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Post subject: Fender ProTube Twin Amp - Output Power Selector Problems
Posted: Thu Jul 29, 2010 12:43 pm
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I have a Fender ProTube Twin amp purchased in 2007. This amp has an FULL-1/4 (100w/25w) output power selector switch.

My problem is when set to the 1/4 or 25 watt position my 4 6L6 output power tubes all overheat and glow bright orange. The tubes got hot enough to discolor the print on the tubes. The volume does not decrease to 25 watts and it makes all kinds of loud deep noises. Also when you engage the tremolo the termolo effect it is almost non existent and the amp output makes more noise.

The amp works fine when the Power selector is in the FULL or 100w position. This only happens when put in the 1/4 (24w) mode.

Has anyone else ever had or heard of this problem? If so can any one point me to a possible fix. I have the schematics for this amp and have elctronic skills, though very rusty, but not with Tube amps.

Just a note all tubes, elctronics are MFG original. This amp has never been worked on.
Thanks


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Post subject: Amp power problem
Posted: Thu Jul 29, 2010 1:47 pm
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Joined: Tue Jun 29, 2010 1:20 pm
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Hi Stratcat80,

If your amp has a warranty, working on it may void that warranty. I think that it is five years.

First, turn off the amp and unplug from electricity! Make sure that the capacitors in the amp are discharged too!

Looking at the schematic, find the switch contacts in the power supply circuit. On the drawing they are at the bottom middle to the right of the transformer, above the filaments, there are two contacts. It looks like they are lowering the supply voltage for the plates for the lower wattage selection with the switch. Starting with the switch itself, check continuity while switching, and look for signs of heat damage on the switch and then the circuit board (discoloration).

You may have a shorted diode in that circuit, or even an open resistor. Start with D200 and check all up to D211 for short or open. Then check all of the resistors in the circuit around the diodes R200 through R208. If you can, it would be a great idea to get a magnifier and check the solder joints on the underside of the board for cracks, and cold joints. Also look at the traces, they can lift easily, especially in areas of heat damage. Bad solder joints are very common and are the cause of a lot of problems. It's even possible that the caps in that circuit are bad, at least check them to make sure none are shorted.

Just a couple of ideas, let me know. :)

.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Posted: Fri Jul 30, 2010 12:05 pm
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Thanks for you reply shimmilou.

That was the area I was thinking of checking first also just wasnt shure. I also didnt realize the the electronics was covered by warrenty for 5 years. Its only the tubes, speaker, cabinet ect thats only covered for 180 days to a year.

Its time to either pull out my trusty Fluke multimeter or go for a long road trip to my nearest authorized Fender service dealer.

Either way i will post back here for all to know the fix for this problem.

Thanks again shimmilou for your help.

Rock On

Stratcat


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