Just found! SWR SM-400! Questions!

[Stompin Tom]

Well, my friend's downstairs neighbors moved out in a hurry and left a bunch of garbage. There was a surprising amount of musical equipment in there, and my friend poached as much of it as possible. Amongst the stuff was this:



He didn't want it, and I play bass in his band... so, now it's mine. I plugged it in and, wow, I can't believe it, it works! Sort of. None of the pots or sliders really work very well, so I decided to take it apart and spray the pots with some deoxit.



I get it apart easily and spray the pots, but then I start noticing that someone had duct-taped some wires... weird, huh?





What's going on here??

It suddenly hits me. Half of the power amp is missing!!



And the top cover has some char marks above where the second power amp was.



My guess is that half of it blew, and the repair guy just pulled out the burned out power amp. The thing's designed to run in stereo or bi-amped @ 200 watts or bridged at 400 watts. Aside from the dirty pots, it worked fine at 200 watts... Now for the questions.

Do I need to worry about this?!? Is this a common failure on this amp? What might cause this and can I possibly rely on this thing??

Any idea if I can drop another power amp unit in there (from, say, a fried sm-400)?

Is this amp worth it? Anyone still using this model and liking it? I play pretty aggressively with a pick and a little distorto in a rock/punk band . Any suggested settings?

Thanks!

[lowfreqgeek]

Yep, you really, really need to worry about it.

In fact, you should get it out of your house ASAP!

Pack it up and I'll send you instructions on how to dispose of it properly. It can only be saved by a cleansing trip to the Southwestern Desert of New Mexico.

Hurry...

[Stompin Tom]

Ha ha. I guess you like the sm-400?

[lowfreqgeek]

Well, I had an early, early, Studio 220 - which was basically an SM-400 without the second power amp (much like what you have) and way underpowered. Then I stepped up to a Bass 350 (actually, it was a "Basic 350", which was renamed after the first year of production), which I had for many years. I eventually moved on to bigger, better stuff. But I acquired and moddified another Bass 350 last year. The "SWR Sound" isn't my thing all the time, but those are just such cool amps that it's fun to have one around. As long as the preamp is working, you can make good use of it.

As far as a free amp goes, keep it and use it in the studio, as a preamp, or for practice. It WILL BE underpowered for most things as it is now, but that doesn't mean it's useless.

Of course, if you decide you don't want it, I'm sure there are plenty of people around here who could figure out some use for it as is.

[barbe-et-hache]

I have a swr sm400s, I paid 400US$ for it but both side are working.

Have fun with it.

[Stompin Tom]

200 watts is not enough? Really?? I've been using a 100watt combo for practice since... high school. Which was a long long time ago. In most situations, it's also been loud enough at gigs. Just lacks a little low end kick, which I blame on the shockingly small cab the 15 in speaker's in. I guess I"ll just have to try it out and see for myself!

I'm also wondering if the 'swr tone' is for me... but i'm not really sure what the swr tone is, considering how much flexibility the preamp seems to have.

[okcrum]

Yeah one power amp is missing. Also, if you plan to gig with that, plan to stick a fan on top of it. The heatsinks are undersized, and short of hacking a hole in the back panel for a smaller fan a la Eden, you're going to need the extra cooling. Those babys run hot.

[Alex1984]

The SWR tone is very modern, quite scooped, but those power amps are sweet and very neutral. The SM-400 will be clean through and through. They run warm, but that's what they're intended to do, the entire case was supposed to act as a heatsink.

The pre-amp is very high quality stuff, but probably not your thing based on your genre.

[duff2]

Great find Tom! Can't beat the price!

Now find someone competent with a soldering iron and get 'em to perform this simple mod:

SWR Aural Enhancer Bypass

After that you will have yourself one sweet, full sounding amp that will no longer seem underpowered. Need some grind in your tone? After the mod you can crank the gain and the aural enhancer, and you can get all the 12AX7 overdrive goodness your ears desire. If your speaker cabinet has a horn tweeter, make sure to turn it off or else the grind will sound like shite.

Good luck!

Duff2

[Stompin Tom]

Very cool Duff!! Now I know so much more about the amp and I'm sure I don't want that scoop in there. Not my thing at all. I've worked on amps before and built effects pedals, so that simple mod will be easy. Thanks for posting!

