Thanks.
Yes, my mic is a Stam SA-47. So it is the same as the Stam in that review.
My heater WAS 4.7. The EF800 that was in it started crackling after about 4 months. Then I started trying to figure out what was up, and I have gotten confused. Some think I need to run it at 6.1. Some think 6.3. Some think I probably need an EF860 in it. And Josh Stam, the designer, thinks I should run it at the factory spec of 5.05. If I bring it up, I wouldn’t know what resistor to use.
I brought it up to 6.1 AFTER putting in a new tube. It sounded fine. Maybe a little more harmonics. Then I spoke with Stam and he said that’s too high because of the pin 2 setup, and it should be 5.05. At 5.05 the mic is quiet and seems to sound great.
You are saying that if I run my crackly tube at 6.3 for a few hours, it will restore it?
Data of 1.1 V, if known, applied to the cathode bias for VF14. However, each tube behaves differently. Moreover, VF14 operated at 55V/50 mA, but was strongly underheated to around 34V/40 mA (in U47). Each EF tube behaves differently as well. You can't rely on a schematic with 1.1 V. What voltage did you measure in the Stam clone (cathode bias)?
The cathode bias sets the operating point, which is also related to the anode voltage. Schematics are valid for VF14; for individual tubes or nuvistors (in U47 13CW4 or 8393), the operating point, anode voltage settings, and other components necessarily change.
Everything also changes during so-called underheating of the tube. Especially modern EF-type tubes do not tolerate underheating very well. VF14 and generally V-type tubes (at 55–60V) used to have a double, helical filament, operated differently, had a different cathode coating, etc., etc.
P.S.
I'll add another experience with a nuvistor, I believe it was 8393, the successor to 13CW4 (13,5V / 60 mA), which the late Mr. Neumann recommended as a replacement for the VF14 tube when it became scarce because Telefunken stopped producing it. Normally, the U47 is powered at 105V / 40mA; for the nuvistor version, Neumann released a document, increasing the voltage to 110V / 50 mA. If I had the nuvistor underheated, it produced strange sounds; it heats up again and works, but these new tubes, including the glass EF ones, are not prepared for excessive underheating. They can behave strangely, produce peculiar sounds, and can be invasively destroyed by improper handling (coating on the internal parts).
First, I recommend measuring the heater voltage of the tube, then try another tube that I know is correct.