Members theblackbear Posted July 22, 2014 Members Share Posted July 22, 2014 I have an early tweed Bassman reissue I bought from a close friend around 1990. It has worked flawlessly till now--the only servicing I've done are a fresh fuse and lightbulb. When I got it he said it had been modded, but the only mod I’m sure of is a solid state rectifier. It has started buzzing on the low E string until it thoroughly warms up, at which time it functions perfectly again The buzz mildly “phases” in and out, like a wah wah. The tube rectifier was included with the amp and I swapped that with the plastic solid state to see if it makes a difference. It actually does, in a way. It seems to make the amp warm up faster, so the buzzing disappears in fifteen minutes instead of a half hour (to say nothing of the difference in tone). Thoughts on the problem? Tube or filter cap issue? Link to comment Share on other sites More sharing options...
Members r0bo01 Posted July 22, 2014 Members Share Posted July 22, 2014 taking a buzz/post buzz reading of your ht (b+) voltage in each scenario (ss vs. tube) would probably tell you something. i needn't warn you about the zap, need i? (if not so --> tech...)could be filter caps, could be tubes (especially power). amp is at least 24 yrs old?if nothing but the service you posted has been done, well...prob shouldn't be that hard to find, good luck with it! edit: it might not even be out of the realm that your ss rect. has an issue, seeing that with the tube rect., buzz cleared up faster (or maybe tube just can endure the problem better than ss)... Link to comment Share on other sites More sharing options...
Members guitarcapo Posted July 22, 2014 Members Share Posted July 22, 2014 I'm thinking it might have a bad/microphonic tube going on...especially if it's an issue that goes away after the tubes heat up a while. Link to comment Share on other sites More sharing options...
Members theblackbear Posted July 22, 2014 Author Members Share Posted July 22, 2014 taking a buzz/post buzz reading of your ht (b+) voltage in each scenario (ss vs. tube) would probably tell you something. i needn't warn you about the zap, need i? (if not so --> tech...) could be filter caps, could be tubes (especially power). amp is at least 24 yrs old? if nothing but the service you posted has been done, well... prob shouldn't be that hard to find, good luck with it! edit: it might not even be out of the realm that your ss rect. has an issue, seeing that with the tube rect., buzz cleared up faster (or maybe tube just can endure the problem better than ss)... Thanks for responding. The buzz is identical at startup with either the ss or tube rectifier. Link to comment Share on other sites More sharing options...
Members theblackbear Posted July 22, 2014 Author Members Share Posted July 22, 2014 I'm thinking it might have a bad/microphonic tube going on...especially if it's an issue that goes away after the tubes heat up a while. Thanks for posting. Are the preamp/AX7's the microphonic tubes? BTW, it's definitely a buzz, not a hum. Link to comment Share on other sites More sharing options...
Members WRGKMC Posted July 23, 2014 Members Share Posted July 23, 2014 Sounds like a power tube issue to me. I'd get new power tubes or at least have the ones in there re-biased. Its very possible you have a weak power tube that isn't coming up to its proper current flow causing an uneven bias. This imbalance will cause an AC hum in the speakers and if its not corrected can lead to catastrophic failure. A matched pair of tubes is very inexpensive so it should be your first thing to troubleshoot. Hum from bias imbalance is very common in Fenders. Often times, even with new tubes properly biased you may get a 1 to 3 second period where you hear a slight AC hum when you kick the standby switch on to apply the B+ power and then it quickly goes away and you only hear a smooth hiss of the power amp running. Its highly unlikely its the preamp tubes. Maybe the inverter but preamp tubes usually get microphonic before they fail. They give off a bell noise or static when tapped. This is a loud combo so tubes don't last very long like they do in a head. If you play regularly I'd change tubes once a year to avoid issues. My buddy has a vintage tweed Bassman and had the rectifier tube go bad and it took out the power transformer. My own Bassman would take out the grid resistors and power tubes if the speaker impedance was too high. Best not to screw around and get those tubes replaced. The amps will sound fantastic and last a lifetime if you maintain them. Those tweeds are one of the best stage amps ever made and highly valued. Ask any sound man with experience mixing those amps and he'll tell you its got a great mojo. If for some reason the issue isn't corrected by the tubes, you'd go straight to the power caps next. The Original Bassman caps would last at least 10 years if not 20. I had a Bassman where one of the 4uf caps leads broke off and it still ran OK. Make sure none of the dimples on the plus end are bulging. That's the sign the electrolyte is leaking. The power caps go bad faster if you "don't" power the amp up. If the amp has been setting unused for several years having the caps replaced may be needed. Again theur are cheap to buy. I bout a set for my Bassman for about $5 each a few years ago and replacing them was very simple. Link to comment Share on other sites More sharing options...
Members theblackbear Posted July 23, 2014 Author Members Share Posted July 23, 2014 Sounds like a power tube issue to me. I'd get new power tubes or at least have the ones in there re-biased. Its very possible you have a weak power tube that isn't coming up to its proper current flow causing an uneven bias. This imbalance will cause an AC hum in the speakers and if its not corrected can lead to catastrophic failure. A matched pair of tubes is very inexpensive so it should be your first thing to troubleshoot. Hum from bias imbalance is very common in Fenders. Often times, even with new tubes properly biased you may get a 1 to 3 second period where you hear a slight AC hum when you kick the standby switch on to apply the B+ power and then it quickly goes away and you only hear a smooth hiss of the power amp running. Its highly unlikely its the preamp tubes. Maybe the inverter but preamp tubes usually get microphonic before they fail. They give off a bell noise or static when tapped. This is a loud combo so tubes don't last very long like they do in a head. If you play regularly I'd change tubes once a year to avoid issues. My buddy has a vintage tweed Bassman and had the rectifier tube go bad and it took out the power transformer. My own Bassman would take out the grid resistors and power tubes if the speaker impedance was too high. Best not to screw around and get those tubes replaced. The amps will sound fantastic and last a lifetime if you maintain them. Those tweeds are one of the best stage amps ever made and highly valued. Ask any sound man with experience mixing those amps and he'll tell you its got a great mojo. If for some reason the issue isn't corrected by the tubes, you'd go straight to the power caps next. The Original Bassman caps would last at least 10 years if not 20. I had a Bassman where one of the 4uf caps leads broke off and it still ran OK. Make sure none of the dimples on the plus end are bulging. That's the sign the electrolyte is leaking. The power caps go bad faster if you "don't" power the amp up. If the amp has been setting unused for several years having the caps replaced may be needed. Again theur are cheap to buy. I bout a set for my Bassman for about $5 each a few years ago and replacing them was very simple. Thanks! Link to comment Share on other sites More sharing options...
Members guitarcapo Posted July 24, 2014 Members Share Posted July 24, 2014 Usually I just get a new set and re-tube the whole amp. I'll do this by replacing each tube and testing the amp after each replacement. If the offending noise goes away after one of these trials... that's a bad tube that I throw away. I keep the rest of the old tubes for back-up. If you gig anywhere you really need to have a back up set of tubes anyway. Link to comment Share on other sites More sharing options...
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