[barbe-et-hache]

Originally Posted by Alex1984

The SWR tone is very modern, quite scooped, but those power amps are sweet and very neutral. The SM-400 will be clean through and through. They run warm, but that's what they're intended to do, the entire case was supposed to act as a heatsink.

The pre-amp is very high quality stuff, but probably not your thing based on your genre.

+1
don't worry about it being hot, it was made this way, no need for an added fan. I even removed the fan that was in my SWR SM400S and I was playing in a metal band with a loud drummer.

[KUNGfuSHERIFF]

Very cool. Wish I would have known about that 10 years ago when I was still running a PB200. I always cranked the crap out of 200hZ in order to get a decent sound. Now I understand why.

[Stompin Tom]

So, any suggestions on a 12AX7 that sounds good in this preamp?

Also, if anyone happens to have a broken sm400 laying around, I'd be interested in it for parts!

[T-Bird]

Hi.

Cool find.


Get the (still) working amp biased in order to avoid the fate of the other one.

I had a SM400 back in the day, liked it, but sold it to finance some other purchases.

I have a Studio 220, and while it's not the worlds most powerful amp, it'll get You by on most situations if you have efficient speakers. If the volume requirements demand more power, the bi-amp pre-outs allow You to use the existing amp to power the frequencies above the cross-over frequency, and to use a separate power amp for the lows.

A great find, congrats.

Regards
Sam

[Stompin Tom]

Yeah, I've been thinking that if it turns out that I need more power I'd probably just add a power amp (like you suggested)... I have a fairly big (6 unit?) rack unit that this will go into anyways. Unless, of course, I luck into a broken down sm400 for cheap.

I once tried to bias the SS power amp section of a guitar amp I had (I thought it was shutting down... turned out to just be a much simpler ground issue)... I don't really remember the details... time for some more research.

[T-Bird]

Hi.

Here's the procedure:

http://mhumhirecords.org/DIYpages/sc...s/SWR/Bias.pdf

You'll need quite a bit of equipment to pull it through with the "cross-over notch" method though.

I had the multimeter adjustment instructions as well, but that was a few computer crashes and a few dead HDDs ago, so perhaps You'll find them floating somewhere in the vast internet void .

If the amp sounds fine and doesn't overheat (badly ), chances are that the amp is biased correctly. Never hurts to check it though.

On the positive side, Your power supply has a breeze with powering just one amp now, so there should be no PS sag and the resulted loss of low freq definition.

Regards
Sam

[okcrum]

If the amp runs hot (and it does, even with correct biasing) then by all means use a fan on it. The bits that are left will last longer. The heat sinks in the 400 are too small for convection cooling. You'd need to set bias current waaay too high to affect the operating temp of the power amp.

Run the amp into 2 ohms at full power with a sine wave for 30 min. or so. You'll see what I'm talking about.

It's common knowledge that cooling extends the life of electronics. It's been that way since there was electronics.

[Stompin Tom]

Fan's not a bad idea, of course. And there's room in there now! I definitely don't have the gear to bias like T-bird linked to... I've only attempted the multimeter approach before... still need to find some directions on that.

Well, I did Duff's un-scooping mod... cleaned the pots... then hooked it up to an ampeg fridge. Worked great and sounded pretty good. It wasn't quite the tone bliss I was hoping for, and I could totally see how a bit more power amp headroom would be nice. Of course, I imagine those old (inefficient?) 8x10s eat up power. As soon as I have the $$ I'll be running through a 2x12 neo, which should be more efficient. We'll see. I'll see how reliable this thing is at practice before I even think about gigging with it.

Anyways, does anyone have any recommendations for preamp settings they like?

[Nightlyraider]

One of the general electronics "rules to live by" includes not using something that has previously set itself on fire without having it fixed.

[JTUK]

Get it to a decent repair guy.

If they know what they are talking about they will say that these are very well built amps... the pre is wired in a bit differently, but all the stuff used is a good quality and well made.

The amps run very hot but that is the design. No need for a fan but if you want to use one, then go ahead.

I have run mine for 20 years without a hitch..and only the fact that it was knocked onto the ground under power and it needed a quick cheap repair..( $50 )

I have changed the tube once and run it bridged into 4.

It would run below that as well without complaining ( not recommened ) and I would say that it is generally a bombproof amp.
Also, a simple repair job could get you a very good and sought after classic amp, IMV.