Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dpm309 on July 25, 2011, 06:54:39 pm
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I am working on a Buddy's '65 DRRI that is having reverb problems. I eliminated the tank, driver and recovery tubes, and checked all of the components near V3 and 4. The amp emits a high pitch squeal when the reverb control is above 3 or so. This happens with or without the tank and guitar plugged in. I do not have the footswitch for it so that is ruled out as well. My next step is to check out the reverb driver and the question I have, is a 022921 (125A20B) equivalent to the Fender reverb driver number 037099? This is the one Doug sells and it says it is for all Fender reverb amps.
Thanks,
Dan
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Doug's transformer is the right one. From what you say it's unlikely that the problem is with the driver or transformer. Look closely around the recovery tube.
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Thanks Sluckey. When I worked on this amp last, I changed out the 47uf and 220uf/100V filter caps due to some strange motor-boarding and distortion sounds. In retrospect, I should have changed out the 3 22uf caps also. These were all Illinois caps that everyone was talking about in earlier post. The consensus seemed to indicate a bad batch of IC filter caps in Fender amps that were manufactured in the 1990's. I will change these out and report back. I did notice that one of these filter caps is connected to V4, the recovery tube.
Thanks,
Dan
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Well, I changed out the 3 IC 22uf filter caps with Nichicon 22uf caps and that did the trick! The lesson I learned is to change out all filter caps, even if only 1 or 2 are bad. This also supports the case against IC caps. I have used IC caps in several of my builds with no problem but I think that Fender (and possible a few other amp companies) must have used a bad batch of IC filter caps sometime in the 90's. Anyway, the amp is working and I will return it to my bandmate.
Dan
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I think that Fender (and possible a few other amp companies) must have used a bad batch of IC filter caps sometime in the 90's.
I wouldn't call filter caps that lasted 10 to 20 years a "bad batch".... :icon_biggrin:
Glad you got to the bottom of your problem!
G
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Wasn't it in the late 90's we had those caps with defective dielectric compounds because of a defective stolen formula reproduced in China? Wait... that was early 2000's wasn't it. Almost everyone got bit by that monster... especially those wanting CHEAP caps.
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While testing the amp, I noticed a slight squeal/feedback sound and it came and went as I tapped V4 (reverb recovery). Swapped it out with a new JJ 12AT7 and the noise went away. The original 12AT7 must have gone microphonic. Before I changed the filter caps, I did substitute the bad tube with the JJ and it did not solve the original problem. I also ran every tube through a B & K 747B tube tester and they all tested out OK. This tube must have been microphonic before I started working on it. I guess that even a B & K tube tester is not fool proof or cannot determine if a tube has gone microphonic.
I was wrong about the age of this amp. It was actually purchased around 2005 so the caps were less than 6 years old before they went bad.
Dan
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I noticed a slight squeal/feedback sound and it came and went as I tapped V4 (reverb recovery). Swapped it out with a new JJ 12AT7 and the noise went away. The original 12AT7 must have gone microphonic.
V4 should be a 12AX7.
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While testing the amp, I noticed a slight squeal/feedback sound and it came and went as I tapped V4 (reverb recovery). Swapped it out with a new JJ 12AT7 and the noise went away. The original 12AT7 must have gone microphonic. Before I changed the filter caps, I did substitute the bad tube with the JJ and it did not solve the original problem. I also ran every tube through a B & K 747B tube tester and they all tested out OK. This tube must have been microphonic before I started working on it. I guess that even a B & K tube tester is not fool proof or cannot determine if a tube has gone microphonic.
I was wrong about the age of this amp. It was actually purchased around 2005 so the caps were less than 6 years old before they went bad.
Dan
The B & K 747-B is a good general purpose mutual conductance tester. I have one myself. I use it to weed out tubes that have shorts or extremely low gain. These testers can check for shorts and check the level of the gain on an arbitrary meter, but they cannot check for microphonics. The best test for a tube once you've determined that it isn't shorted is to test it in the circuit of the amp you intend to use it in and see how well it works and sounds. Even with the nice military Hickock testers, or some of the more modern testers that New Sensor, Vacuum Tube Valley, and Eurotubes use, that is still the case.
Greg
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Sluckey,
My bad! :BangHead: I had V3 and V4 switched. I put the original 12AT7 into V3 and 12AX7 into V4 and the amp is running great. I tapped both tubes and did not get any signs of microphonics. Would this tube switch have caused the feedback/squeal?
Thanks for catching my mistake.
Dan