Re: Grid Bias Adjustment on 475A


 

Jeff,
I haven't followed this thread for a while, so
Just a few points:
- 0.7 V at the base of Q1306 is close enough to 0.6 V as mentioned in the SM, I don't see your point ("only"). If anything, a slightly high base voltage possibly indicates a harder drive of Q1318's base, resulting in a more negative cathode voltage.
- What are you using as a HV probe and what measuring instrument (DMM?), i.e. what load are you putting on the cathode voltage when measuring? The cathode voltage cannot provide a lot of current into your probe while measuring cathode voltage.
- It is unlikely for R1303A to drift low. Also, I can see several paths spoiling measurement of R1303A in situ, to a degree depending on the voltage that your ohmmeter applies.
- I guess you know/realized that R1303B and R1303A together form a voltage divider; the voltage across R1303A should be about - 24M5/500k times the 50 V across R1303B. If R1303A were about 13M7, as you measured, I would expect the cathode voltage to be about - 1400 V, not -1900 V as you measured. Again, it's quite possible that you measure -1900 V because you're loading the cathode supply with your HV probe, and/or the voltage divider that results from the combination of the probe and your DMM (?) isn't correct at the range you're using; many DMM's have different input resistances at different ranges. 11 MOhm is a common value, as opposed to the 10 MOhm you'd expect on all ranges.
- Did you check C1302 for low leakage? If you study the schematic, you'll see that a leaking C1302 will result in a less negative cathode voltage with the regulation (R1303A/R1303B) in equilibrium. You could just (temporarily) replace it with another cap in the same capacity range.
- The less negative voltage (-2.1 V) at the collector of Q1308 than the SM mentions suggests that the base of Q1318 is driven harder than with -3.2 V, which would result in a more negative cathode voltage than intended, not less negative, as you indicate.
- The collector of Q1308 being so far away from the power supplies surrounding it (+ 5 V and - 8 V), suggests regulation is happening

Raymond

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