On Sunday, 29 December 2019 19:29:49 UTC, wrote:
A little over a year ago, or was it two, I bought a Nobsound tube
amp. The amp is a type A and operates in the ultra linear mode. It
uses 1 5U4C Russian rectifier tube, two Chinese 6N9P tubes and two
EL34B tubes. I don't kinow if they are Russian or Chinese.
When I first powered it up everything went smoothly but after maybe
an hour or so it made this loud static like noise. I promptly turned
it off.
Deciphering the directions I realized the amp was made to operate on
110 volts, not the 125 typical in my house. Oops. So I wired up a
transformer in buck configuration to get 110 volts and tried the amp
again. And it has operated fine since.
Until a few days ago. Once again the loud static noise. Changing
the volume changed nothing and I shut the amp off. After it cooled I
pulled each tube out and then re-inserted it thinking that maybe a bad
connection may have been the problem.
After again powering the amp up it is once again performing
flawlessly. So what gives?
I have read about red plating, how it damages tubes pretty fast,
and am thinking that maybe I damaged the tubes running them at the too
high voltage for a while. But why does the amp sound great after it
has a chance to cool off? Since the latest incident I have run the amp
for about 4 hours straight so I don't think the problem is heat
related. I mean just because the amp is hot it doesn't necessarily
mean it is going to act up.
Both times the amp made this noise it was getting its input from a
built in bluetooth reciever. I thought this might be the problem but
since the noise is volume control insensitive it seems to me that the
problem is probably unrelated to the input.
The noise is LOUD too. I mean hurt your ears loud. I never turn the
volume up as loud as the static noise was. When I say static I mean
scratchy crackling noises, not white noise. Kinda like if someone was
dragging one wire across another.
Unfortunately I could not tell if the noise was coming from both
speakers. I just ran over and turned it off the first time it
happened. The second time I tried turning down the volume and then
turned it off.
Should I be shopping for tubes? And could it be any of the tubes?
There are no tube testers local to me. The closest I know of is an
hour and a half drive away.
Thanks,
Eric

Most likely cause is a bad connection somewhere. The fault is most likely after the volume ctrl, though it might be on a secondary supply line. Connections can be prodded with something insulating. A resistor in series with the speaker would keep the volume & Pdiss down. Keep an eye on supply current that it doesn't get excessive during fault.

A failing tube, cap or carbon resistor are also possible. Less likely deteriorating insulation. Comparing the voltages in the 2 channels while it faults might help home in on where it's going awry.


NT