Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Pastortom on August 27, 2010, 04:02:09 pm
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Lookin' for some input.........
Generally speaking, would a "weak" intensity on a Fender Vibrato mean a "weak" optoisolator, or are there more variables that commonly cause the weakness?
Again, it works......but the intensity is just very weak......full intensity (10) acts like a 1 or 2 setting.
What say ye?
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Could be a bad solder, a weak tube or a weak neon/LDR module. Begin by checking/reflowing board connections, then try changing the 12AX7 in the oscillator stage with new or known good tubes, then try a new roach. Less probable causes: defective passives - caps, resistors and pots.
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I'd change the tube first. You need a really strong 12AX7 in that circuit.
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Begin by checking/reflowing board connections, then try changing the 12AX7
Why on earth would you open up an amp and start reflowing joints before just plugging in a different tube?
(It's nearly always a weak oscillator tube)
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I have also seen it where the bulb was not lined up with the opto in Fenders
Kind of rare, but it can happen.
You can realign the parts and re-heat shrink them.
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I'd change the tube first. You need a really strong 12AX7 in that circuit.
Agreed, but I've just worked on two vintage DR's suffering from weak vibrato because of board-related problems. Both were fitted with recent tubes and functional roaches, both were fine after a few reflows.
Changing the 12AX7 in the vibrato section is usually sufficient with recent amps.
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I have seen more than one amp with no cathode bypass cap on the oscillator tube. Result was weak oscillation, slow to start.
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How old is the amp? The light sensitive resistor in the roach will lose its effectiveness over time, diminishing the effect, but definitely check the tube first.