High pitched sound from the amplifier

[troplin]

Since a few weeks I can hear an intermittent very high pitched sound from the amplifier itself (not from the speaker). As far as I can say it only happens after the amp has run for a while. It's not very loud but enough to hear it from my hearing position which is quite near.
Otherwise it sounds fine, but this bothers me.

Any pointers/hints where to start?

 

[Paul Birkeland]

I would start by checking the DC voltages listed in the manual, just to confirm that the amp is operating properly.

-PB

 

[troplin]

Side A:
2: 89.6 V (OK)
3: Started at 18V but then rising up to 25 V (Should be 15-18V)
4: Rising up to 10.5V where it remained somewhat stable (Should be 0 V)
5: 0 V (OK)
6: 366 V (OK)
7: 381 V (OK)
10: 350 V (OK)

Side B:
16: 80.8 V (OK)
17: 15.57 V (OK)
18: 0.012 V (OK)
19: 0 V (OK)
20: 372 V (OK)
21: 388 V (OK)
24: 369 V (OK)

Heater:
H2/H5: 3.125 V (OK)
H4/H7: -2.937 V (OK)


So there seems to be a problem on side A, especially Terminal 4. Last time I measured these voltages both sides were much more consistent.

 

[Paul Birkeland]

3: Started at 18V but then rising up to 25 V (Should be 15-18V)

There's a 249K resistor between 4 and 5, is yours well connected?  Terminal 5 is at 0V, and the resistor between 4 and 5 will force 4 to 0V.  I would guess that this resistor has come loose.  The other option is that the 0.1uF coupling cap is leaking and no longer doing its job properly, but this would be the first bad coupling cap I've encountered.

The problematic voltage that you have at 3 is a result of the issue you have at 4, it doesn't need to be focused on or diagnosed separately.

 

[troplin]

The 249 K resistor connections look and feel solid. The Resistance between Terminal 4 and 5 is spot on 249 KOhm when the amp is cold. I made sure to measure at the bottom lug of the terminal strip s.t. I don't touch the resistor leads directly.

I took some more measurements and they are interesting:

• The voltage at Terminal 4 actually starts at 0 V but is then climbing slowly and steadily. After maybe 5 minutes it was at 7 V, and still counting. I didn't wait for it to reach 10.5 V
• After turning off the amplifier I was measuring the resistance between 4 and 5 again, and it was maybe 150 K, but quickly climbing up to 249 K. It was probably even lower at first because I waited a few seconds before taking the first measurement. At that time there was no voltage left between 4 and 5 that would skew the measurement.

If this was a bad joint I'd expect a more erratical behavior and not a slow and steady climbing voltage.

The coupling cap (That's the smaller one with the yellow seals on the side of the terminal strip, right?) looks fine from the outside. How do I check if it is still ok?
[EDIT]I just remembered that my multimeter has a capacitance mode: The cap measures 112.5 nF which I guess is ok. The capacitor on the B side measures 113.5 nF for comparison.[/EDIT]

 

[Paul Birkeland]

You can just remove one end of the cap from its terminal, then fire the amp up again and see if terminal 4 behaves properly.  You should also swap tubes to see if that non zero voltage moves.

 

[troplin]

You should also swap tubes to see if that non zero voltage moves.

Yes, that's it. The bad voltage follows the tube. I guess I can trash that tube now. Any idea what's wrong with it?
That's already the second tube going bad since I built the amplifier two years ago. I didn't expect to wear out one tube per year. Is that just bad luck or could there be something wrong with the amplifier that isn't visible in the voltage checks?

 

[Paul Birkeland]

It's more than likely bad luck.  I have gotten 40,000+ hours out of these tube types before they started going bad. 

There are some conditions that can cause weird deaths like this.  It would have to be a connection that's just tight enough to work most of the time, but not so solid that it works reliably in the long term.

For example, if the 680 ohm resistor mounted by each 12 pin socket isn't well connected on one end or the other, tube damage (and amp damage) can occur.  The same goes for that 249K resistor, and the ground connections that tie them into ground. 

Oddly enough a leaky coupling cap is a little less menacing, but it would indeed eventually cause some permanent tube damage as well.

If you notice that tubes are dying in the same side of the amp every time, that's an indicator that there's some intermittent issue in the amp.