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Guyatone Flip500
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Old 9th June 2019, 11:50 AM   #1
Carlosraj is offline Carlosraj  United Kingdom
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Default Guyatone Flip500

Hi


A friend send this amp to check its condition. It is a very old amp and I could not get the schematic . It use a single 12ax7 and two 6ca7eh valves . There is sound but very distorted. The left side power tube gets very hot while the right side is much less hot. Even after interchanging the tubes, it is the left side tube that is getting hot and the right side less hot. I removed the valves to measure the resistance of the output transformer. From center tap(red) to right(brown) reads 77 ohms. From center tap(red) to left(yellow) reads 11 ohms. The reading to left side seems very low. Is this reading normal for an output transformer. Please assist.


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Old 9th June 2019, 12:25 PM   #2
Steve Morley is offline Steve Morley  Canada
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If all the parts are original chances are resistors have drifted, coupling caps are leaky and filter caps aren't filtering. If it is really old and wired point to point and not a PCB then it should be fairly easy to trace out the circuit. Make your own schematic, scan it then post it here. Steve
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Old 9th June 2019, 12:40 PM   #3
Carlosraj is offline Carlosraj  United Kingdom
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I have checked all the resistors and caps, they are all OK. What I am concerned is whether the resistance readings on the output transformer are normal. I totally isolated the output transformer (took it apart from the amp) before taking the readings.
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Old 9th June 2019, 04:16 PM   #4
aut0m4tic is offline aut0m4tic  United States
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I've had this exact problem on a few of the console amps I've worked on. It's always boiled down to a bias problem. Coupling caps from the anode of the preamp tube to the grid of the output tubes have been the culprits for me. Even leaking 1 volt is a dealbreaker, you a negative voltage on the output tube grids to keep them out of meltdown.
I cant speak about typical xfmr resistance readings, but where they are part of the supply, and not part if the control, I doubt that's the cause of your overheating problem.
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Old 9th June 2019, 11:35 PM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by Carlosraj View Post
...center tap(red) to right(brown) reads 77 ohms. From center tap(red) to left(yellow) reads 11 ohms....
While 11r may be low, this is NOT why one tube is overheating.

As Steve and others say, read voltages at pins, suspect old/bad capacitors and resistors.
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Old 9th June 2019, 11:39 PM   #6
PRR is offline PRR  United States
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Not the 500, but possibly related:
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File Type: jpg ga2000_schema.jpg (90.7 KB, 19 views)
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