Tube amplifier loses bias on one tube and starts humming.

[Matze]

So far, I have been very satisfied with my Destiny eXperience MKII. However, I have now noticed a hum on the left channel. I thought it might be due to bias. While measuring and adjusting, I found that the third KT88 tube has no bias. What could be the reason for this?

I have also discovered that the humming occurs as the tubes warm up. When measuring bias while the tubes are not fully warm yet, there is voltage present on the third KT88, which continues to rise as the temperature increases. As long as there is voltage on the third KT88, there is no humming. Only when all the tubes are nearly warm does the humming occur, and at that point, the third KT88 also has no bias voltage. Additionally, I have read that perhaps a preamp tube could be responsible for the behavior. Currently, the amplifier is equipped with 4 x KT88, 2 x 12AX7, and 2 x 6N8P tubes.

 

[OldHvyMec]

Swap a valve that will bias with the one that won't. If the valve that wouldn't bias still won't in another position, the valve is bad. If you
can't bias that position it's likely a problem with that location and will need repair. You can swap left/right with all the other valves. Swap the
small valves one at a time right to left and see if in some way they are causing the problem. I'm not looking at a schematic so I'm limited.

Bias down the valves before swapping and then back up after the swap. 20 minutes tops you'll know if its a valve vs a circuit (usually a resistor)
for the valve that has failed.

You can lose a power valve that can damage the circuit also, BUT once again the old valve swap will confirm one, the other or both.

A 5 minute warm up is all that's needed for a bias check and you can get picky with the bias once you've found the problem. I alway bias LOWER
with new valves and make sure they hold for at least 24 hours. Then bias where I want. I tend to bias on the higher side for lower listening levels
and bias lower for reference level playing.

If you suspect a circuit issue remember there are some HIGH voltages when plugged in. Unplugged many times caps that aren't discharged will
give you quite a jolt. Discharging caps can be exciting for a rookie. They can offer a pretty good pop if your not ready. I use a 40 watt bulb,
alligator clips and a switch to avoid arching at the cap connections. I HATE arch/burnt spots inside a case, it always makes me wonder.

BE CAREFUL if your not familiar, the one handed approach (one in your pocket and one hand you work with) might still bite BUT it's path is not through
your heart. I'm not a fan of getting zapped AT ALL.

Happy hunting.

 

[Matze]

Thank you for the quick advice. I could have figured out the tube swapping on my own too. Sometimes, however, one is like an ox in front of the mountain... Fortunately, there are such good forums where even with such specific topics, help is quickly available. For now, I swapped tube 3 with 4 and cleaned the contact pins in the process. I thought that couldn't hurt ;-) Interestingly, the humming is completely gone, and even after the tubes fully warmed up, it doesn't return. Even after readjusting the bias, the humming stayed away because all four KT88 tubes still have bias voltage.

I will remember your procedure. Also, thank you for the information about the level of bias voltage. I once experimented with it and found that the tube amplifier sounds brighter when the bias is high, and softer at the lower end, conversely. Now I understand that it probably also relates to the listening volume.

I will test it over the next few days and see if the humming returns or has been eliminated.

 

[solderdude]

Some tube sockets do not 'clamp' the pins hard enough or good enough.
Have no idea if this is the case here or not but seems to have been a 'contact' issue.