cytania Posted November 10, 2011 Share Posted November 10, 2011 The amp is a SWR SM-500 and the cab is a Barefaced Bass Compact. Used to sound very hi-fi, allow lots of tone shaping and cut through the guitars no problem. Last few months it has become increasingly muffled and prone to farty distortions on mid-range notes. Often these problems seem to lessen after a few hours practice or bizarrely by turning up the master volume slightly. Last night switching to a shorter cable improved things some but not a enough to say 'problem fixed'. The SWR has one tube in it's preamp stage, the manual mentions 'breaking glass' noises if it needs replacing, could this be my farty sound? There's also a hum noticeable at home that wasn't there when the rig was 'new' (the amp is third hand). Looking inside I see no obvious nasties (no dust, no patched up wiring) but the tube is unmarked in any way, not sure if this makes it the original SWR provided one, in which case replacement is way overdue. I guess I'm asking this so I don't become 'the guy who had his tube changed when actually the guitars were just too loud' but against memory of things the amp feels underpowered and easy to get farting the cab. The speaker doesn't look damaged and I'ver certainly not played it overdriven or distorted for effect. I used to be able to fiddle with the EQ, say add in lot's of low end for a double bassy feel, but now I keep things flat as any variation seems to provoke distortion. Your impressions sought, ta. Quote Link to comment Share on other sites More sharing options...
BassBod Posted November 10, 2011 Share Posted November 10, 2011 That should be a great combination - my first thought is either preamp valve (easy change) or something like component decline (capacitors?) in the power amp. You can get a good 12ax7 for under £15 (Sovtek etc) and if that doesn't help at least you've got a spare for the future. SWR seemed to use Groove Tube branded valves..but they were just badged (or rigorously selected and tested, depending on your level of cynicism) valves from a number of different makes. Those small preamp valves can last a long long time, or can be busted by the amp getting a good thrashing in transit, just depends on how it travels and a fair degree of luck. Bad news is it probably needs a proper test by a good tech...good news is they are traditional technology, put together by people, so they can be brought back to "as new" function by anyone with the right knowledge and experience. One other thing - those amps can push out a lot of very low frequency (really low, under 30hz) that can upset the best of speakers and waste a lot of your power on stuff you can't hear. If you are playing at all loud use the eq to cut these lowest frequencies and it may clean things up. As a rough test you can push on the strings over the pickup area (muted with your hands) and watch the speaker cone - if you see it moving dramatically (with your normal eq/volume settings) then it may be a good idea to take some lows out of your sound. Hope that helps Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted November 10, 2011 Share Posted November 10, 2011 Have you tried with a different bass? and is your bass active? might be worth changing the battery in the bass if it is active - the low powered and farty sound is often down to a low preamp battery. If not that, then you can try changing the pre-amp valve like BassBod said. You can get some great valves from watfordvalves.com Failing that, its probably a job for a tech. Quote Link to comment Share on other sites More sharing options...
cytania Posted November 10, 2011 Author Share Posted November 10, 2011 Thanks for the helpful comments chaps. Changed the bass batteries but they were fine and the effect happens with my backup Ibanez. Today it's very much like a fuzz box effect and even at low volumes, but no clipping indicated on the lights. Also tried dropping frequencies but no effect on the fuzz. I've also had time now to try it just turned on unconnected to anything else and it got quite hot which the manual suggests means it needs rebiasing. Hope to get seen by a professional tech very soon as Xmas gig coming up. Quote Link to comment Share on other sites More sharing options...
BassBod Posted November 10, 2011 Share Posted November 10, 2011 I've got the SWR service "biasing procedure" somewhere - PM me your email and I'll see if I can send it? I'm sure any good tech could do it anyway, but always good to have the makers' instructions. Quote Link to comment Share on other sites More sharing options...
umph Posted November 10, 2011 Share Posted November 10, 2011 Yeah gut instinct says power amp problem, valves don't usually die slowly and gracefully. Would be worth sticking a nice one in there anyways. Quote Link to comment Share on other sites More sharing options...
cytania Posted November 18, 2011 Author Share Posted November 18, 2011 Well, just got in and setup the gig rig. Plunka Plunka! Hot Rox have sorted my SWR amp out nicely. Apparently the problem was the input jack. Cost fifty quid but well worth it as that's one of the last things I would have checked. So no tubes replaced and the Compact is wonderfully powerful again. Like new or possibly better :-) Quote Link to comment Share on other sites More sharing options...
