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Topic: Intermittant Twin Reverb |
Dave Meis
From:
Olympic Peninsula, Washington, USA
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Posted 9 Nov 2015 2:43 pm |
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'79 Fender Twin 135. Turn it on, flip the stand by and it plays beautifully...play a while..a few minutes to a long time, and the the tubes go cold, but the pilot light is still on. I've had the chassis out and poked and prodded for a loose wire, cold joint ect., but no joy. The most recent service was to replace the 'hum balance pot', which goes to V2. It was some time back, but with it being intermittent, it could be related? Sometimes the power can be restored to the tubes with a judicious 'whack' to the cabinet, but not in a reliable fashion. Any ideas that don't involve an incendiary device?
TIA, Dave |
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Lane Gray
From:
Topeka, KS
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Posted 9 Nov 2015 5:40 pm
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I'd bet you have an easily found thermal fault in the power supply, like perhaps the heaters, or less likely that high voltage.
Describe the symptoms to your tech, and I wager it'll take the minimum charge.
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Ken Fox
From:
Nashville GA USA
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Posted 9 Nov 2015 6:00 pm
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The fact that the power light stays on indicates the power supply is fine. The 6.3 VAC supply for the tube heaters is also used for the pilot light. The 6.3 supply usually goes to the first power tube on the left as you face the rear of the amp. It then goes to in parallel to the remaining power tubes and then to the preamp tubes. As I recall on that model the heater wiring goes from the last preamp tube to the 100 ohm, hum balance pot, which is center tapped to ground. Hum balance is accomplished by varying the power supply rails to ground. On older Fener Amos this was done with either a center tap off the 6.3 VAC section of the power supply transformer to ground or by creating a center tap to ground via two 100 ohms resistors, one coming from each side of the power on lamp to ground.
So if all the tubes are losing heater/filament-voltage then look for a cold joint at the start of the area where the loss first occurs. If some heaters are on and some off then that will tell you exactly the area to look at.
Most definitely sounds like a cold solder joint |
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Ken Fox
From:
Nashville GA USA
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Posted 9 Nov 2015 6:42 pm
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| You are correct on the balance control from V2. My memory was off there. |
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Dave Meis
From:
Olympic Peninsula, Washington, USA
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Posted 9 Nov 2015 11:51 pm
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Thanks guys! It was ALL of the tubes when they would go cold. When it would go out, I could poke around between the pilot light and the first power tube and it would come back on. so I started at the pilot light and, while I didn't find an obvious reason (broken wire, dull solder, ect.), I re-flowed the joints and so far, so good. Heater pairs are running 5.9 VDC all the way through, so I'll leave it apart and run it all day tomorrow and see what happens. I will say it's never been so quiet!  Thanks again for your help! Dave |
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