Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Pastortom on November 09, 2010, 02:34:54 pm
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I have a non-working vibrato on a 1972 Twin Reverb...........
I've checked for a bad RCA on the foot pedal input, and it's OK.......I checked the driver tube and socket, and it was OK, BUT I put in a new 12AX7 anyway to be sure......Still nothing......I just changed the roach, and STILL nothing.......
...and when I say "nothing", I mean this: When I adjust the 'intensity' pot upwards, the amp volume goes DOWN a little at a time, about the same as if it WERE working properly......BUT, no oscillation at all.......zilch........it seems to be responding, but no oscillation.(not even a faint "tick".....no oscillation at all).
If I turn the circuit on and off, you can hear the audible difference.....'Off' is a little bit louder overall.....'On' a little lower.....Again, it's like the circuit is ACTIVE, but not oscillating.
I've been told the tube actually does the oscillating, with the roach just feeding the rest of the circuit.........I THOUGHT that tube 5 (counting from the right rear as #1) was the vibrato driver........
If I'm missing something simple here, please advise............I installed the roach in what appeared to be the same position as the old.....The new has a silver dot on one end....the old does not......Is there a specific polarity on these opto's? (possible to solder it in "backward" ?)
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Silver dot should be facing the tubes, not facing the pots.
(http://www.tubesandmore.com/cemirror/inv/vibrato_ldr.gif)
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Well then, it's in there correctly.........Good joints........should be working. So which tube is the vibrato driver? Do I have it correct?
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Don't trust just using the pedal to activate the trem.Get an RCA cable from your home stereo and plug one end in and ground the other end with a jumper wire.
V5 is the trem tube.You need to check to make sure the tube has voltages at all the pins.If the cathodes don't get ground somewhere the tube won't oscillate.
Take a look at the layout and see where the trem pedal plugs in?Ground it internally if you have to.You could even have a bad female RCA jack on the rear panel.You need to investigate the obvious first before you start throwing parts at it.
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I never trust a footswitch..........it acts the same whether or not I use a switch or a shorted plug.
I checked the RCA for an obvious problem and didn't find anything........I'll check it again, and I guess I'll have to check the individual pins of the V5 socket.........There's a brand new 12AX7 in there as I said, so I doubt the tube is the problem. (no difference after I changed from the original).
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What about the intensity pot ground?No ground,no worky.
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Just checked that pot ground.......it's excellent........
Checked all of the pin connectors and joints on V5.......
Really weird..........Turn the intensity up, the volume goes down gradually....(in other words, the pot is actually working in some way), but no oscillation.......new tube and new roach.......no difference from the old tube and old roach.
Anything else such as a resistor in that circuit that might cause such a thing?
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Turn the intensity up, the volume goes down gradually
Almost sounds like the neon light inside the roach is glowing continuously, rather than flashing. Try this... Unsolder the roach leg that connects to the INT pot. With the amp powered off, measure the resistance of that dangling roach leg to chassis. Should be verrry high. Now turn the amp on and repeat the resistance check. Does the resistance become some steady but lower value? Or do you get an erratic reading? Or is the reading the same as you measured with the amp powered off?
A roach that is working properly will be flashing at the trem speed. You may be able to see the flash by looking carefully at the end of the roach that faces the tubes. You may have to use a razor knife to slice a small hole into the heat shrink tubing to see the neon bulb. Don't worry, you can use dark fingernail polish to cover the hole when you have the neon flashing properly. Or just use electrical tape.
Almost any component in the V5 circuit can kill the trem, but not many are likely suspects. Tube, roach, and jack/footswitch are most likely suspects.
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What are your voltage readings on v5?