Tube Noob troubleshooting question

[Erik Tracy]

I have an older Audio Note OTO Line SE which I have swapped back into my system now that the weather is cooler.

It worked fine when I got it used months ago - then when the weather got hot, I decommissioned it to listen to my Luxman L509X.

This morning with everything back in and warmed up for 30 minutes - nothing, nada. All connections checked, rechecked, verified.

The tube compliment is
x1 12AU7/ECC82 (NOS Mullard)

x2 12AX7/ECC83 (Platinum Gold Lion)
x4 EL84 (Sovtek)

All tubes were glowing and warm except the 12AU7.

Should that tube glow and be warm after 30 minutes, or has it possibly gone bad?

I have no backup tubes to swap in to try. But, I may have to get a second set.

Thoughts? Things to try?

Thanks!

 

[Al Casey]

Remove the 12AU7, clean the pins and put it back.
If its filament is not glowing, replace the tube.
By the way, never move a tube amplifier while the tubes are still hot, as this could cause internal damage.

 

[Ken Boyd]

If you have a VOM meter you could check the continuity of the heater filament.

 

[Erik Tracy]

I pulled the 12AU7 and took some shots


The "back" looks discolored


Pins cleaned with deoxit, put back in socket - nothing.

 

[ogodei]

The discolored back is normal, thats called getter flash.

Something simple? Verify your connections into the amp again and make sure you've selected the correct source.

Upstream cables all connected? What else did you change during warm weather?

 

[Erik Tracy]

If you have a VOM meter you could check the continuity of the heater filament.

So pins 4 & 5 separately to 9?


With the 12AU7 back in - still no glow or warmth after 20 minutes

 

[Erik Tracy]

The discolored back is normal, thats called getter flash.

Something simple? Verify your connections into the amp again and make sure you've selected the correct source.

Upstream cables all connected? What else did you change during warm weather?

Every cable is good, every connection verified. Multiple sources (CD and TT) have no sound.

 

[Palustris]

The heater should glow after one minute. Without a schematic it is difficult to know how the 12AU7 is connected to the heater winding. The easiest remedy is to install a new 12AU7.

 

[Lavane]

The clear top 12AU7's I have don't show much of a glow at all. I can barely see it at the top of the tube. The filaments are buried inside the structure pretty good. At first I thought one of them wasn't lighting up at all. I had to put my hand around the tube to get it dark enough to even see it.

 

[Erik Tracy]

The clear top 12AU7's I have don't show much of a glow at all. I can barely see it at the top of the tube. The filaments are buried inside the structure pretty good. At first I thought one of them wasn't lighting up at all. I had to put my hand around the tube to get it dark enough to even see it.

After 20 minutes, the tube wasn't even warm to the touch (with power off).

 

[Erik Tracy]

New 12AU7A on order from Tube Depot and already got a shipment notice.

Wait and see.

 

[Ken Boyd]

So pins 4 & 5 separately to 9?
View attachment 3646585

With the 12AU7 back in - still no glow or warmth after 20 minutes

That is correct, continuity should be good, open is bad. First test, pin 4 to pin 9 and then check pin 5 to pin 9.

 

[Al Casey]

@Erik Tracy
What is the result of the continuity test?
Remove the output tubes (EL84) and try the following:
- Replace the 12AU7 with one of the 12AX7s and check which one lights up
- Turn off the amplifier and let us know

 

[Erik Tracy]

That is correct, continuity should be good, open is bad. First test, pin 4 to pin 9 and then check pin 5 to pin 9.

I read 7.2 ohms on both pin 4 to 9 and pin 5 to 9.

 

[Mr Boochie]

I read 7.2 ohms on both pin 4 to 9 and pin 5 to 9.

Are you getting 6.3VDC across pins 4 & 5 and 9 of that 12AU7 socket? Measure with the tube out.

I presume the 600R resistors on the 12.6VDC rectified heater line drop the voltage to 6.3VDC.

 

[Al Casey]

I read 7.2 ohms on both pin 4 to 9 and pin 5 to 9.

If, after swapping the tubes, the 12AX7 does not light up in the 12AU7 socket, the problem may be due to a poor connection at the socket.

 

[Erik Tracy]

I'll get back to you on those.

The day has moved on to Christmas lights.

Thanks to everyone here for the help - much appreciated!

 

[Mr Boochie]

If, after swapping the tubes, the 12AX7 does not light up in the 12AU7 socket, the problem may be due to a poor connection at the socket.

Check the leads from the PSU. Check the solder points on the 12AU7 socket. Do a wooden chopsticks test with the DVM hooked up and see it the voltage jumps with any tapping.

Good luck!

 

[Al Casey]

Are you getting 6.3VDC across pins 4 & 5 and 9 of that 12AU7 socket? Measure with the tube out.

I presume the 600R resistors on the 12.6VDC rectified heater line drop the voltage to 6.3VDC.

The voltage of the small tubes heaters are 12.6V; the 600 ohms are the cathode resistors

 

[gadget73]

yeah 4 to 5 so 12v. What an annoying way to draw a schematic.