I've posted recently about some problems I've had neutralizing the finals in my TR-4. I think in the last post I made about it, I stated that the original 12pF (C77) that's across the variable capacitor, C76, tested completely bad ("No, unknown, or damaged part"); however, it occurred to me that this tester (a "T7") isn't spec'd to test below 20pF so the original may still be good.
In any case, I ordered some replacements (a 10pF, a 12pF, and a 15pF) and they arrived today. I tried the 12pF and got basically the same results as the original (a semi-null with C76 fully open). I then decided to try a 2pF SM I happened to have and that gave me a semi-null with the plates fully closed so I figured the sweet-spot must be somewhere between those two values.
Since I didn't have anything on hand at around 7pF, I connected the 15pF and the 10pF in series to obtain 6pF across C76 and that gives me a null with the plates virtually at the half-way point so now I'm searching for a suitable capacitor in the 6pF range but I'm not finding anything.
Anyone know where I might find a suitable capacitor? If I can't find something, I may just stick with the two capacitors in series but I'm not very fond of a "flying splice" like that.
What I'm really wondering about is why this rig needs only half of the original value capacitor in that place. I know that Steve has mentioned a brand of tubes that apparently needs a little more capacitance but haven't heard anything about needing less and so I'm wondering what could be causing that. As far as I can tell, there are no other capacitors in parallel with those two but several other capacitors connected to that part of the circuit in one way or the other. Any suggestions as to anything I might look for or maybe I should just chalk it up to the rig being 60 years old and several factors could be at play because of aging parts.
Thanks,
Barry - N4BUQ
