brensabre79 Posted December 16, 2013 Share Posted December 16, 2013 I recently got hold of a Peavey AlphaBass. Built in 1989 it's an all-valve bass amp loosely based on the Mesa D-180, well a poor mans version they say... It's 160 watts into 4 or 2 ohms. I have used it for a couple of gigs, I'm loving the weight of the all-valve sound and it's got a lovely clean tone. But I've found that it's not quite got enough oomph. I can get it loud enough, but it starts to distort a bit too much - especially on the lower notes. If I back off the pre-amp volume to get rid of the distortion, then it's sitting just on the too quiet side of the mix. We don't play really loud either. My Carvin BX500 is usually on 1 master volume, maybe 2 for a big gig. I've got the Peavey all the way at 10 on the post volume control (apparently this is merely an attenuator for the power amp which is always on full, so 10 is the recommended setting once you have set up your tone. Now, the 6L6 power amp valves in this are all original Peavey USA rebadged Sylvanias from 1989. One of them has been replaced with a JJ, which may not be helping as it's probably not matched, but the rest are as old as the amp. I swapped the pre-amp valves already (3x12ax7/ecc83 and a 12AT7), and whilst the tone has changed slightly, the distortion hasn't. As an all-valve noob, can anyone tell me if putting in a brand new set of 6L6s will help me eek just a little more clean volume out of this thing or am I wasting half the value of the amp on valves when it's always going to be slightly too quiet? Should I re-bottle it? Or look for something else? Cheers! Quote Link to comment Share on other sites More sharing options...
Telebass Posted December 16, 2013 Share Posted December 16, 2013 Is it actually supposed to have 12AX7s? Pretty high gain valves, great for guitar. Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted December 17, 2013 Author Share Posted December 17, 2013 It is indeed. Except for the Phase inverter (12AT7) I think most bass pre-amps with a valve in have 12ax7/ecc83. Certainly all the ones I've owned, Hartke, Genz Benz, Orange, Carvin and this Peavey. My Markbass compressor pedal has an ecc81 though. I can get the pre-amp clean, no problem, but then it is not quite loud enough so I end up breaching into distortion to keep up volume wise towards the end of the set (we start quiet and finish noisy) I suppose I'm trying to find out if old, worn out valves will lose a bit of power and/or distort a bit more compared to new ones, or is it the case that they either work or they don't...? Quote Link to comment Share on other sites More sharing options...
Billy Apple Posted December 17, 2013 Share Posted December 17, 2013 (edited) I recently had my Matamp GT200 in for repair. It was under-powered and at first they put it down to old power valves (3 years old). Matamp told me that over time power valves to degrade and lose oomph. Depends on use, but the longer and louder you go the quicker they wear out. However, they tested all the power valves and they were fine, further digging revealed a dodgy pre-amp valve (ecc83) which was replaced and power returned. So yes, older used power valves will lose watts, but then again so will other factors. I'd get it to a tec for a health check Edited December 17, 2013 by Billy Apple Quote Link to comment Share on other sites More sharing options...
6v6 Posted December 17, 2013 Share Posted December 17, 2013 [quote name='brensabre79' timestamp='1387271866' post='2309978'] I suppose I'm trying to find out if old, worn out valves will lose a bit of power and/or distort a bit more compared to new ones, or is it the case that they either work or they don't...? [/quote] They definitely do lose power over time, when the power tubes are realy worn you can lose a *lot* of headroom and definition. If you replace them, make sure you take it to someone who can bias it correctly, as running tubes too hot will reduce lifespan and headroom. A decent tech will also be able to identify if there are other headroom-limiting issues, ie if it's actually the power section saturating as opposed to one of the preamp stages or phase splitter, which could be remedied by swapping for lower gain tubes, or tweaking the circuit/components. Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted December 17, 2013 Author Share Posted December 17, 2013 Thanks! It's a non-adjustable bias, but as per my thread in the Repairs section, I might have to get this modded. It's reassuring that changing the power tubes (25 years old) will probably get me a bit more clean headroom. I changed all the pre-amp valves already, but point taken about swapping ecc83 for ecc81 - won't this just mean I have to turn the pre-amp gain up louder to get the same amount of gain though? I'm not really driving the gain stage much to start with, but with the post vol on full, I have to turn up the pre to get the level i need... Quote Link to comment Share on other sites More sharing options...
