thinman Posted April 15, 2008 Share Posted April 15, 2008 (edited) I've got a problem with a Hughes & Kettner Bassbase 250 head. The gain seems to fluctuate up and down such that it might run loud for say, 10 seconds, then get a lot quieter for a similar period. The loud/quiet periods vary. There's no noise or extra distortion apparent when either loud or quiet and the transition is very rapid. It doesn't seem to make much difference where the input gain is set - the same thing happens. There doesn't seem to be a problem with any of the pots - they're all quiet - and the jacks seem OK. The bass is fine in another amp. The preamp is valve only and I wondered if that might be the problem, i.e. it's got old and the heater's gone a bit cranky - I think the valve has been replaced in the past - I'm not 100% certain of that but it does look quite new. The amp itself is quite old - maybe 16 years. I'll order another as they're only cheap and easy to replace but has anyone had similar problems? Edited April 16, 2008 by thinman Quote Link to comment Share on other sites More sharing options...
Lfalex v1.1 Posted April 16, 2008 Share Posted April 16, 2008 Is it a 12AX7 in the pre? Pity it doesn't have a SS gain stage, as you can tell if it's the valve by using the SS side (Trace/Peavey/Hartke etc.) I had a 12AX7 go microphonic in my Trace after about 12 months. The replacement's lasted another 10 years! They should be easy to swap in and out, just do it with the power off and remember to treat valves like Halogen headlamp bulbs - don't touch them with your bare hands. Quote Link to comment Share on other sites More sharing options...
thinman Posted April 16, 2008 Author Share Posted April 16, 2008 [quote name='Lfalex v1.1' post='177865' date='Apr 16 2008, 01:40 PM']Is it a 12AX7 in the pre? Pity it doesn't have a SS gain stage, as you can tell if it's the valve by using the SS side (Trace/Peavey/Hartke etc.) I had a 12AX7 go microphonic in my Trace after about 12 months. The replacement's lasted another 10 years! They should be easy to swap in and out, just do it with the power off and remember to treat valves like Halogen headlamp bulbs - don't touch them with your bare hands.[/quote] Yes - it's an ECC83/12AX7. I've got another one on order. I think the valve was replaced about 12 years ago but the amp's had little use in that time until I got it recently. Reading a lot of manufacturer's websites they recommend preamp valve replacement about every 3-4 years but I think that's assuming a fair bit of use. The previous owner recalled a similar problem either with this one or another identical amp and suggested cleaning up the headphone jack and the effects loop jacks as they got a bit crudded-up. Haven't done that yet but will try that first and before I replace the valve. I wondered if anyone else has had a similar problem that was proven to be the valve. Mine doesn't seem to be microphonic. Are you sure about not touching the valve? I thought that with halogen lamps it's because they run far hotter and a foreign substance can cause irregular heating and breakge. I didn't think valves got as hot and they have stuff printed on them which must be as bad as a bit of finger grease. Quote Link to comment Share on other sites More sharing options...
thinman Posted April 21, 2008 Author Share Posted April 21, 2008 Well, it turned out to be some cracked solder joints on the board where over time the front panel must have wobbled around. They were quite easy to see with a magnifying glass and a bit of resoldering sorted it. Quote Link to comment Share on other sites More sharing options...
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