Pre Amp Valves. SORTED

professorben

So I think I need a couple of new Preamp valves for my Mesa Studio Pre

Its making a whistling high pitch noise when I'm not playing through it, this is only on the lead channel and predictably only at high gain settings. 

Sooo
Any suggestions on types and places to get?

4x 12ax7
1x 12at7 I'm guessing this drives the reverb tho?

@icbm I know you are familiar with Boogies. 

Any advice?

Cheers

ecc83

Until IC stirs! Don't buy anything yet. Swap the 12AX7s around (number them with a sharpie) since it will only be one that has gone miccy, if indeed the problem IS a valve.

Yes, the AT7 is probably in the reverb circuit. Don't plug it in anywhere else and don't put an AX there* just removing it will eliminate it.

*I am probably being over cautious and the Mesa has properly over rated loads but I do not advocate swapping valve types willy-nilly. ICBM will no doubt comment.

Dave.

ICBM

Yes - it's unlikely it's more than one valve. I wouldn't just change all of them for no good reason. It will be one of the 12AX7s. As Dave said, simply swapping them around may fix it, if the problem valve is then in a less sensitive position.

If they're all a bit microphonic and you can't fix it like that you need a new valve. I would buy a standard Chinese 12AX7 of any brand - they're cheap, reliable, and sound good. The Studio Preamp doesn't have any special requirements like 'cathode follower' positions which can make some types less suitable. Simply replace each 12AX7 in turn with the new valve and see if the problem stops.

It's also possible that it isn't a valve, although not that likely - so I wouldn't spend a lot of money on a full set of valves until you know that.

digitalscream

@professorben - I've got a few spares, if you just want to try a fix. I should have a 12AT7 somewhere, too.

professorben

@ecc83 @ICBM ;
i swapped the valves over from one position to the next, no difference.

@digitalscream yeah if you have a coup,e of spares I'll have em off you, btw that Velocity is available as and when.

digitalscream

professorben said:

@ecc83 @ICBM ;
i swapped the valves over from one position to the next, no difference.

@digitalscream yeah if you have a coup,e of spares I'll have em off you, btw that Velocity is available as and when.

OK, cool - I'll pop over to yours at some point. Mainly because I can now.

professorben

digitalscream said:

professorben said:

@ecc83 @ICBM ;
i swapped the valves over from one position to the next, no difference.

@digitalscream yeah if you have a coup,e of spares I'll have em off you, btw that Velocity is available as and when.

OK, cool - I'll pop over to yours at some point. Mainly because I can now.

Awesome, ill pm you my deets.

DJH83004

The 12AT7 is the reverb drive / return, Mesa combine this into one valve rather than Fender who use two. You can safely remove this just to eliminate anything in the return circuit. As a start point, rather than swapping the 12AX7s around, you should really replace them one at a time with a known good valve. Then its a case of splitting the circuit down to isolate the fault. From memory the graphic equaliser section is solid state, do the sliders have any effect on the HF noise?

professorben

DJH83004 said:

The 12AT7 is the reverb drive / return, Mesa combine this into one valve rather than Fender who use two. You can safely remove this just to eliminate anything in the return circuit. As a start point, rather than swapping the 12AX7s around, you should really replace them one at a time with a known good valve. Then its a case of splitting the circuit down to isolate the fault. From memory the graphic equaliser section is solid state, do the sliders have any effect on the HF noise?

No only the pre gain volume and tmb controls. 

professorben

Ok so rather embarrassingly I've sorted it, after swapping each valve out in turn, and checking, I just thought......oh did I flip the ground switch???


yep, lifted the ground on the preamp and all is sorted.