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Old 13th May 2013, 06:34 AM   #1
irext is offline irext  Australia
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Default O/P Transformer DC Resistance Check

I am repairing a Bugera 6262 120W guitar amp which has 4 x 6L6 output tubes in a fairly typical layout. The fault is low o/p power and I strongly suspect the output transformer as DC resistance checks of the primary show a significant imbalance between the centre tap of the primary and either anode connection. One side reads about 39 ohms and the other 27 ohms. Before I lash out and buy a replacement has anyone else had a similar experience with a faulty transformer and how much of an imbalance in the resistances is deemed acceptable. All other DC voltages are well within range and max power o/p in it's current state is around 15W measured across an 8 ohm dummy load.
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Old 13th May 2013, 07:51 AM   #2
Enzo is offline Enzo  United States
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Such imbalances are fairly common. There are two ways to wind a center tapped winding like your primary. One is to wind half the turns, conect the tap, then finish winding the other half. This results in the same number of turns on each half, but the later ones are wound around the earlier ones, so each wind is longer wire than the inner ones. The transformer works on turns ratios, not resistance, so it doesn;t matter so much. But the longer wire means more resistance. AFter all, the resistance you measure in any transformer is just the resistance of the wire.

The other way to do it is to wind it with two pieces of wire at the same time. That way both windings will travel the same path and be of the same length. YOu then tie them together to make one center tapped winding. That results in even resistance, but is more expensive to manufacture.

If I recall correctly the Peavey Classic 30 OT measures something like 90 ohms and 120 ohms.

It is conceivable you do have a bad transformer, but it would more likely be two turns shorted together, and your meter would never detect that, so the mismatched resistance is probably not the clue. But We cannot say never...

If it were mine, I'd be checking the signal level at the grids of those 6L6s to see if they are being driven. I;'d be checking their screen grid to make sure all have B+ on them. If a tube fails, it often takes out the screen resistor for that tube. I'd check to make sure the phase inverter has B+ on both plates.

IN fact before any of that, I would plug my guitar - or other test signal of your chosing - into the FX return jack and see how that sounds. That feeds into the power amp bypassing the preamp. Strrong and clear or still weak?

FLip the speaker impedance selector switch back and forth a few times to scrub its contacts, any help?

PLug guitar into the front, and run a spare cord from FX send to FX return. ANy help?

Last, plug guitar into the front and run a cord from the FX send to some other amp and speaker. Sound OK over there or same problem? That tests the preamp.

Go over to geofex.com, and under Tech Tips find the Transformer Short Tester. It is EXTREMELY simple to make and use, costs very little for the few parts, and folloow the directions.


But I think you will find the circuit has a problem. I could be wrong. MY primary suspect would be the little transistor T1. It is just to the right of the CLEAN VOLUME control VR11A on the schematic. I have no idea where on the board it is. There are not many transistors. If it fails or is stuck on at its gate, you will have your very weak signal.

That is a mute transistor, and it quiets the amp briefly as you switch channels to prevent loud noises during transition. I'd probably just remiove it and see if the amp wakes up.
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Old 13th May 2013, 10:33 AM   #3
irext is offline irext  Australia
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Thank you so much Enzo. Your reasoning behind the resistance imbalance makes perfect sense. I had never stopped to think about how the transformer is constructed but indeed the turns ratio is paramount not the length of copper to achieve it. I will do some more checking but so far all anode,cathode and screen voltages are present and correct. I ran the amp into a 100w 8 ohm dummy load with a signal gen attached and could only manage about 28 Watts. The 6L6's were glowing purple at this stage which may suggest a lot of energy being dissipated somewhere (o/p transformer?). More checks to do but I think I'll add a shorted turns tester to my test gear. Many thanks for the info and suggestions.
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Old 14th May 2013, 02:36 AM   #4
irext is offline irext  Australia
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Problem solved! Found no volts on one of the anodes of the phase splitter 12AX7 due to an open circuit 100K resistor. A bit fiddly to replace as the whole preamp board has to be removed but once done the amp is working normally with loads of o/p and no hum. The hum should have been a clue as it was not being cancelled out as the o/p was effectively single ended not push pull. Once again thanks Enzo for stopping me from barking up the wrong tree.
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Old 14th May 2013, 04:17 AM   #5
Enzo is offline Enzo  United States
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yeah, well I have barked up so many wrong ones myself over the years, it feels good to point out a right one now and then.
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Old 14th May 2013, 12:10 PM   #6
irext is offline irext  Australia
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Built the shorted turns tester. Works a treat!
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