cytania Posted December 11, 2011 Author Share Posted December 11, 2011 (edited) The problem is back again. A metallic tinge around most notes at any volume. Took my amp and cab over to a friend's yesterday. His amp drove the cab cleanly. My SWR drove his cabs fine. Indeed the buzz wasn't much in evidence. We tightened the driver and grill mesh screws on the compact and had a nice cup of tea. This morning I tried it out the rig and it's back; glassy, metallic shimmer that almost modulates. I'm taking it back to Hot Rox as I don't think their repair got anywhere near the mark. Maybe they tightened the input jack but the erratic nature of the problem shouts 'valve' at me. They can fix this under the original charge Edited December 11, 2011 by cytania Quote Link to comment Share on other sites More sharing options...
cytania Posted December 11, 2011 Author Share Posted December 11, 2011 Just sent Hot Rox the following; "On the 11th November I dropped my bass amp in for repair. At the time I suggested the preamp tube be changed but when I picked it up on the 18th I was told the input jack was the causing of the buzzing. The buzz had gone but soon reappeared and the amp was still underpowered. I paid £55 for the repair and am very unhappy to still have a bass amp fit only for a quiet metal band. Ideally I would like a full refund however if you wish to look at the amp again I would like a timescale as my and my friends repairs have taken a long time. I understand that amp problems can be erratic but I feel that my descriptions of the nature of the problem and suggestions for it's solution have been ignored. It would have been much cheaper to purchase a 12AX7 valve myself." Quote Link to comment Share on other sites More sharing options...
Monckyman Posted December 12, 2011 Share Posted December 12, 2011 Good luck! Quote Link to comment Share on other sites More sharing options...
cytania Posted January 8, 2012 Author Share Posted January 8, 2012 Just adding the punchline, it's not the preamp tube, fitted one myself and although the sound is wider (presumably a tad more gain) the fartiness is still with. Bad driver now most likely. Quote Link to comment Share on other sites More sharing options...
Musky Posted January 8, 2012 Share Posted January 8, 2012 Just a thought, but have you tried cleaning the input jack? I had a problem with an amp which transpired to be a dirty input jack. The symptoms were completely different to yours but you never know, especially as HotRox identified it as the problem in the first place. Quote Link to comment Share on other sites More sharing options...
Merton Posted January 8, 2012 Share Posted January 8, 2012 Bad driver sounds odd given the cab is fine with other amps. I reckon dodgy pot on the amp somewhere...? Quote Link to comment Share on other sites More sharing options...
Prime_BASS Posted January 8, 2012 Share Posted January 8, 2012 This is the worlds most odd problem. Especially if the map was fine with other cabs and the compact was fine with other amps. Have you tried all your leads? Could also be a bad electrical insulation problem. Should also try going into the ampo striaght through the effects loop. This usually bypasses the pre-amp stage (the master volume will also need cranking up as the pre-amp help brings the signal voltage up) if it's fine it's definitly not the power amp or cab. Where abouts in notts are you, me and me dad are pretty nifty with this stuff, also I could try my stuff out with the cab to see if it is the amp. Quote Link to comment Share on other sites More sharing options...
cytania Posted January 9, 2012 Author Share Posted January 9, 2012 Thanks for the offer PB, I'll take you up on it if I start getting really crazy. When I was last in Hot Rox I tried out an Orange Terror Bass with my cab and that did give the buzz too. My friend's amp is 300w and fairly wooly sounding so may not be causing enough of a peak to bring out the fault. As regards the inputs I've tried both the passive and active jacks with no difference, Hot Rox declare the amp is OK for what that's worth. Hopefully Alex will come up with some more ideas. I've been wondering about getting half a speakon made up with the other end bare wires and then connecting this direct to the driver, thereby removing the cab connector. I can see why alot of the long time players here have several amps, spare cabs, the odd driver etc. Had thought of popping over to Bass Direct to try amp/cab pairing with stuff there but it's a bit cheeky... Quote Link to comment Share on other sites More sharing options...
Prime_BASS Posted January 9, 2012 Share Posted January 9, 2012 Unless you had been absolutly ragging the compact with 1000 + watts I find hard to believe its the driver in the cab. By input jacks. I mean the effects loop, into the return slot. Quote Link to comment Share on other sites More sharing options...
cytania Posted January 10, 2012 Author Share Posted January 10, 2012 Hi PB, just tried the effects loop thing you mentioned, despite my being full of cold. Great idea as essentially it's a third input jack. The problem is still there with it though. If it's the amp then the problem is excessive peakiness, the kind of super-tweeter top that 'presence' controls add. Which means the Orange Terror Bass also kicks out alot of crest/peak energy. Think fret buzz, but none of my other maps show up anything. Going to get an opinion when I'm over this cold and get a mixer fault looked at by a local tech. Quote Link to comment Share on other sites More sharing options...
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