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Topic: Fender amp reverb problem |
Rick Batey
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Posted 13 Oct 2009 2:37 pm |
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I've got a gig in a couple of days, my Fender combo's reverb has gone west, and I've no time to get it to a tech.
The amp is a '73 Pro Reverb. Amp works fine but there's suddenly no reverb, just a loud increasing hum as I turn the reverb knob up. I cleaned and re-tensioned the connections to the pan: no dice. Has V3 or V4 gone down? Both are glowing. How can I tell which one's died? Anything else I can do... or will it just be a reverb-free night? I'm not to be trusted inside a tube amp... but any suggestions welcome. |
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 13 Oct 2009 3:07 pm
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You can swap V1 for V4 to test the return tube, Both 12AX7s (7025). I would not advise swapping V3 for the phase inverter Just incase it is in fact bad(12AT7). Check that one with a known good tube although a 12AX7 will work there in a pinch so there you go with V1 again. You may have lost the reverb driver.
Did you remove the pan from the bag and clean/tighten the connections there? |
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 13 Oct 2009 3:11 pm
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| I forgot to memtion that you can run the amp with V1 removed for testing. Makes a cool spare at a gig too. |
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Mike Fried
From:
Nashville, TN, USA
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Posted 13 Oct 2009 3:25 pm
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A "loud increasing hum" sounds like it's most likely either an open hot connection in the reverb's "output" cable or the output side of the reverb tank has gone "open". It's also possible that V4 has gone bad - try swapping it out and see what happens (if you don't have spares, pull V1 or V5 and try it in the V4 socket). (BTW, the tubes lighting up only indicates that their heaters are working, not that it's functioning ok.) If that's not it, it's likely a problem with the cables or the tank (most likely if it stopped working after it had been moved). It's much less likely V3, but you can try a 12AX7 there to test if you like.
If the tubes are ok, try replacing the cables with a pair of stereo rca cables from your hifi or Radio shack, etc., making sure that you maintain the correct inputs/outputs.
If that doesn't fix it, it's likely the tank. Remove the tank from the bag and the bottom cover from the tank (four short phillips screws). Look at the thin wire connections going from the spring assembly to the "rca" jacks on the tank enclosure and see if any of them have broken (a common problem). If you have a low-power (15-25W) soldering iron (and rosin-core solder) and can strip the wires, you can resolder them yourself.
If none of this fixes your reverb, it's a problem inside the chassis, probably at the input to the reverb recovery section of V4. That's gonna take a tech...
Good luck!
_________________
Visit my music page at http://facebook.com/drfried |
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Rick Batey
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Posted 13 Oct 2009 3:31 pm
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Thanks, Rich. Yes, I had cleaned the connections inside the bag. I tried the tube swaps you suggested, but with no improvement. I guess that's the limit of my fixing abilities unless you have any other brainwaves.
Good point about V1 - I actually haven't had a tube in that position for a long while, it lives in my spares bag!
Appreciate the help.
Whoa - I posted at the same time as Mike - I'm going back in. |
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Rick Batey
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Posted 13 Oct 2009 4:03 pm
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Success!
It was one of the thin wires from the spring assembly to the jacks.
I'm back, and boingin'. Thank you, Mike. |
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Mike Fried
From:
Nashville, TN, USA
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Posted 13 Oct 2009 5:01 pm
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You're welcome, Rick. I'm glad (and not surprised) that was the problem. Those little wires have always been the Achilles Heel of those spring reverb tanks.
BTW, removing the V1 tube raises the gain (and lowers the headroom) of the other channel a tad, as they share a cathode resistor/bypass cap for each channel's second gain stage. You may or may not want the increase, but if you're looking for the "cleanest" possible tone V1 should be in there.
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Visit my music page at http://facebook.com/drfried |
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