Quote:
Originally Posted by bperboy
Okay, I put the D2 back in, so I still really don't know if that was the problem, but I measured resistance from cathode to ground on both tubes, and they both can be adjusted to 1.1k ohms, so I'm gonna go ahead and assume that both the trimpots are in working order. I'm gonna go test voltages one more time, and then if you guys have any other suggestions...
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The idea of putting D2 back in is that the resistor did not fix the problem. I'm out of ideas because everything checks out. There isn't anything left to measure.
You rechecked resistance between grid and ground?
Have you tried measuring bias after removing the buffers?
I would question the tube socket. If you have 1K between the cathode and ground, 0-50K between grid and ground, your heaters are working, your tubes are good, and the CCS is good, with about 24V on the input side, and connected to the tube plate, the thing has to bias.
Did you stick wire in the socket and measure resistance from the wire? I took a VD RN55 resistor, bent about 1/2" back, and tightened the loop with a pair of pliars. I then spread the V shaped lead a little and stuck it in a couple tube socket pins. I was able to get good reliable readings after clipping my meter probe on with a grabber clip.
In an old thread, I recall you were using a $3 DMM that gave you very strange readings on your Steps. Did you replace the meter with something better?
If you cannot do the above with a bent lead, you may be able to pull the tube up an 1/8" or so, enough to maintain contact with the socket. You could then stick a probe tip in and check the resistances we discussed. I can't try it directly with mine because it is cased, but I lifted the tube a bit and it still heated up. If one the key pins is open to ground, that may be your problem (or you may just have the tube pulled too far out).
I have one idea left, pretty much a Hail Mary Pass and it will take a little courage. What I am suggesting below I actually did on my Millet and it even fired up afterwards....
This will test the CCS.
1. Remove the buffers
2. Remove the tubes
3. Bend the lead of a 5K resistor as I suggested above and put it in the tube socket, Pin 7 (plate).
4. Put one end of an alligator clip lead on the other end of the 5K resistor.
5. Here is the scary part. Clip the other end of the alligator clip lead to ground.
6. Put the DMM across R5, measuring volts.
7. Power it up.
You should measure about 0.56V across R5, assuming you used a 0.56ma CCS. If it is significantly higher, say more than 1 volt, your CCS is probably shorted. If it is significantly less, some other problem.
You do not actually need the 5K resistor. I grounded the plate right to ground when I tried this (I knew I had a good CCS), and then tried it with the 5K resistor just to ascertain the measurement would be as I thought. The 5K resistor protects R5 in case the CCS is shorted, limiting the current to about 5ma or so (just under 1/10 watt). If you don't have a 5K or so resistor use something lesser but more than 2K and don't leave power on too long, especially if your voltage across R5 is more than one volt.
I can't guarantee you won't blow something up but I tried this on mine and from the above you can see I tried to anticipate the worst case (shorted CCS). If I were in your shoes I would do this, but do it very carefully.
I hope you are learning something from all this
