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Author
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Topic: Fender Pro Jr. pre amp tube problem
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Kevin Ruddell
Member From: Toledo Ohio USA
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posted 14 November 2005 04:52 PM
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I bought a new Fender Pro Jr. amp at the local dealers last spring, it's the 15 watt tube amp with a volume knob and tone knob and 10 inch speaker . The EL 84 stock tubes sounded awful as did the pre amp 12AX7's . I replaced them with EH tubes and there was a huge improvement in sound. Much less tinney and midrangey. On my Peavey Classic 30 tube amp I put a JJ Tesla 12AT7 in the first preamp tube slot, and it really smooths things out because of the lower gain compared to the12AX7 . I spoke with Lloyd at Peavey before doing this . One of the guys at the local Fender dealer has done this on their amps also. When I tried it with my Fender it sounded great but started cycling a high pitched whistling sound after a few minutes. When I lightly tap on the end of the tube the high pitched noise increases in intensity but performs with no problems when inserted into my Peavey Classic 30. I've tried this a few times and the same problem occurs . Has anyone else had this problem ? I'm going to try another manufacturers 12AT7 tube but not sure why it works with no problems in the Peavey but not the Fender. |
Jay Fagerlie
Member From: Lotus, California, USA
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posted 14 November 2005 05:13 PM
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Hey Kevin,
It sounds like parasitic oscillation, and it is usually attributed to the way the connections inside are routed, either the wire or circuit board runs.
If you are inclined, put the squealing tube back in the amp and move around the wires around that tube. You can use a chopstick, or any NON-CONDUCTIVE probe. Be careful, there's some deadly juice in there!
Sometimes just moving one lead away from another will stop the problem.
The first thing I would do is to try another tube.
Jay[This message was edited by Jay Fagerlie on 14 November 2005 at 05:14 PM.] |
Ken Fox
Member From: Ray City, GA USA
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posted 14 November 2005 05:18 PM
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I would think both amps are PC board, not hand wired. The tube sounds microphonic to me. One amp is likely higher in plate voltages and brings out the worst of that tube! I would try another tube. A micphonic tube will often work in the later gain stages of an amp but be very prone to show its mocrophonic tendancies in the early gain stages of the circuit. |
Jim Sliff
Member From: Hermosa Beach California, USA
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posted 15 November 2005 04:57 PM
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They're both PCB amps.Hard to tell what the problem is, but I agree with the idea of parasitic oscillation - just an unknown cause. It needs a visit to the tech. If you replaced the stock EL84 tubes with EH tubes, you replaced them with the same thing - just a different label. But they vary wildly in quality, so it's not a big surprise. Advice - never replace all your tubes at once if your amp sounds bad. Start with one preamp tube, and work one at a time - it's the only way to pin down where a problem may lay in the circuit. The "shotgun" approach is a bad idea. |
Kevin Ruddell
Member From: Toledo Ohio USA
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posted 16 November 2005 03:01 AM
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Thanks for your comments guys . I'm going to try another tube in the Fender and see if solves the problem . I know most of the Russian tubes have quality that's all over the place . Generally my experience with them has been a negative one . I just assumed that the tubes that cost more are the same ones rrelabeled but sifted through to remove all the bad ones that the Soviet Technology spits out . All of the Sovteks in my Peavey and Fender sounded awful . The Czech JJ Tesla tubes I've used sound more like the GE tubes I used in my Fender amp back in the seventies |
Kevin Ruddell
Member From: Toledo Ohio USA
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posted 02 December 2005 05:31 PM
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I just got some tubes from Torres Engineering after trading e-mails . They were of the opinion that the Pro Jr. is a poorly designed amp . I changed to a 12AY7 tube in the first preamp slot and it really smoothed the attack out, although I have to turn it up some more than with the higher gain 12AX7 . I tried a 5751 Sovtek but it just whistled . The 5751 tube works out really well in my Peavey Classic 30 though. I had a trebley sound on my Peavey that was really annoying and just noticed I had only changed 3 of the 4 Sovteks to JJ Tesla EL84's . I bought a Groove Tubes EL84 from the corner music store but didn't care for it that much . Torrres sent me another Czech Tesla EL84 and that works great . That shrill top note problem on the Peavey is totally gone . I don't need an eq pedal now. |
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