Tube Amps / Music Electronics
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|3/19/1999 6:11 PM|
||Silver face Showman Reverb|
I have a Showman Reverb that's kicking my butt. It has low output and is distorting at too low a volume. I have changed; all tubes, all filter caps, screen grip resistors (one was burned). I'm beginning to think output transformer,but, there is no difference in the resistance reading from either side of the center tap. Has anybody had a problem like this. I'm not a newbie but I'm stumped.
|3/20/1999 7:04 AM|
Jay: This sounds like one of Charles sunday Teasers[but it ain't sunday] What was the amp sounding like before changing caps and things? Don't let it kick your butt!I get like this at work sometimes, and have a nut that won't come off, and has been on for twenty years[ but this is barges] I tell the nut you might as well give up cause your coming off! But I have to be careful cause I work on gasoline tanker barges. and they can blow up! I've seen a few, and not much left around the area.But back to your amp, I would check voltages, and wireing, see if you put something in the wrong place. Check tube sockets for arcs, or bad conections. And sometimes new tubes can be bad.It just may take some detective work, check the coupling caps, and resistors to see if they are ok.Hows the speakers? Just start at the preamp first tube, check voltages, see if something is not what it should be.There should be others that will give some ideas too. Push on the board and tap around to see if it comes and goes. let us know what you find, and we'll see what we can come up with. Make sure the screen grid was put on the right pins. Everyone makes mistakes, and sometimes when you move things off the board you put it in the wrong hole, its easy done.And one other thing, be sure the speaker is plugged into the right jack, not the extension, you will hear it but not very loud, its easy done, everyone has done this at one time or another.. [Richie]
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The Ultimate Tone, Volume III by Kevin O'Connor
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|3/21/1999 10:34 AM|
Not ignoring all of Richie's good advice, but are you familiar with systematic troubleshooting, the technique I was trained in when the dinosaurs roamed the earth? The idea is to feed in a singal and either follow it from the input to where it dissapears (or isn't strong enough, in your case) or start at the output and see where it appears.
All you need is a signal source and a detection device. The signal source can be a audio generator (ideally) or just a low voltage generator. Take a filament tranny, hook a 100k potentiometer across it and adjust it for around 150 mv (0.15vac) using your DMM (even better would be to use a bride rectifier off the secondary and put a 0.1 uf cap in series between the rectifiers and the pot - this gives you a 120 hz source which is closer to the guitar frequency range). Now take this 150mv input and feed it into your amp using a 1/4 plug. To detect the signal, use your DMM to measure the AC signal voltage at each tube grid and plate, anywhere you don't get sufficient is an area to concentrate on (a scope is a better detector if you've got one). "Sufficient" gain is a little variable and I'm not at home with the Showman schematic in front of me right now. But, the first stages should be about 30-35 and the following stages should be about 20 (I think) and I believe that the Fender PI is about 15 for each 1/2 (Doc, Richie, Bruce, RG, et at, help! I'm running on faulty memory here).
Anyway, I hope you get the idea. This way you aren't randomly checking voltages and changing parts but instead determining the weak stage that needs attention.
|3/21/1999 2:26 PM|
Nice tech tip!
|3/25/1999 12:31 AM|
I'm with GW. In an old tube amp, a no-signal or low-signal condition is a snap to troubleshoot if you're methodical about it. Put a signal in, check the plate and grid of each stage with a scope in sequential order from input to output, and you'll find the defective stage within minutes. A couple of quick DC voltage readings at the grid and plate will tell you if it's a dead tube, open plate resistor, or whatever. If you approach troubleshooting in a random fashion it could take hours to track down a simple problem.
|3/25/1999 12:38 AM|
By the way, the old "circuit disturbance test" described by Jack Darr in his book (and reproduced in the Boob Toobs book) is a great way to troubleshoot a no-output condition with nothing more than a voltmeter. Start at the output and check plate, screen and grid voltages going back to the input. You should hear a pop in the speaker whenever you apply the voltmeter to the circuit, and the pop will get louder as you work back toward the preamp. When you don't hear a pop, then you've found the bad stage! It's simple, quick and effective.
|3/21/1999 2:21 PM|
Are you getting the correct bias readings?
Have you checked the neg. bias supply circuit?
I had a friends 61 Showman in for a cap job. After I had done the cap job I checked some voltages before I put the power tubes back in. My neg. voltage from the bias supply was at -27.....wrong,should be -55 at least. Someone before me had replaced the 50uf cap in bias supply circuit with some half rate quality 100uf/100v cap. The cap had gone bad and was causing the neg. voltage to be wrong.
This is a case where I made the mistake of trusting another techs "newer replacement part" instead of replacing it in the first place.
I don't know if this has anything to do with your problem.......but its the only Fender Showman story I have!
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