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Author
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Topic: Twin blowing fuses
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Gordon Black
Member From: burns,oregon,usa
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posted 05 March 2002 11:56 AM
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I've got a Silver Face Twin that keeps blowing the 4 amp, slow-blow fuse. This occurs when the Master Volume is cranked anywhere above 5, regardless of where the channel volumes are. Thanks to other Forum members, I've got a book coming on trouble shooting/repair, but before it gets here...anyone have any ideas? Thanks! |
John Floyd
Member From: Somewhere between Camden County , NC and Saluda S.C.
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posted 05 March 2002 01:25 PM
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Check any of the 6L6 power tubes glowing blue, Indication that they are gassy, also Check for Burned Resistors around the Power amp Tube Sockets(6L6's),Set the bias Adjustment to the center of its travel, (This will have to be adjusted later anyway by a tech, if you have to replace power tubes) Try Replacing the Tubes, and as a last resort The output transformer. Sounds like your power amp section is faulty.  Caution!!! There are High Voltages in the Fender twin and you should be very careful When troubleshooting problems with this amp powered Up. If your are inexperienced in troubleshooting electronic equipment, Its probably best to take it to a Professional, a lot less expensive than losing your life. ------------------
John - Camden Recording and Sound Services
Megatele
[This message was edited by John Floyd on 05 March 2002 at 01:50 PM.] |
Tony Prior
Member From: Charlotte NC
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posted 05 March 2002 01:48 PM
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Gordon, you didn't say how old your Twin is , but John's post is right on. I would only add that you should replace the Power Supply Filter Caps as well. If I still had my schematics I would tell you exactly what they are ..but...they don't live here anymore.. This is definetly a power supply problem not uncommon to Twins that have been workin' hard.TP Good luck |
John Floyd
Member From: Somewhere between Camden County , NC and Saluda S.C.
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posted 05 March 2002 02:09 PM
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Tony
Definitely the Power Supply Filter caps too.
I can't remember what the Cap Value is seems like they are 20 mfd @ 600 Wvdc, Could be 40 mfd. Its been about 26 years since I last worked on a Twin, and I guess that shows my age and memory problems  Gordon
Its also a good Idea to replace all of the Coupling Capacitors in the preamp section too, but as a result of doing this, Your tone will change back to what the amp sounded like when it was new. Sure will clean them up. If you elect to do this, be advised that Old speakers may have some problems dealing with the increased power that your amp will be putting out and may need replacing or reconing. Everything deteriorates over years of use in an amp including the voice coils in speakers. I have seen instances of doing a complete overhaul including cap replacement and the customer would be back the next day complaining only to find out that his speakers needed replacing also. Hard to convince a customer that he isn't being scammed when this happens. Just the way it really is sometimes ------------------
John - Camden Recording and Sound Services
Fender Megatele |
Ken Fox
Member From: Ray City, GA USA
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posted 05 March 2002 03:15 PM
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If your filters are bad (open) you'll have a lot of 120 Hz noise in the output, I doubt you would blow a fuse over that. The tubes all need to be tested (especially the output tubes). You can run the amp on two output tubes at a time to test it. You must run it with the inner two or out two power tubes in place. If you have a bad tube causing the fuse to blow, this might help you isolate it. I use a variac and an amp meter to test for such problems, as you likely don't, then the other method might help. There are numerous other things that could cause these problems. The amp needs to be diagnosed and repaired properly to assure a good future for it! It is especially important to check/replace the 470 ohm screen grid resistors at the 6L6 tubes, these take a beating. The 1500 ohm grid stop resistors at the 6L6 tubes get a beating as well from the heat. Plate load (100K ohm) resistors can cause a lot of noise (crackling problems). Poor or open cathode bypass caps (25 mf at 25VDC) cause low power and poor low end response. There's a lot to do when putting a tube amp back in good working condition, but it is certainly rewarding. If your amp is a Silverface or 80's Blackface, no bias control will be available (unless it has been modified). Bias can be correct by changing the bias resistor or modifing the amp (adding a true bias adjustment). Other things could be going on there as well. You could have partially shorted primary windings in the output transformer (hard to confirm without a true impedance meter). [This message was edited by Ken Fox on 05 March 2002 at 06:34 PM.] |
George Kimery
Member From: Limestone, TN, USA
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posted 05 March 2002 06:18 PM
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My Evans amp was blowing fuses like crazy, until I talked to a factory rep. I was using the fuses that have the little flat metal lightning bolt design instead of fuses with a wire filament. When I changed to the wire filament fuse, everything was fine. This is probably not your problem, but I thought it might be of interest to someone on the topic of fuses blowing. |
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