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Fender Twin Reissue Issue

 
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ID Crisis



Joined: 25 Aug 2006
Posts: 2366
Location: Quad Cities, USA

PostPosted: Thu Jul 22, 2010 5:39 pm    Post subject: Fender Twin Reissue Issue Reply with quote

I have an odd issue on a Fender Twin. The Tremolo wasn't working which I repaired by replacing Resistor R146. See schematic link below

http://support.fender.com/schematics/guitar_amplifiers/Twin_Pro_Tube_Amp_SchE45.pdf

This took care of the tremolo. It also needed new power tubes, which I replaced with JJ's. Checking the bias at the test points on the back of the amp I get 323mV (recommended is 60 mV) turning the pot does not have much effect on it.

Also I am running the amp with the switch at 1/4 power. If the switch is in full power mode I am getting a loud hum and the 2 power tubes closest to the OT start red-plating.

I know the tremolo works by varying the bias on this amp, so not sure if it's all related or not.

The other thing is, it sounds like this amp has run on low power. He said they rehearse in a place where someone wired it themselves and when he runs his amp there the pilot light is dim (not the case at his home or in my shop. I am guessing the whole band may be connected to one circuit and the Twin is not getting enough current. He also said they recently played a gig running off of a generator.

Anyone seen this before or have any ideas?

Thanks!
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tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Sat Jul 24, 2010 8:12 am    Post subject: Reply with quote

the tremolo in that amp is an optical LDR-based tremolo, not a bias-vary trem. Shouldn't have anything to do with your bias woes.

Running an amp on low power can be bad for it.

As you've figured out though, there's something in your bias supply that's whack. I'd start there and trace the circuit until you find out what's wrong.

Obviously, on a reissue, this is going to be something of a PITA. They're not "service-friendly" compared to the originals.
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ID Crisis



Joined: 25 Aug 2006
Posts: 2366
Location: Quad Cities, USA

PostPosted: Sat Jul 24, 2010 10:36 am    Post subject: Reply with quote

tangelolemon wrote:
the tremolo in that amp is an optical LDR-based tremolo, not a bias-vary trem. Shouldn't have anything to do with your bias woes.

Running an amp on low power can be bad for it.

As you've figured out though, there's something in your bias supply that's whack. I'd start there and trace the circuit until you find out what's wrong.

Obviously, on a reissue, this is going to be something of a PITA. They're not "service-friendly" compared to the originals.


Looking at page 2 of the schematic, the tremolo intensity control is directly connected to the bias circuit.

They truly are a PITA. The circuit board seems to be glued to the standoff posts. I was fortunate to be able to get to R146 through one of the tube socket cutouts and didn't have to remove the board.

I'll keep checking the bias circuit.
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tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Sat Jul 24, 2010 12:49 pm    Post subject: Reply with quote

For some reason I had assumed you were talking about the Twin Reverb Reissue. Obviously looking at the schematic you linked to, you're talking about the "Pro Tube" Twin Reverb, which I've never worked on.

I don't know that amp well and I'm going cross-eyed trying to read the schematic sideways! Very Happy

I'd still just go into the bias area of the amp and start tracing the circuit.

Sorry I'm not more help.

Good luck.
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Structo



Joined: 21 Nov 2004
Posts: 26415
Location: Salem, OR

PostPosted: Sun Jul 25, 2010 9:05 am    Post subject: Reply with quote

I see on the schematic that it calls for 60vpp @ 9Hz.
That doesn't make much sense to me because the bias voltage to the power tube grids should be negative.


Check out TP54 on the bias supply. This is part of the half/ full power switch section.
On low power it should measure -25.2vdc
Full power should be -63vdc (I think, it's blurry)

If those two voltages are off, then I would inspect the components coming off of the bias tap on the PT, work backwards from that test point if the voltages are wrong.
It could be that switch or other R207, R208, or any component from the PT forward.

I hate working on newer pcb amps!!!

Plus, if what you say about the kid running this thing on low power or off of a generator, it could be a combination of things that will be difficult to trace.

I look at all the solid state crap on these amps and think, why?

Good luck.
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