'78 Fender Twin MV

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'78 Fender Twin MV

Postby Alain Benoit » Sun Jul 24, 2011 2:47 pm

The customer wanted a general servicing including, replacement of all tubes, bias check, replacement of all four output sockets, foot pedal for the tremolo, and diagnosis of reverb intermittancy issue.

Tube complement is 2 x 12AT7, 4 x 12AX7 and 4 x 6L6GC.

Old output sockets.
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At the bottom left hand corner you'll see the main bias supply resistor. The schematic clearly calls for a 3k3 value here. As you can tell this one is a 33k.
A value of 33k dropped the bias voltage to approximately -38VDC, at this voltage the lowest achievable idle current was well over 70% of maximum plate dissipation.
Seeing as this component looks possibly original, this amplifier would have been cooking output tubes since 1978.
Having the opportunity to select a new value I went with 2200 ohms. This gave me around -45VDC for an idle current of near 60% at the halfway point of the potentiometer's travel. 60% is considered average. FYI, B+ was 435V.
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Old preamp tubes.
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New preamp tubes.
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Old output socket wiring.
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New Sockets. Grid resistor, 470Ω 2W Metal Oxide. Bias feed resistors, 1k5Ω 1W 1% Metal Film.
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One service tech had installed 220k filter cap bleed resistors.
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Faux center tap. 100R, 1%.
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1 Ohm resistor from cathode to ground to measure bias.
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New bias feed lines. Teflon wiring.
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New output tube socket wiring. Also tidied up the AC wiring. New grounded line cord. And took the unfused utility outlet and the ground select switch out of service.
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Re: '78 Fender Twin MV

Postby macrae11 » Mon Jul 25, 2011 8:24 am

Looks pretty gungy. Seeing as how hot the output tubes were running, did it still sound like a twin? And since it was an evil twin, was putting it back closer to spec actually an improvement? ;-)
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Re: '78 Fender Twin MV

Postby Alain Benoit » Mon Jul 25, 2011 11:21 am

I thought it was actually in pretty good shape. What makes you say that it was an 'evil' twin? I always had referred to the red knob twin as the 'evil' twin.
Did not hear the before. Still haven't heard the after.
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Re: '78 Fender Twin MV

Postby Mathieu Benoit » Mon Jul 25, 2011 11:34 am

Internet wrote:The Twin Reverb II was replaced by The Twin, commonly referred to as the "Evil Twin" due to the addition of both a separate gain channel and a switchable overdrive on the clean, in 1987. It featured a dual output switch, allowing the user to select either 25 watts or 100 watts of output. The Twin was designed for maximum versatility for live or studio use with features like speaker jacks with selectable impedance, Low-z/XLR output, a "Powwr Amp Thru" feature that bypassed the preamp circuit and a buffered effects loop. Both clean and overdrive channels could be activated simulateously and reverb could be turned on/off for each channel. Earlier Twin Reverb amps were known among musicians to be best suited for loud, "clean" tones. The "Red Knob Twin" was made until 1994; it has since been succeeded by "Twin Amps" of Fender's current Protube line.
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Re: '78 Fender Twin MV

Postby Mathieu Benoit » Mon Jul 25, 2011 11:37 am

macrae11 wrote:Looks pretty gungy. Seeing as how hot the output tubes were running, did it still sound like a twin? And since it was an evil twin, was putting it back closer to spec actually an improvement? ;-)


The thread title does say '78.
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Re: '78 Fender Twin MV

Postby macrae11 » Mon Jul 25, 2011 12:59 pm

Sorry my bad. I always knew the evil twin as the red knob jobby too, but I was under the mistaken impression that they started manufacturing it much earlier than they actually did. Just basing the condition on the corroded tube sockets and some of the wiring issues, particularly image 571.
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Re: '78 Fender Twin MV

Postby Alain Benoit » Mon Jul 25, 2011 1:08 pm

Funny how I knew exactly which image you were referring to.
I have seen Fenders of this age in alot worse shape but few in better. Remember Aaron's Twin?
That being said what surprised me was apparent bleaching of the wiring, almost as if it has been exposed to UV radiation for extended periods.
On the other hand it had been serviced in the past by a qualified tech. The main filter bank had been replaced, some with Mallorys, all electrolytics on the preamp board had also been replaced. The cathode to ground bias measurement resistors along with the screen and faux tap resistors had also been replaced. Also as a nice surprise, 220k filter bleed resistors had been installed.
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Re: '78 Fender Twin MV

Postby Alain Benoit » Wed Jul 27, 2011 9:04 pm

Here is the tremolo pedal that the client wished built.
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After this picture was taken I remembered that the reverb and tremolo jacks on these older twins are RCA, so I changed it.
Image
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