My Ampeg SVT Cl is doing weird stuff

[FletcherML]

Hi guys,
I've got an Ampeg SVT CL that is having some weird issues. I'm wondering if it's related to the tubes.
I replaced my tubes a couple months ago and have pretty much not gigged at all since then.
When I turn it on, it makes a hum sound for about 15 seconds. No matter how long it is on standby first. If I start playing, the hum goes away. My initial reaction is that the tubes aren't warming up as fast as they should.
Any thoughts?
Replace all the tubes?
Set it on fire?
I leave for a month and a half long tour soon. Don't want it dying on me then.
Thanks y'all.
Fletcher

 

[Floyd Eye]

Are you sure it isn't the fan? It's pretty common for the fans to be a little noisy when you first turn the amp on.

 

[hodgy]

Is it properly biased?

 

[Jerrold Tiers]

Sounds a bit like one of the stages is getting a big pulse when the high voltage is applied, possibly due to a problem with the coupling capacitor circuit somewhere... I suspect a resistor from a grid to ground may have a loose connection.

What may be happening is that the capacitor has noplace to send the charge that it gets when the voltage rises.... and the result is a temporary DC offset which cuts off a tube and you hear the power supply "ripple" as hum.

After you play it for a bit, things can even out.

Do you notice any distortion right when you start playing? Especially if you don't wait for the hum to go away.

 

[FletcherML]

Hey I checked for distortion, it's definitely not there. Just sounds normal through the hum. I don't know anything about the electronics of this stuff so I won't be able to fix that alone. Sad.

 

[JimmyM]

Doesn't sound super expensive to fix if that's the problem at least.