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Ampeg V4 screen resistor open


 
3/20/2006 4:30 PM
AtomicMassUnit
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Ampeg V4 screen resistor open
Hey guys, I'm working on an Ampeg V4 and basically, the power amp doesn't work. I'm not getting any voltage on the screens. The 470 ohm resistors on the power tubes are all fine, but the single 470 that feeds them has a problem. If the power tubes are out, I get the correct voltage on both sides of it. When I put the power tubes in, it goes open. My question is, is there anything that could casue that other than bad power tubes? The reason I ask is because theyre a brand new set of JJ 7027a's and I really hate to pay another 80 bucks for a set, especially since I have no idea why these would have gone bad. Unfortunately, I don't have another amp that takes 7027s or a tube tester to check these.
 
3/20/2006 5:09 PM
loudthud
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Check that pin 1 on the 7027 sockets is not grounded. If it is, this would indicate that someone modified the amp to use EL34's. Pins 1 and 4 are (should be) connected internally on a 7027 so if you plug those in after the mod, the screen resistors will be damaged. Your description suggests that the 470 ohm resistor that feeds all the screens is damaged and is much higher than 470 ohms.
 
3/20/2006 11:11 PM
AtomicMassUnit
loudthud.... Pin 1 is grounded! There were definitely some goofy mods done to this amp, and apparently that's what this is. On all the power tubes pin 1 is connected to pin 8, grounded. I'll disconnect these, replace the resistor feeding the screens, (the resistance is infinity across it, totally open), and see if these tubes have survived. Thanks! I'll report back tomorrow...
 
3/21/2006 8:34 AM
AL
That amp may have been rewired to accept another tube (6L6?). That's fairly common with Ampegs. They used some odd tube choices and some of them were nearly impossible to come by for years - and some still are. Check all your tubes for rewires.  
 
AL
 
3/21/2006 9:33 AM
loudthud
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Just a lucky guess. Good thing it didn't originally have 8417's. In that tube, pin 1 is connected to pin 3, the plate.
 
3/21/2006 11:02 AM
AtomicMassUnit
Thanks guys! loudthud, that fixed the problem. I clipped pin 1 and replaced the screen resistor and all is well. This has been in my house for almost a year now, and I've had to track down all the wierd things that someone else did to it. There were places where the pcb traces were purposely cut, one of which was a plate for a preamp tube! There were parts removed all over the place, whoever worked on it just randomly pulled parts and cut traces. Blown OT, no power tubes, a bunch of different things... I am so happy to get this finished. Thanks so much for the help!
 
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