should a valve look a bit blackened?

I got a valve pre from China on Friday and is sounds nice, but today it developed a strange problem where the left channel does not go completely quiet when the volume is at minimum but the right is quiet, and the right stays quiet while the left rises as you turn up the dial and then suddenly the right kicks in.

Anyway I had a look inside, dont know much about these things but one of the valves looks a bit burned. Would this cause the problem or will it be the volume pot?

 

does the valve look brown anywhere ....or is it just silvered near the top.

What you desribe is a cheap volume pot [maybe !] one channels track is out ....though this doesn't discount a faulty valve.

get a set of decent replacment valves ....what are they by the way ?

I had a chinese int valve amp ...the driver tubes died with in a month! uk purchased groove tube valves although costing as much as the amp where still working perfectly after 18 months.

 

Apparently the valves are:

6N3
6N11 x 2
6Z4
6N1

The amp is a Xiang Sheng 708B

I think it's the 6N1 that looks a bit burnt inside.

 

Swap the tubes across the channels, if it moves with the tube you have a tube problem, if not its something else

 

You talk much sense ... though it appears there is only one of said tube

 

Yeah, not sure that would work.

Anyway, I think the main problem is as Zanesh said the cheapo volume pot. Today it seems to be working a bit better, the right hand channel is actually the one that cuts in first. Something must be wonky inside the pot I think.

btw to change the pot does the volume knob just pull straight off?

 

It's normal for there to be some blackening/silvering inside the glass envelope. This is a feature not a bug. There's a sacrificial bit of metal inside, often magnesium, called a getter. This vaporises a bit and reacts with any stray gas around, such as oxygen liberated from the glass, whereupon it plates out. This is to preserve the good vacuum so that the electrons flying around can do their thing instead of hitting gas molecules.

 

Ah yes. D'oh. Just pull them all out and jam them in to a different socket. Usually works for me.

 

Is that safe? They wont blow up will they?

 

they might if they are power amp tubes, pre-amps you can swap like for like.