Tube failure mystery: Solved, I think!

[sookwinder]

-Also, there was some oscillation between 1.9V and 3.5V on the lead coming off the 25 MGD capacitor going to pin 8 of V2

what are the voltages like at pins 1 and 6?
Do they oscillate as well.
what about the voltage at pin3?

What time period is the oscillation?
1 sec
0.1 sec
10 sec?

[wproffitt]

-Also, there was some oscillation between 1.9V and 3.5V on the lead coming off the 25 MGD capacitor going to pin 8 of V2

what are the voltages like at pins 1 and 6?
Do they oscillate as well.
what about the voltage at pin3?

What time period is the oscillation?
1 sec
0.1 sec
10 sec?

So, I replaced V2 as soon as I got home on Friday and the amp could not sound better. I checked the voltages tonight and here’s what I got:

-Coming off of the 25 mfd cap in question (the one going to pin 8 of V2, I got a steady 3.5 V when the layout calls for 2.4. No oscillation. The other day, with the old tube, it was oscillating at the rate of about 1/2 a second, or so it seemed. Now, steady as can be with the new tube.
-on that tube itself, I got:
Pin 1: 245 V (should be 230V)
Pin 3: 0V (bad reading? What should I be seeing?)
Pin 6: 242 V (should be 230V)

I think all is well. The amp certainly sounds awesome. I may be able to score some old RCA or Telefunken tubes from my coworker that I know have tested as A-OK, too! Wouldn’t that be nice?

[sookwinder]

Pin 3 and Pin 8 should be connected and give the same voltage, as they are both connected to the same cathode resistor/cap combination.
Are pin3 and pin8 connected in your amp?

[wproffitt]

Thanks, David! Pins 3 and 8 are indeed connected per the layout. I was just having a tough time getting a reading with my borrowed multimeter on pins 3 and 8 for this tube, but I can’t say why. Would it be functional if these weren’t getting any juice?

[øøøøøøø]

Your voltage readings are few % high, but that's relatively normal with modern power. Your amp was expecting to see 117v coming from the wall.

Judging from your readings, it's probably seeing ~124, which is not too out of the ordinary since line voltages have certainly crept upward since the 1960s.

[wproffitt]

You can buy Ruby Tubes at a lot of online tube shops (e.g., AmplifiedParts.com - they have always been good to deal with). I believe that the Ruby Tubes are primarily Chinese tubes but they are tested before being resold. Tube amp Doctor does the same. There's also https://www.kcanostubes.com/. I am pretty sure that he tests his new manufacture tubes. There probably are many others, also.

You pay a little more for pre-tested tubes but usually get a no-questions kind of warranty (or should).

Jthomas, thanks for the tube website. Buying tubes really does feel like a crapshoot sometimes. I did pick up a TAD 12AX7 for the parts drawer. Let’s hope I don’t have to plug it in any time soon!

-also, thanks,, Brad. I figured that the higher voltages in the amp probably had something to do with the higher wall voltage. Also, the wiring layout for this 1955 relic makes it clear that the voltages are all meant to be within a 20% margin of error on either side, so I figure things are probably fine. The reassurance of someone who has done this a few times is nice, though!