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Post by Maddog on Sept 2, 2018 11:40:47 GMT -7
When I'm driving my 1961 Bassman pretty hard, I can strike a hard single note or chord, and the volume takes an instantaneous dive. It doesn't completely kill the volume, but gets noticeably quieter for maybe a half a second. It recovers almost as fast as it falls off....like I said, the entire cycle takes less than a second.
I De-Oxit'ed everything....no change
I re-tubed the whole complement GZ34, 5881 (x2), and all the 12ax7's in the preamp........no change
I changed speaker cabs and cables.... no change
This amp was serviced by Michael Clark (Clark Amps) about 3 years ago....he replaced the filter caps and tweaked some other stuff....
I'm thinking I've got a filter cap that is croaking and cannot hold/release the reserve power needed when I'm pushing this baby....(but I'be been wrong before!
I appreciate your thoughts and ideas....
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Post by purpletele on Sept 2, 2018 18:48:06 GMT -7
When I'm driving my 1961 Bassman pretty hard, I can strike a hard single note or chord, and the volume takes an instantaneous dive. It doesn't completely kill the volume, but gets noticeably quieter for maybe a half a second. It recovers almost as fast as it falls off....like I said, the entire cycle takes less than a second.
I De-Oxit'ed everything....no change
I re-tubed the whole complement GZ34, 5881 (x2), and all the 12ax7's in the preamp........no change
I changed speaker cabs and cables.... no change
This amp was serviced by Michael Clark (Clark Amps) about 3 years ago....he replaced the filter caps and tweaked some other stuff....
I'm thinking I've got a filter cap that is croaking and cannot hold/release the reserve power needed when I'm pushing this baby....(but I'be been wrong before!
I appreciate your thoughts and ideas.... It's the second fuel filter that you didn't know about. There's a story there. Lee, ask your grandson Lincoln and he will tell you, your probably right grandpa, but the filter caps were just changed. We'll Lincoln, sometimes when an amp sits for a few years funny things can happen. I can't think of anything else that would do that.
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Post by mudman on Sept 2, 2018 19:27:12 GMT -7
I know the 6g6 is solid state rectified, could one of the diodes be giving up the ghost?
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Post by Maddog on Sept 3, 2018 0:22:45 GMT -7
I know the 6g6 is solid state rectified, could one of the diodes be giving up the ghost? Mine is a 1961. In that year (only), it was tube rectified....GZ34 ...I put a new one in with all the other tubes....No change...
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Post by mudman on Sept 3, 2018 5:21:40 GMT -7
Gotcha, my mistake. I would also look for a cold solder joint. As the amp heats up (or under stress from a hard hit note) you could have a joint opening up. In any case it sounds like tech time.
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Post by Deleted on Sept 3, 2018 5:57:44 GMT -7
Sounds like it wants a little professional attention after sitting all alone for so long.
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Post by premiumplus (Dave) on Sept 5, 2018 18:16:16 GMT -7
Power supply filter caps are pretty new, but they can fail. I also like the idea of a cold solder joint, but there's another thing that might be happening. Fender's circuit boards are wax impregnated cardboard of some sort, and "Fenderitis" happens when they get moisture in them over the course of many years. All sorts of symptoms can arise from that issue. I had one that I could actually measure a few megohms of resistance when probing the board with a VTVM. It's a long shot, but something to think about. Question, does it hum more than usual when idling?
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Post by Maddog on Sept 5, 2018 19:45:25 GMT -7
Power supply filter caps are pretty new, but they can fail. I also like the idea of a cold solder joint, but there's another thing that might be happening. Fender's circuit boards are wax impregnated cardboard of some sort, and "Fenderitis" happens when they get moisture in them over the course of many years. All sorts of symptoms can arise from that issue. I had one that I could actually measure a few megohms of resistance when probing the board with a VTVM. It's a long shot, but something to think about. Question, does it hum more than usual when idling? No, since Clark tweaked it, it's dead quiet....I'll open it up this weekend and get a look. This amp lives in a fairly dry climate, and hasn't been out of the house in years.....still, is it (fenderitis) a matter of actual moisture, or corrosion secondary to moisture??
The reason I was going with bad filter, is it sounds great until it's working hard and instantly needs a little more juice but can't get it so it drains all it's got, then it instantly corrects the dip in volume, like a cap that drops its voltage and recovers, but doesn't have enough stored potential to supply the increased demand...FWIW....
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Post by mudman on Sept 5, 2018 19:59:35 GMT -7
Power supply filter caps are pretty new, but they can fail. I also like the idea of a cold solder joint, but there's another thing that might be happening. Fender's circuit boards are wax impregnated cardboard of some sort, and "Fenderitis" happens when they get moisture in them over the course of many years. All sorts of symptoms can arise from that issue. I had one that I could actually measure a few megohms of resistance when probing the board with a VTVM. It's a long shot, but something to think about. Question, does it hum more than usual when idling? No, since Clark tweaked it, it's dead quiet....I'll open it up this weekend and get a look. This amp lives in a fairly dry climate, and hasn't been out of the house in years.....still, is it (fenderitis) a matter of actual moisture, or corrosion secondary to moisture??