6v6 Posted December 17, 2013 Share Posted December 17, 2013 [quote name='brensabre79' timestamp='1387283419' post='2310154'] Thanks! It's a non-adjustable bias, but as per my thread in the Repairs section, I might have to get this modded. It's reassuring that changing the power tubes (25 years old) will probably get me a bit more clean headroom. I changed all the pre-amp valves already, but point taken about swapping ecc83 for ecc81 - won't this just mean I have to turn the pre-amp gain up louder to get the same amount of gain though? I'm not really driving the gain stage much to start with, but with the post vol on full, I have to turn up the pre to get the level i need... [/quote] I would definitely get it modded so it can be biased correctly - some manufacturers (like Mesa) claim that their fixed bias amps which aren't adjustable never need adjusting, but IMHO this is wrong, all it does is restrict the tubes you can use and/or force you to run with the tubes not biased 100% correctly. Looking at the block diagram in [url="http://assets.peavey.com/literature/manuals/80300980.pdf"]the manual[/url] is interesting - it doesn't look like the "post vol" is really a master volume as such as there are several gain stages after it, it's kinda a weird arrangement. If it were mine, I'd get the power tubes replaced and properly biassed, then experiment with the fx loop and poweramp in jacks, to figure out if more headroom is available by tube swaps anywhere in the preamp and gain recovery stages (basically figure out which stages are overdriving by isolating them with the volume controls and jacks) If the power tubes really are 25 years old, I think you may be surprised how much more headroom changing them will give (assuming they are knackered) Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted December 18, 2013 Author Share Posted December 18, 2013 Thakns 6v6 I'm not sure if the power tubes are knackered or not to be honest, 5 of them are the original Sylvania/Peavey Super 6 "Made in USA" and one of them is a random JJ 6L6 - which struck me as a cheap fix by the previous owner for one that had gone. I was told the post volume is merely an attenuator for the power amp and as such can be run at full for a clean sound as long as the pre-amp isn't distorting. I certainly found this to be the case, but ended up having to increase the pre to get enough volume, at which point it distorted - and not in a nice way. After about 3 weeks, Peavey Customer support came back with a very short email to say I could change the valves without bias adjustment. I could also substitute KT66 if I wanted to without adjustment - I'm not sure if I'll get any more clean power from these though...? Quote Link to comment Share on other sites More sharing options...
6v6 Posted December 22, 2013 Share Posted December 22, 2013 [quote name='brensabre79' timestamp='1387360468' post='2310992'] After about 3 weeks, Peavey Customer support came back with a very short email to say I could change the valves without bias adjustment. I could also substitute KT66 if I wanted to without adjustment - I'm not sure if I'll get any more clean power from these though...? [/quote] This is bad advice IMO, KT66's draw significantly more heater current than 6L6s, and they are extremely unlikely to bias correctly (without some adjustment) in your amp. They will probably work, but unless you're very sure on the power transformer specs, I personally would not risk it - the KT66s will draw 2.4 *amps* more from your heater filament of your power transformer (0.9A vs 1.3A, 6 tubes). It might work for a while, but IME running heater supplies too hot eventually results in power transformer failure. Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted December 23, 2013 Author Share Posted December 23, 2013 Dont worry. I've ordered at matched set of JJ 6L6s, a bias probe and the two resistors I need for the bias mod. I'm just waiting on the potentiometer then I'm going in for surgery (having discharged the caps first). That should be my boxing day sorted then Quote Link to comment Share on other sites More sharing options...
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