New amp day (Vox Night Train) - Strange buzz, need input please

[C-Martin]

Hey guys, I just scored a demo Vox Night Train through Sweetwater. This little amp rocks, however i think it has an issue. I know some of you guys have one or have at least tried one out so I'd like to have your opinion on this. I'll try to explain it as best as I can.

When fully powered on, the gain is above noon and the volume is about 10%, I put it back on standby and it takes the amp about 10 seconds to go completely mute as any tube amp does. Once the amp is completely muted there is a buzzing sound that emerges and raises in volume for about 5 seconds and then is sustained for about 15 seconds before it goes away and then the amp goes back to being completely muted. I also noticed that when I play with the volume knob the noise raises or goes away, oddly when cranking it to max the noise goes away but when lowering it to 0 the noise gets louder. I hope there isn't something wrong with it, i don't really want to have to ship it back but i will if the amp is defective. The buzz isn't that loud, but it is loud enough to notice it and has gotten me worried.

I've already contacted Sweetwater about it and a tech is supposed to contact me but i thought I'd ask on here and see if this isn't just normal behavior for lunchbox amps. This is the first mini amp I've ever owned. What do you guys think about this?

 

[Sloan]

Are you using the tubes that shipped with it?

With any tube amp, tubes are the first suspect.

 

[C-Martin]

Are you using the tubes that shipped with it?

With any tube amp, tubes are the first suspect.

Using the stock tubes, yes. I've never done tube swaps on an amp myself so i never touch that. Ive always been told it's dangerous and should be left to a specialized amp tech because of re-biasing required.

 

[jangoux]

the Vox NT is cathode-biased, no need to rebias EVER. Just swap the tubes and u're ready to go

 

[C-Martin]

the Vox NT is cathode-biased, no need to rebias EVER. Just swap the tubes and u're ready to go

Sweet! that's good to know, thanks for the info. That makes 2 amps i own that i don't need to haul to a tech to do tube changes. Night Train and my Mark IV (which apparently is self biasing, or so i read somewhere).

 

[Sloan]

^yes!
Browse the internet for more info on tube guitar amps and educate yourself - you will not regret it.
- Biasing is like setting the idle on a car, you don't want it too high or too low.
- You only have to bias output tubes (el84, el34/6ca7, 6l6, kt88, etc..) on amps that are fixed bias. Generally Class A amps, like most of these amps in the small amp craze, are cathode biased (self biasing).
- You do not have to bias preamp tubes (12ax7/ecc83, 12at7, etc...) - it's pretty safe to actually even swap these out with the amp on for troubleshooting purposes. Just keep the amps volume down as there will be pops which could possibly damage a speaker if too loud.

 

[jangoux]

The mark has a fixed bias - there is a resistor in place of a trimpot for biasing. Well, you dont NEED to rebias, but having it modded would probably improve your tone, just like on Rectos and 5150s that have fixed bias. Mesa stock bias is usually too low. Btw, the cathode biasing on your NT is another thing, no need to worry about that