SWR SM 500 valve change ?

[Nostromo]

Hi Basschatters,

Is there anyone out there who understands the circuitry of and has changed the 12AX7 valve inside a SWR SM500 amp ?

As I understand it this amp only has the one valve . . .which I guess must be part of the pre amp side of the circuitry.

My SWR SM500 has been mega reliable since new and is working just fine at the moment . . . but its getting on in years and has had one hell of a lot of use . . . . so I cant help thinking it must be about time to change the valve in it before it gives up and breaks down half way through a gig ? . . . .

Im sure, that if it had been a full valve amp then the valves would have needed changing a long long time ago ?

I was just wondering how easy a job this is, and whether its just a question of literally unscrewing all the little screws around the edge of the top panel, and then removing the top panel, changing the valve, and then screwing the top panel back into position ? . . . or . . . . . whether there is any electrical setup / bias / trim setup adjustments to be undertaken or checked if/when the valve is changed ? ?

If its the former, ie, a simple matter of physical replacement, then I'm minded to do it myself . . . . but I dont have a proper understanding of all the bias stuff that applies to some valve amps ? . . and so, if its the latter, what I dont want to do, is to inocently renew the valve (but not understand or undertake any trim adjustments) thinking I'm gonna be giving it a new lease of life . . . only to find I've possibly made it less reliable than it currently is ? . . or possibly making it run hot or something - which I've read in the user manual can happen if the valve goes away from its correct bias setting ?

At the moment it is not running hot (well not for an SWR SM 500 anyway . . . as they do run warm. Mine is running at the same temperature it always did since new)

Anyway . . . . any advice very much appreciated.

Cheers,

[obbm]

Pre-amp valves need no setting-up or adjustment.

With the amp switched off and disconnected from the mains,

remove cover
unplug old valve
plug-in new valve
replace covers
job done

Just because it is old and has been used a lot doen't mean it's worn out. These types of valves usually go on for decades.

[Nostromo]

Hi obbm

Thanks for your concise advice.

If you were me, would you bother to change it ? . . . . . I've been trying to work it out, and I reckon the amp has done about 2500 to 3000 hours work probably nearer 3000 ? . . . mainly running in mono mode into a 4ohm load so getting warm.

Cheers,

[obbm]

[quote name='Nostromo' post='774619' date='Mar 14 2010, 07:55 PM']Hi obbm

Thanks for your concise advice.

If you were me, would you bother to change it ? . . . . . I've been trying to work it out, and I reckon the amp has done about 2500 to 3000 hours work probably nearer 3000 ? . . . mainly running in mono mode into a 4ohm load so getting warm.

Cheers,[/quote]

Firstly how you run the output stage has no bearing on the pre-amp valve, it's just a stage in the pre-amp. To give you an idea there are valves made in the 1950s and 1960s that are still performing well today after years of use, in fact some are highly desirable.

If the amp is reliably delivering the sound you had when you bought it then don't mess with it. If you are unsure then you could always get another valve and try it - I do it occasionally - and if it's better then you've cracked it. If there is no change the at least you have a spare for the future.

[BassPimp66]

Hi,

I own a SWR SM 400 and I have changed the original Groove Tube valve. It's a 5 min job.
I have changed to something supposedly more vintage sounding , but I can really tell the difference to be honest.

I would advise you buy a valve in case yours die. But, if ain't broken, don't fix it.

Cheers,

BassPimp