Low level amp hum

[jpervin]

I have a Tungsten Crema Wheat that recently has developed a low level hum after it has been on for about 20-30 minutes. It's inaudible when I'm playing but will gradually fade back in when not playing and left on. If I turn the standby switch off the hum will stop. Turning the standby switch back on and the hum will gradually come back.
FWIW, I substituted the stock JJ GZ34 rectifier tube for a ANOS Sylvania 5V4G to get a bit more warmth and sag. I've had the Sylvania in the CW for about 3 months. The hum started a couple of weeks ago.
The other tubes are Marconi 6V6Gs and RWN Neuhaus 12AX7s, which were all NOS when I installed them.

What's the best way to diagnose and fix this problem?

Thanks!

 

[Darkness]

It's time for some tube swapping goodness. You know the drill, start with v1 and move on down the line until the hum stops. If it still hums after swapping ALL tubes with working, non-microphonic tubes, then it's time to call Adam at Tungsten. His customer service is top notch. He's also a cool cat to talk to.

 

[jpervin]

It's time for some tube swapping goodness. You know the drill, start with v1 and move on down the line until the hum stops. If it still hums after swapping ALL tubes with working, non-microphonic tubes, then it's time to call Adam at Tungsten. His customer service is top notch. He's also a cool cat to talk to.

I'll try the tube swapping thing.
In the meantime, I think I may have found at least part of the problem. I'm almost always playing through the Bright channel, Input 2, and don't usually turn up the Normal channel volume. I tried turning up the volume on the Normal channel and it started to hum and the hum got louder as the volume increased. By the time I got to around 10 or 11 on the dial, the CW was howling like a herd of cats in heat!
I have a feeling I'm gonna be giving Adam a call. But I'll try swapping tubes and see what happens.

 

[DT7]

I'll try the tube swapping thing.
In the meantime, I think I may have found at least part of the problem. I'm almost always playing through the Bright channel, Input 2, and don't usually turn up the Normal channel volume. I tried turning up the volume on the Normal channel and it started to hum and the hum got louder as the volume increased. By the time I got to around 10 or 11 on the dial, the CW was howling like a herd of cats in heat!
I have a feeling I'm gonna be giving Adam a call. But I'll try swapping tubes and see what happens.

Sounds like a shorting jack at the Normal channel input isn't shorting.