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Topic: Odd Fender Twin Reverb Problem |
Jerry Van Hoose
From:
Wears Valley, Tennessee
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Posted 22 Dec 2013 4:57 pm |
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One of my Fender Twin Reverbs, an original 68 "drip-edge", has developed an odd problem. Whenever it's setting flat on the floor, it works great & sounds great. Sometimes, I use an "Ampwedge" and slightly tilt it backward, raising the front about 2", maybe even less. As I previously stated, when setting flat, no problems whatsoever. However, when it's slightly tilted backward, it develops a small pulsating hum and the entire amp acts as if it's "microphonic". You can actually tap the top, side or back of the amp and it will amplify the tap through the speakers, just like a microphonic pickup. Again, if it sets flat on the floor, perfect, no pulsating hum, nor the "microphonic" problem previously mentioned. If it's tilted forward or flat on the floor, no problem, just whenever it's slightly tilted backward. Any ideas or suggestions would be greatly appreciated.
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GFI w/Steeltronics 507, Profex II, Nashville 112’s, Hilton, George L .155’s, BJS, Steelers Choice |
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Lane Gray
From:
Topeka, KS
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Posted 22 Dec 2013 5:09 pm
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I suppose I coulda waited for an amp guru, but I'd suspect a barely microphonic tube. Tap the various tubes and I bet one of them is the cause.
I swapped out one of my preamp tubes for a 12 AT7, and now the whole amp is microphonic. The AX is going back in it next time I get a tuit of adequate circularity.
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2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Rick Abbott
From:
Indiana, USA
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Posted 22 Dec 2013 6:24 pm
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I'd venture a guess that a tube is not microphonic, but that a tube socket needs to be cleaned and retensioned. Take off the tube shields and then turn on the amp (flat on the floor). Then turn it on and plug a guitar into it; I just set the guitar in a stand next to the amp. Strum the open strings and then wiggle the first preamp tube in a circular motion (might want a thin glove?) Do this for all the preamp tubes and the 12at7 phase inverter as well. Be very slow and careful, listen for the noise you describe.
After this, let us know what happened. If it's a dirty socket, you can fix it. With advice and caution.
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RICK ABBOTT
Sho~Bud D-10 Professional #7962
Remington T-8, Sehy #112
1975 Peavey Pacer 1963 Gibson Falcon |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 22 Dec 2013 9:54 pm
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Unplug the reverb and see if the problem stops... you might need to pad the bag a little better. Also, you might need a new reverb tank... the dampers in them get old and they get crazy microphonics. If the tank/bag gets bound up you'd have the problem you cite. I've had good luck replacing the bag with a bag made from thin insulating foam and packing tape... like bubble-wrapping your reverb tank. I can't see how tilting an amp would affect a microphonic preamp tube, but anything's possible... part of the charm of these beasties, they're very organic.
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Too much junk to list... always getting more. |
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Tim Marcus
From:
San Francisco, CA
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Posted 23 Dec 2013 1:16 am
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Gotta be a loose connection somewhere - if the whole amp is microphonic with both channel volumes down to 0 and reverb down it would suggest something in the output which can be bad for the amp
Does it happen with all the volumes down?
My guess is when tilted back a contact is broken or made. Remember fender circuit cards absorb moisture and can warp over time - a resistor or capacitor may have fractured from the warped board causing oscillation or a change in bias when tilted. I'd get it to a tech
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Milkmansound.com |
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Bruce Derr
From:
Lee, New Hampshire, USA
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Posted 23 Dec 2013 6:01 am
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| Another vote for the reverb cage. Pull the cage out and look for loose or broken springs, including the suspension springs. I don't think a tube's 'microphonicity' (is that a word?) would change that much from being tilted, but that would be my next suspect. Easy to check for by tapping each one with a pencil. |
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Dave LaSalle
From:
Ontario, Canada
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Posted 23 Dec 2013 6:15 am
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I'm with Lane on the tube thing .Had the same thing with my Twin.
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Politics:Just vote .79' Sho-Bud "Super Pro"Webb,Fender Steel King, |
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Jerry Van Hoose
From:
Wears Valley, Tennessee
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Posted 23 Dec 2013 8:20 am Problem Solved
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Thanks everyone for the many suggestions. I appreciate it more than you could ever realize. I recently had the amp re-capped & at the time, the amp tech told me that my original tubes tested better than any new ones that he had in stock. My amp expertise is limited to removing & replacing a chassis after a cap job, changing out tubes, replacing a speaker or reverb tank....absolutely nothing electronically beyond plugging in the power cord, ha. The reverb tank was my first suspicion & easiest to check, so I unplugged it & the problem still remained. Next, I carefully began tapping & checking tubes with a pencil. Thankfully, all of the original RCA pre-amp tubes were ok, however, whenever the 6L6 next to the end of the chassis was tapped, the noise worsened/stopped, then began again. So, I removed all 4 of the RCA 6L6's and replaced them with a new matched quad of JJ's from the Vibrosonic. They've been powered up now for over an hour, tilted front/back & sideways, noise gone, problem seems to be solved. I've wrapped the other 3 RCA 6L6's in bubble wrap and will store them away. Should I try to find an RCA 6L6 to match those 3 to re-install or go with a new matched quad set of foreign origin? Once again, I want to thank all of those that replied as I greatly appreciate all of the suggestions and consideration. Follow up....out of curiosity, I just now installed the suspect RCA 6L6 in my other Fender Twin Reverb for about a 10 minute duration, same problem, same pulsating/microphonic noise. Then, removed & re-installed prior tube, all ok. So, it was definitely the tube.
 Wish I had a new Milkman amp  .
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GFI w/Steeltronics 507, Profex II, Nashville 112’s, Hilton, George L .155’s, BJS, Steelers Choice |
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Lane Gray
From:
Topeka, KS
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Posted 23 Dec 2013 12:40 pm
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Before I'd have swapped out the tube, I think I'd have tried retensioning the socket, and it's possible that there's nothing wrong with the old tube, but the new one just fits a bit better in the old slightly loose socket. I had a 6L6 that acted weird but retensioning the socket helped it a lot.
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2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Craig Baker
From:
Eatonton, Georgia, USA - R.I.P.
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Posted 23 Dec 2013 1:39 pm
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Jerry,
Save that "bad" 6L6 for me. I can use it as a spare for my antique Heathkit CW transmitter.
Warmest wishes for a wonderful Christmas.
Craig Baker 706-485-8792
cmbakerelectronics@gmail.com
C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024 |
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Jerry Van Hoose
From:
Wears Valley, Tennessee
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Posted 23 Dec 2013 2:05 pm
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Lane, that's a good idea, so I tried the suspect tube in my other Twin & the Vibrosonic, different positions too but with the same outcome. It's definitely the 45 yr. old tube. Presently, all is well, thanks to everyones replies & suggestions. Well, maybe except for the Vibrosonic that's now missing 4 power amp tubes, ha. I've decided to order a new matched quad set (JJ's). Craig, oh yes, I remember Heathkit, enjoyed their catalogs too, knew a guy that had one of their guitar amps. Merry Christmas!
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GFI w/Steeltronics 507, Profex II, Nashville 112’s, Hilton, George L .155’s, BJS, Steelers Choice |
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