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Posted: Wed Mar 31, 2010 5:37 pm
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James, can you tell me what you'were doing when you were shocked ? Was the amp running ? Do you have a decent multi meter ? I think that if you're going to pursue this problem, a more methodical approach is needed. You shouldn't just throw parts at a problem, although replacing all those resistors wasn't a bad idea. It just wasn't focused on the hiss you found at V4. I don't believe that you ever tried a new quality tested tube in that position. I have a 12AX7 & 12AT7, both are matched and tested good quality tubes, that I use just for testing purposes. What I'm getting at is that the tube must be taken out of the equation before checking out the surrounding components and wiring. Art

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Posted: Thu Apr 01, 2010 5:29 am
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Hey Art,
I got distracted while checking the bias. A stupid mistake of sticking my hand in to remove alligator clip without turning off the amp. It will never happen again. I got new tubes yesterday evening and plan on letting the amp run for awhile with the new 12AT7s and see how she does. I agree with stratele. The hiss might just be natural. It did improve with the new resistors, especially on the Normal channel. I was just concerned about the sound when tapping V2. If that's nothing major, I think it should be a go. Thanks for all your help. Now I'll turn my attention to the foot pedal. It doesn't seem to do anything.


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Posted: Thu Apr 01, 2010 5:31 pm
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OK James, lesson learned!!! I think Stratele was referring to the action around V2 as normal, not the hiss noise. But yeah, let the amp stabilize for a while. My old Twin was extremly susceptible to the chopstick probes, even the circuit board was lively. I think it has to do with the board and wire insulation drying out and becoming brittle. But that's just a guess. Anyway, does your circuit board have a pretty good wave to it ? If so there is a 100ohm resistor in the feedback loop that may need resoldering. Have fun with the foot switch, that should be fairly straight forward , after all your work on the amp. Art

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Posted: Thu Apr 01, 2010 6:02 pm
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Hi Art,
Still at it. I'm not sure what you mean by a 'wave' in the circuit board or where the resistor is but I'll get to that next. Today's lesson was in making sure the reverb tank is plugged in before leaving the amp on for two hours and all knobs on 10. Came back to a blown fuse, fried reverb tank tube (brand new) and the smell of burnt components in the morning. Ah, yes. I think I fried the reverb driver, there was a glob like a melted brown crayon under it and a little on the outside. I'll get a replacement tomorrow. Good thing is (I guess) the hiss seemed to be toned down a bit. Might have to live with it. And, how simple is a footswitch? Simple replacement of parts? Hey, thanks for the help. I might be taking one step forward and two steps back, but I'm getting there.
James


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Posted: Thu Apr 01, 2010 9:29 pm
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James, can't say this is dull!!! OK, here come the questions. Was the reverb tank unplugged at the back of the chassis or down at the tank itself? The brown ooze could either come from a leaking electrolytic capacitor(but I thought you replaced all of the old electrolytics on the circuit board) or cooked wire insulation. Either way, since the fuse blew, we're looking at a short circuit. You said you got a book by Gerald Weber, does it have the diagram for the current limiter ? You're gonna' have to track down the source of the ooze, that's just a visual job. So you said the ooze came through the V3 tube socket and stuck to the tube ? If so, I'm guessing the tube shorted and overheated the wire leads on the socket. At this point, I would remove all of the tubes, until you find the short. Some basic continuity and ohmage tests will be necessary for all of the components surrounding this tube. Hopefully you have just dodged another bullet. Actually, leaving the amp on for 12 hours shouldn't be a problem. Art

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Posted: Fri Apr 02, 2010 3:20 am
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Art,
Sorry if I don't explain things clearly. The V3 tube wasn't oozing the brown stuff, it was just 'frosted' at the top, like someone dipped it in white paint. Actually, it was on the inside. The brown stuff was inder the reverb driver (the small transformer? ) set in the middle of the preamp tubes. There was also a bit of it on the top side of the driver. I'll check again to see if any resistors have melted. And the reverb tank was left unplugged from the chassis.


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Posted: Fri Apr 02, 2010 8:14 pm
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Ahhh James, can't say I like the sound of this. Are you telling me that the reverb transformer oozed ? I can only guess what cooked. But with your luck it's just the transformer and the driver tube. If you want, you could pull all the tubes(with a new fuse) and turn on the amp and standby in the play posistion. By all means only use the switches with one hand. If the fuse blows we're looking at a dead short. James, at my work space I have a 3/8" rubber pad under my feet and a wooden work bench with a 1/4" rubber pad that the amp and wooden amp supports rest on. It's no joke being safe!!! Art

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Posted: Sat Apr 03, 2010 6:27 am
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Art,
I hear you. One shock is enough. The other day when this happened, I took a chance and changed the fuse, connected everything with a different V3 and turned it on. Nothing blew but it seemed to have no reverb. That's why I think it's just the transformer (fingers crossed). So I got another transformer yesterday, a Hammond replacement and, naturally, I have questions before I put it in. Namely, the two pairs of wires (red/blue and black/green) coming out of the tranny are the same but they are reversed from the original. The come from different sides. So, do I wire them to the same places as the original according to the colors or do I go by which side of the tranny they come from. That would mean not going by colors. Any of this make sense? Simple. Where black/green comes out of the original, blue/red come out of the replacement and vice versa. Connect the same as original or not? Whew!


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Posted: Sat Apr 03, 2010 12:43 pm
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Hey James, the amp stayed on? That's good news. You would follow the same color code as the original. Red to the power terminal, blue to pin 6 of V3 socket(pin 6 and pin 1 are connected by a jumper wire), green to the center terminal of the reverb input jack, and black to the chassis ground connection. Tried to find an old reverb trans, to see what you mean about how it looks , but no luck. This should work out fine. Art

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Posted: Mon Apr 05, 2010 5:33 pm
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Art,
Replaced the tranny today, biased the output tubes again and she's playing great. The hiss is still there, better with the replacement resistors and I'm going to live with it. I'll call it 'character' of the amp. Okay, so my last thing to learn is how do I check the footswitch? It seems to do nothing, it doesn't turn reverb off or on and I have no idea what the vib side of the footswitch is supposed to do. Any tips? Oh, and thanks again for all your help, comments, guidance. Couldn't have opened up this amp without encouragement. Peace.
James


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Posted: Mon Apr 05, 2010 9:30 pm
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This can't be the same James I know. He hangs out in those amp crisis circles. Hey, glad to hear things are working out with amp! Can't help but notice that you called your amp"she." I used to call my old Twin,"The Queen of Fullerton." Dressed her in a blond cab, wheat grill, brown faceplate, ivory chicken-head knobs, and an amber pilot light. Yeah, she was quite demanding of my attention. Anyway, the footswitch has two simple switches(SPST). You can test them with your meter(ohms), a probe on each terminal, and see what kind of action you get(it's real obvious). Check the ground on the reverb switch in the foot pedal and the continuity on up the cable to the outside of the plug. Most failures, that I've run across are either the switches or a break in cable. On the reverb side the center conductor cannot have any contact with the outside shielding conductor. Art

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