#124 Questions

[jfs322]

*Deleted*

[Bombacaototal]

Weirdest thing ever, was playing the amp, noticed a bit more hiss then usual, then an increase in hiss and the amp went quiet but pilot was still on. I opened it up, measured the HT fuse and it was fine, still replaced it, replaced the mains fuse, measured all diodes which were ok. I noticed voltages were higher all across, I replaced the bias cap. Then I realized that the tubes were not glowing at all, none of them. I measured the heaters in each tube and I get about 3.2VAC on each side from the tonestack all the way to the last power tube. I don't have a filaments fuse and my PT has a CT which is grounded. I am honestly so confused, how can I have heater voltage but not light up the tubes?

I then went on to remove all tubes (pre and power). Then added new power tubes without the preamp tubes, and it's the same.

Has anyone ever encountered a similar issue?

[bepone]

not possible to have heater voltage ok and in the same line tubes (None of them) not glowing. this would imply that sockets are shitty. but all of them in the same time? not possible.

[sluckey]

Not weird at all. With a center tap connected to ground it's possible to have an open circuit in one heater lead between the PT and the first tube socket and still measure 3.15VAC from each heater pin to ground. But if you measure from pin 2 to pin 7 of a 6L6 you will see that you do not have 6.3V ***ACROSS*** the tube so it can't light up.

Since pilot is lit but no tubes are lit the open circuit is between the pilot and the first tube in the string. Most likely a bad connection at the pilot light assembly or at the first tube in the string. Less likely a broken wire.

Take home... Always measure filament voltage ***ACROSS*** the tube pins, not from a tube pin to ground.

[martin manning]

Take home... Always measure filament voltage ***ACROSS*** the tube pins, not from a tube pin to ground.

But it's ok to measure from each side of the parallel string to ground if all of the devices on the string have been removed.

[Bombacaototal]

Many thanks all for the suggestions, yes must be an open circuit somewhere. My pilot is actually wired at 240V from the mains, so the heater issue would not show there. I will definitely check between the PT and the first power tube and will report back!

Thanks for the advice on measuring the voltages if the filaments across.

[sluckey]

Many thanks all for the suggestions, yes must be an open circuit somewhere. My pilot is actually wired at 240V from the mains, so the heater issue would not show there. I will definitely check between the PT and the first power tube and will report back!

Since the pilot is not on the filament string, it's possible that the open is inside the PT. I suggest measure for 6.3v directly across the PT filament leads first.

[Bombacaototal]

Just measured the filaments coming off the PT at the socket, PIN2 and 7. I'm getting 6.28VAC

As I was pressing with the probe the sound came back, for sure a cold joint


Resoldered the PT filaments at the first tube socket and all good now. Thanks for the learning everyone