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Old 27th December 2008, 02:20 AM   #1
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Default Bias problem- 1 Socket shows extremely low bias!

Ok, I'm working with a Fender Supersonic amp:

http://www.fender.com/support/amp_sc...nic%20Combo%20(Updated).pdf

I thought I'd put the schematic up front, as people often have a hard time finding it. Anyway...

I've run into a problem with the amp, where one socket (V8) has started showing an extremely low bias of -0.04 ma no matter what tube I put in it! I've used a couple of sets of tubes that match up fine in other amps, but keep getting this weird -0.04 ma reading in just 1 socket of the Fender Supersonic. Does anyone have any ideas about what might cause this? I'm not really sure where to start looking for this, even; negative bias circuit? Any help is greatly appreciated, and I'll be checking this forum at least once a day if anyone has any ideas! Thanks!
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Old 27th December 2008, 02:52 AM   #2
llwhtt is offline llwhtt  United States
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First check D36 and R141, if the diode is shorted you'll get no or very little reading but the amp will probably work just fine. If the resistor is open you'll probably see .6-.7 volts no matter what you do. How does the amp sound? If the amp sounds bad check the screen resistor R143, if it's open V8 isn't conducting and therefore your low reading. While you're in there check voltage on V8 pin 5, should be a negative DCV. Should be same as V7 pin 5. Check voltage on V8 pin 3, it will be close to 500VDC if the tube is not conducting. Be careful of HIGH VOLTAGE and slipping probes!!!!!!!!!!!!!! If all else fails tighten the pins in the socket and check for bad solder joints. I'm not sure how this particular Fender is constructed but if it's like the Hot Rod/Blues/Devilles etc. the problem might be broken solder joints on the PCB tube sockets.

Craig
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Old 27th December 2008, 03:07 AM   #3
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Cool, I'll check those tonight! The Supersonic is a little nicer than the HRD/HRDx's in that none of the tube sockets are PCB-mounted!

IMPORTANT: 1 thing I just noticed (and can't believe I didn't notice before) is that the tube in V8 doesn't get nearly as hot as the other power tube (V7). It does appear to come on, and reads a plate voltage of 479 (vs. 483 in V7), but I can grab seconds after turning the amp off if I want to, whereas the tube in V7 is too hot to touch for at least 5 minutes or more!

But yeah, for a relatively new amp I'm surprised how many times I've had to tighten the pins in the sockets already; I lost sound completely for a while when V5's socket was getting too loose! But yeah, I've just done that with the power tubes (V7 & V8) a few days ago. R143 and R141 were the first things I checked, btw, and they were fine last week when this problem first started, but I just checked to see if they were getting the correct ohm rating while still in the circuit. R141 does have a suspicious looking brown spot in the center, however...

Thanks a lot for your help, Craig!

--Billy
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Old 27th December 2008, 03:12 AM   #4
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http://www.fender.com/support/amp_sc...0(Updated).pdf
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Old 27th December 2008, 03:24 AM   #5
llwhtt is offline llwhtt  United States
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Actually the first thing to check is the heater, is it lit??? Basically if V8 isn't getting hot, it's either the heater or it's not doing anything. The amp probably doesn't sound right if this is the case. A brown spot on that type of resistor is not good, probably open.

Craig
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Old 28th December 2008, 12:46 AM   #6
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Great, thanks for all the help! I'm replacing R143 and R141 this week, as they're both 1 watt resistors and could probably stand to be upped to 2 or 3 watts, anyway. Hopefully that will solve the problem, but if not, I'll move on from there until I find something!
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Old 31st December 2008, 02:42 AM   #7
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Well, I changed the screen resistor at R143 to a 5 ohm, and the amp seems to be running fine now! Does any one have any ideas of what would cause a screen resistor to burn out on a Fender amp that's only 2 years old (it was clearly burned, almost completely brown outside)? Is it just from playing a high volumes, or is the 1 watt resistor that was originally in there really too low wattage for that position? If anyone has any ideas of examples of things that could cause this, I'd love to hear them so I check things to make sure I don't have the same problem again! Thanks for all the help from everyone who wrote on topic, it was really helpful!!!
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Old 31st December 2008, 02:46 AM   #8
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whoa, I meant a 5 WATT, not 5 ohms! It's still 470 ohms~!

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Old 31st December 2008, 02:08 PM   #9
kevinkr is offline kevinkr  United States
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I've had similar problems in a number of hifi amps I've built over the years, it could have been an issue (excessive screen current) with a particular tube that was in the amplifier at some point, and the resistor was probably a bit undersized particularly if you like a heavily overdriven sound - as the tube starts to saturate the plate voltage can drop below the screen voltage and when that happens more screen current flows and dissipation in the screen resistor increases.. (No screen resistor can result in very interesting fireworks in your output tube(s) under these conditions due to nearly unlimited current flow.)

Based on your comments I would recommend you check and tighten the tube sockets on a regular basis or consider finding some better quality drop in replacements.
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Old 31st December 2008, 02:50 PM   #10
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You haven't mentioned what "type" of resistor(s) burned. If they were film resistors these have very little tolerance for overloads, even momentary. Film resistors act like fuses when overloaded. Especially metal film types. That's why they're call flameproof resistors. For screen resistors this is a good place for carbon composition types like Allen Bradley. They can smoke and still continue to function. If you're burning out 5 watt wire wounds, then it's probably a result of todays imported garbage tubes.
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