Tube Amps / Music Electronics
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|2/8/1998 6:49 AM|
|James||Deluxe Reverb problem|
My B/F Deluxe reverb is acting strange. When I have the volume on 4 or higher, the lower notes seem to distort. The sound resembles a blown speaker. There is a distorted sound riding above the fundamental sound, but it fades out faster than the note. I have changed speakers as a test and the symptoms persist with different speakers. I have changed the 6V6s and preamp tubes and the distortion is still there. I checked all the voltages per schematic and they are very close. I have the 6V6s biased at 25MA with the cathode method. I re-soldered all joints. All electrolytics have been upgraded. I hope one of you have experienced this before and can help me out. Thanks
|2/9/1998 11:33 AM|
Have you replaced the coupling capacitors from the 12AT7 plates to the 6V6 grids? Leaky coupling caps can cause a symptom like this. Is the grid bias voltage stable (pin 5) on both 6V6's?
|2/10/1998 9:15 AM|
Thanks for your response, Doc. I did as you suggested, changed caps from the at7's plates. I even replaced the entire driver circuit, and the buzz is still there. Do you think the output transformer could be the culprit?
|2/10/1998 12:21 PM|
It could be the output transformer. One side of the primary could be bad, and the speaker isn't seeing a complete push-pull waveform summation. If you had a 'scope & signal generator the waveforms could be checked at each stage to locate the problem area.
You could try this, before disconnecting all the leads for a winding test--
Pull one 6V6, turn the amp on and throw the standby after the filaments are lit. Play the guitar and listen to the sound. Turn off the amp, pull the 6V6 (gloves?) and put the other one in its socket (or just use the hot one you pulled). Repeat the warmup & play sequence. Test only long enough to listen at a few volume settings. You don't want to overheat the transformer by long exposure to imbalanced primary current.
Was the sound different side-to-side? If it was an easy choice you can bet half the primary is beat, and you'll need a new OPT.
If you couldn't really decide, then disconnect all leads. I think you could leave the secondary leads connected if you insert an open plug or cable in the speaker jack to open circuit it. Go to R.G. Keen's effects web page and get the schematic for a simple transformer tester (it uses a neon bulb & a battery) and test your windings for shorted turns.
If the OPT tests good, I guess I'm stumped at this point. It's not easy to fix these weird ones over the phone.
|2/11/1998 8:48 AM|
could also simply be loose windings in the OPT. Dave Funk covered this in a VG article in '97 (May or June ???). just try a new OPT trans.
|2/11/1998 6:52 PM|
I checked the output transformer per R.G.'s website. The voltages were close, so that rules out the shorted theory.
Jeff, I don't have access to the article. Was there a way to correct the loose winding? I'd be interested in trying to save this tranny, if possible, might damage the vibe if I changed it!
|2/11/1998 8:50 PM|
yeah -- if you have access to some high-tech vacuum equipment (which Dave does, and the rest of us don't). all i'm saying is that, since you've swapped out everything else, the next logical swap is the OPT. there are plenty of them around -- from used to current-production Fender to boutique hand-built by various guru's.
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