anyway, and hold them in reserve for the next time I go under the hood.
Last edited by sKiZo on Sat Nov 16, 2013 9:10 pm; edited 1 time in total