Tube Aging

[atul.mehta6]

Hi,

I had a question on whether tubes age sharply in a couple of years. So I had built a Crack+SB kit a few years ago. While doing the final safety checks these were the voltages I measured:


OA: 104 V
OB:  108.5V
G: 0V
B+: 188 V

Now I remeasured these the other day (since I plan to sell it later) and these are the voltages.

OA: 114 V
O: 112.3 V
G: 0V
B+: 181.4 V

It still works and sounds fine. The resistance checks are fine and all the LEDs light up. But the spike is voltage seem a bit strange. I am wondering whether I did a shoddy job soldering and if it is still safe to use (some values are 12-14% more than expected).

So I got curious and I got my S.E.X 3.0 with the C4S upgrade. This has been in use since 2 months. When I had built it, I measured the voltages as below:

OA: 69.2 V
OB 70 V

but now both have dropped to ~ 66 V. And there was no replacement of tubes or anything with that (unlike the crack).. Note that all the 4 LEDs light up just fine and there are no issues with the sound. Why do the voltages shift like this and is it something to be worried?

Thanks in advance!
Atul

 

[Paul Birkeland]

In the SEX amp with the C4S load, the plate current and cathode bias voltage are fixed and the plate voltage wanders.  Lower plate voltage in that situation means more emission, so the drop from 70V to 66v indicates that the tubes are breaking in and working better than when you first tried them.

In the Crack, the 6080 grid voltage depends on what the 12AU7 is up to, so the voltages you have don't really mean much without knowing the voltages on terminals 1 and 5.  If the voltages on the plates of the 12AU7 do not move but the cathode voltages on the 6080 increase dramatically, that is also a sign that the 6080 emission is going up a bit.  If you find that OA and OB on the large board go down and get really close to OA and OB on the small board, that's a dying 6080.  If you find that the OA and OB voltages go up on the big board and also that the OA and OB voltages on the small board go up, that has more to do with the 12AU7 wearing a bit.

 

[atul.mehta6]

Thank for the reply Paul. Good to know that the S.E.X is performing better every day. Coming back to the Crack + SB, I replaced the tubes to what I had originally when I did my initial tests. These are the voltages I see:

T1 --> T5: -3.4 V (moves a bit around that number 3.41 - 3.45)

Large Board:
  OA: 110V
  OB: 111.7V
  G:  0V
  B+: 182 V

Small Board:
  OA: 182 V
  OB: 182V

I am attaching a pic of the build if it helps..

 

[Paul Birkeland]

Large Board:
  OA: 110V
  OB: 111.7V
  G:  0V
  B+: 182 V

Small Board:
  OA: 182 V
  OB: 182V

This is an impossible set of voltages.  The OA and OB on the small board won't be 182V.

 

[atul.mehta6]

When you're right, you're right Paul..

I remeasured across the small board and I see
OA: 82V
OB:  78.6V

The voltages on the large board remain more or less the same.

Dont really know what happened when I measured earlier.. Maybe I need glasses