I finally finished my 36w. Ill post pics tomorrow.
It sounds great and I'm sure my next amp will go more smoothly. I know I have learned from some dumb mistakes I made on this one.
Just a couple of questions though.
I have played it for about 5 long sessions and it has worked fine. Last session I started getting a crunchy distorted sound at higher levels on the clean channel. Also, the other day and was playing at low levels and it started fading in and out. I suspect output tubes. Does this sound like that type of problem? I must admit that I did made some rookie mistakes and a few of the tubes had some arcing during the testing stages.
Second question. Is is possible for a tube to be "good" according to a tube tester and still have a problem? I tested these tubes ( EL84) not long ago and they all were good ( I have 6). But when I put one tube in particular in the amp, ( XX00 ) it and the tube next to it start heating up and glowing after a minute or 2. Sound cuts out and I obviously turn off. I switched positions of that tube( 00XX ) and the sillyness followed it.
Amp finished. WITH PICS
Moderators: zaphod_phil, Daviedawg, Graydon, CurtissRobin, colossal
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macombaland
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Amp finished. WITH PICS
Last edited by macombaland on Tue 03/02/10 1:08 pm, edited 2 times in total.
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CurtissRobin
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If you got unpleasant sounds from the clean channel but the other channel was OK then the problem will be in the clean channel preamp.
Fading in and out appears (based on your descriptions) to be a power stage problem, possibly connected to your tube question.
Tube testers have limits and you may have encountered one of those limits. When the problem follows the tube it's a good bet that the tube is the problem. Had the problem stayed put it would be inside the chassis somewhere. Since you have six tubes, identify each with a Sharpie and swap tubes until you've determined which is the bad guy, then toss it.
KennyO
Fading in and out appears (based on your descriptions) to be a power stage problem, possibly connected to your tube question.
Tube testers have limits and you may have encountered one of those limits. When the problem follows the tube it's a good bet that the tube is the problem. Had the problem stayed put it would be inside the chassis somewhere. Since you have six tubes, identify each with a Sharpie and swap tubes until you've determined which is the bad guy, then toss it.
KennyO
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macombaland
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I am concerned about buying new power tubes and having them go bad just like these did.
Like I said earlier, I was probably a little rough on those first tubes during testing.
The amp plays for a few minutes and then starts going bad. I can turn it off for a minute then back on and I am able to play again. Is this a symptom of bad power tubes?
Last thing. I would like to get to the bottom of the bad tubes and it is hard with 4 slots( and maybe more than 1 bad tube) . I want to run 2 power tubes. I have a 16ohm speaker and I hear that you can remove 2 tubes and use the 8 ohm output tap. Good or bad idea?
Like I said earlier, I was probably a little rough on those first tubes during testing.
The amp plays for a few minutes and then starts going bad. I can turn it off for a minute then back on and I am able to play again. Is this a symptom of bad power tubes?
Last thing. I would like to get to the bottom of the bad tubes and it is hard with 4 slots( and maybe more than 1 bad tube) . I want to run 2 power tubes. I have a 16ohm speaker and I hear that you can remove 2 tubes and use the 8 ohm output tap. Good or bad idea?
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CurtissRobin
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The sequence of playable -- goes bad -- turned off awhile -- playable again,... says you have a thermal problem, i.e. a part heating up and failing. It's most often a tube but if it's taking out tubes it could also be a resistor (particularly those that heat up because they carry substantial current) or a cap positioned alongside something hot. If the cathode resistor is the culprit your tubes could be redplating severely and quickly dying.
Pulling two tubes raises the impedence so you'd use a 16 ohm speaker on a (normally) 8 ohm tap. You could also just leave the 16 ohm speaker on the 16 ohm tap. It'll affect the sound a bit but not the life of the amp unless you use it that way for a long time and a lot of hard playing. For your testing you'll be fine either way.
KennyO
Pulling two tubes raises the impedence so you'd use a 16 ohm speaker on a (normally) 8 ohm tap. You could also just leave the 16 ohm speaker on the 16 ohm tap. It'll affect the sound a bit but not the life of the amp unless you use it that way for a long time and a lot of hard playing. For your testing you'll be fine either way.
KennyO
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macombaland
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I have identified a pair of tube, which when installed caused the parallel tube to red plate. Upon replacing those two i get no visible plate glowing but the amp does shut off.CurtissRobin wrote: It's most often a tube but if it's taking out tubes it could also be a resistor (particularly those that heat up because they carry substantial current) or a cap positioned alongside something hot. If the cathode resistor is the culprit your tubes could be redplating severely and quickly dying.
KennyO
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CurtissRobin
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OK, how about doing some voltage measurements? Make a table with a column for each pin of each power tube (use two tubes, not four, exclude heaters and unused pins). In the top row record the voltages immediately after you power up and switch off the standby. Play for a while then read and record the voltages again. Repeat from time to time until you hear the amp doing its thing and you'll surely see a voltage or two stray significantly from its original value. That change points to the troublemaker part.
KennyO
KennyO
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macombaland
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OK i figured it out. Did the measurements and they were the same before and after.
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What it turned out to be was a loose solder joint. The joint was being held together by the heat shrink tubing and when it would get warm in the case, the heat shrink would sag, causing the cut outs and power drops. Kind of a funny problem.
I do have a couple of other questions though.
1. Is it normal for the dirty channel to have considerably more gain than the clean channel?
2. When I flip the switch that is connected to the "mid" pot, it cuts my volume in half on both channels. That is the only state in which the TMB pots actually work however it isn't loud enough.
Are these things normal or are the wiring problems?? Thanks
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What it turned out to be was a loose solder joint. The joint was being held together by the heat shrink tubing and when it would get warm in the case, the heat shrink would sag, causing the cut outs and power drops. Kind of a funny problem.
I do have a couple of other questions though.
1. Is it normal for the dirty channel to have considerably more gain than the clean channel?
2. When I flip the switch that is connected to the "mid" pot, it cuts my volume in half on both channels. That is the only state in which the TMB pots actually work however it isn't loud enough.
Are these things normal or are the wiring problems?? Thanks
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