Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jjackson7 on March 29, 2012, 11:02:40 pm
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I have a friend's Jet City 100H that has malfunctioned. During band practice the volume dropped to about a quarter of what it was. After checking the obvious (cords, guitar, cab) we decided to change tubes. I had some known good 6L6 and 12ax7s laying around so we changed all and still the same problem. I posted the voltages from the tubes on the voltage charts here under Jet City 100H. There are no obvious burnt resistors anywhere in the amp. I chopsticked around to see if I could get the volume to come back - no luck. We went from the effects loop send into the effects loop return in my amp - plenty loud. Then went the other way with it - same problem. I guessed the problem was a bad solder joint somewhere in the power amp. Re-flowed the tubes and output transformer primary - same problem. I'm running out of ideas. But am willing to try any suggestions. My tools are limited but I do have a good meter Fluke 179. Thanks in advance for any help. I have a pdf of the schem for the 50 watt if that helps.
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Been searching earlier posts. It seems there is probably a resistor open somewhere. I'm going to keep searching. Any other tips would be great. Thanks.
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Had one of these in my shop with the same issue... Thought is was fixed and then it wasn't. Found that the OT died and replaced it. Do you have a spare lying around to swap it out for a test?
PM me if you want some more info about a replacement.
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The only OT I have laying around is from my bassman 70. Is there a way to test the OT besides unsoldering and ohming? I checked primary and from center to each side is 15.6 ohms. I didn't unsolder the secondary. Where there any measurable indications that the OT you replaced was bad? I don't think my friend is going to believe he needs a new OT but I haven't found anything else even remotely suspect (I am relatively new to this though.)
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Okay found this article. http://www.geofex.com/ampdbug/outtrans.htm. (http://www.geofex.com/ampdbug/outtrans.htm.) "Sometimes a primary will have enough resistance that the transformer does not burn up or burn the wires in two , but will just bog down, have low power and sound bad." This seems familiar though the tone of the amp is the same. I don't have a variac to do the advanced test but I'll look around for one to borrow. I would be interested to see the results of the test even if it is fine.