The reason I was going with bad filter, is it sounds great until it's working hard and instantly needs a little more juice but can't get it so it drains all it's got, then it instantly corrects the dip in volume, like a cap that drops its voltage and recovers, but doesn't have enough stored potential to supply the increased demand...FWIW....
I don’t think it’s the board as you’d probably have more issues and noise going on, but it’s definitely something to check. Could be a filter cap but just unlikely if they’re all new as or 3 years ago. Time to open her up and inspect it. What you’re describing to me sounds like a bad joint, but never rule anything out until it’s checked. Start simple and work your way up. Tubes, Jacks, switches, ground(s), shorts in the wiring, solder joints, caps, resistors, trannies, etc.
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Post by sharkboy on Sept 6, 2018 1:55:54 GMT -7
As above, my first suspicion is in the power supply. What you’re describing sounds like full power isn available- kind of a brown out- extreme sag.
Because of effort, my diagntic routine leans toward laziness: what is easiest to check first, though my first guesses are caps, rectifier, PI.
I normally keep way more tubes on hand than I will ever need. Swapping in tubes back to front by function and then trying to repeat the playing condition of failure (output first, PI, then pre tubes) is a non-invasive way of testing what is easily tested. Then, look at connections, soldering. Then, sockets. Then caps. It is also possible that you have even a resistor degrading. Sad to say, the transformers are not out of the picture, as the laminations can break down.
Please let us know how it goes.
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Post by premiumplus (Dave) on Sept 6, 2018 5:46:08 GMT -7
The way I heard it described, "Fenderitis" is driven by retained moisture in the board.
Since it's quiet I agree that the board probably is okay. Especially since it's been in a dry climate. If the filter caps had lost capacitance there would be increased hum so they are probably not the issue either. You said you had deoxited everything, I assume that the power and rectifier tube sockets are included? An oxidized tube socket pin (one carrying high current) could be suspect. It does sound like there is an intermittent failure that is driven by high current or high vibration conditions. Is it possible that the power supply is being shorted to ground for an instant at high volume? That would explain the quick decline in power and 'almost as fast' recovery you describe. As a troubleshooter this is where you don't leave any possible failure mode out.
It is possible you'll find a bad tube socket too. Inspect the wiring carefully and ensure that there is good clearance between pins and wiring. I'd look around the tube sockets for a burnt area between pins where a high voltage arc has occurred and carbonized, this could be a place where high voltage is arcing repeatedly. This usually happens on tube sockets where an arc was originally induced by some conductive foreign object left in the chassis during manufacture or maintenance.
If the amp has a choke it's possible that it's got an intermittent short to ground too.
I'm really interested in how this resolves, Lee. Good luck with it!
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Post by bgkyt1 on Sept 6, 2018 11:27:12 GMT -7
when they replaced the filter caps did they also replace the bias cap and the caps (.1?) in the power section (after the phase inverter)? it might not be the culprit here, but probably should be replaced.
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Post by Maddog on Sept 6, 2018 14:32:55 GMT -7
when they replaced the filter caps did they also replace the bias cap and the caps (.1?) in the power section (after the phase inverter)? it might not be the culprit here, but probably should be replaced. I had already replaced the bias caps with NOS (yellow astron) caps (like 50+ y/o caps) to main the originality.....I had to go thru several to get 2 that were up to spec. Maybe, in retrospect, that wasn't such a great idea, huh? Yes, Power section caps were replaced...
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Post by premiumplus (Dave) on Sept 6, 2018 15:17:22 GMT -7
The phase inverter caps, if they get leaky (talking about leaking DC voltage here, not fluids) are going to throw the bias off on the power tubes. That could be your problem right there. You need to consider putting some polypropylene caps in there. If you can, test the Astrons for leakage. Power the amp up, in play mode, and put your negative DMM lead on the chassis and measure for DC voltage on the side of the caps going to the grid (pin 5) of the output tubes. Any DC voltage there is bad. You'll make that measurement with no input to the amp. Be careful, you've got high voltage (around 450-500 vDC) on pin 3.
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Post by mudman on Sept 6, 2018 16:01:09 GMT -7
when they replaced the filter caps did they also replace the bias cap and the caps (.1?) in the power section (after the phase inverter)? it might not be the culprit here, but probably should be replaced. I had already replaced the bias caps with NOS (yellow astron) caps (like 50+ y/o caps) to main the originality.....I had to go thru several to get 2 that were up to spec. Maybe, in retrospect, that wasn't such a great idea, huh? Yes, Power section caps were replaced... Astrons leak like crazy. Replace those. If you want to maintain the look, slice them open, stick the new cap inside, replace. Any 50 year old cap is suspect even if it tests ok
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