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Static from tubes?

Discussion in 'Amps and Cabs [BG]' started by jondog, Sep 15, 2002.


  1. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    Hi, I've got a '78ish Fender Musicmaster Bass amp. It's a little tube practice amp (and it sounds great on g****r). I moved it out of storage recently, and now there is a lot of static. It has 1 12ax7a and two 6au5s in it. I cleaned the pots and tube sockets w/ deoxit, no luck. I swapped out the 12ax7a, also no luck. I can still hear the bass, just not clearly through the noise. The tone knob works and affects the tone of the static. Any thoughts on what I can do? I really prefer to fix things myself if at all possible. Thanks, Jon
     
  2. Sounds like the capacitors are leaking. Its very common with old tube amps especially if they have been sitting around collecting dust. I have changed them on a number of amps, most recently my guitarist Fender Pro reveverb. You can do it but be carefull. you can get zapped even while it is unplugged. read up on it before attemping it.
    Benton
     
  3. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    I found this advice in my search . . .
    __________
    >A power supply cap will either create a bad 60 cycle hum through the speaker or blow the power supply fuse when it craps out.

    The fastest way to determine if it's a filter cap problem is to carefully bridge a known good cap across each filter cap and see if the problem clears up with the new cap in the circuit.

    This is definitely not a do it yourself project.

    Carry your amp to a good tech and get it done properly.

    Pkr2
    >
    --------------

    I don't have hum, all I have is static, not like a tv, but sort of like the ocean. It varies a bit and gets louder as I turn up the volume.
     
  4. I may not be a pro but I've seen and repaired this a number of times especially after storage. Humming is caused by bad supply caps but he descibes it as

    " I moved it out of storage recently, and now there is a lot of static. "

    It would be as much trouble to test them as replace them probaby more. The Fenders are real easy to work on and can't get much simpler than than three tubes. No reverb, tremolo, tone controls or second channel to deal with.

    I like to have all my new parts before I remove any and it always seems to take replacing all of them even the ceramic discs and especially the electrolitic caps.
    Once there all replaced the sound justs opens up. Just don't get zapped!!!!!!!!!
    Benton
     
  5. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    ok thanks. Are you saying I should replace all the parts?
     
  6. Before even attempting to repair it yourself, give Dan Torres at Torres Engineering a call. I have the same amp you describe, and I was going to recap it myself. When I called to inquire about the parts, Dan informed me that there is a very expensive power supply cap in there that is an unusual, hard to find design. If that cap is blown, you should re-assess keeping it.

    BTW -- that amp is my number one guitar amp, right now. It sounds especially good at low volume with a TS-9 Tube Screamer, using guitars with either humbuckers or single coils. Mic'd up, it's sweet. A great recording amp, and great mic'd into a PA, too.
     
  7. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    Thanks, I just emailed Torres engineering. How can I know if the expesive cap is the bad one?

    Yes it is an awesome guitar amp. I would be very bummed if I had to get rid of it. I haven't tried micing it through the PA in a live setting, but I've recorded w/ it and wahoo! I've read about mods to make it louder, but I'm not sure if they work on the 6aq5 version and I wouldn't want to mess up the tone. I also thought about seeing if it would push a 16 ohm extension speaker, but now that I've got static all that is on hold.
     
  8. It's probably not the expensive power supply cap (multi section one). I don't think mine even has a multi-section cap, but mine is a later one that uses 6V6 tubes.

    Anyway, sounds like a bad 12AX7, or a noisy 100K resistor. The fact that the tone control changes the sound of the crackle tells you that the source of the noise is in front of the tone control.

    [​IMG]

    Chris
     
  9. And I'm referring to the 100K resistor on the tube side of the 0.01uF cap, not the 100K resistor on the Tone control side of the 0.01uF cap.

    Could also be a bad solder joint, but that probably wouldn't occur during storage. The carbon comp resistor could easily absorb moisture during storage though. Was it stored outside or in a basement? Or anywhere humid?

    Chris
     
  10. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    Good, thanks. The before the tone control advice makes sense. I swapped the 12ax7, but I guess I'm not 100% sure of my spare (old & used) so I'll try some others today. If not I'll start on the 100k resistor. My schematic shouldn't look very different from the 6v6 one you posted, right?
     
  11. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    It was in storage in a basement that flooded, but it wasn't on the floor and didn't get wet. Plenty of damp air I'm sure. Then it rode 650 miles in the back of a trailer. I tried to cushion it but I'm sure it took some jolts. I should be nicer to my gear . . . At least the stuff that sounds good . . .
     
  12. Yeah, the 6AQ5 model is not much different around the lone 12AX7. You probably won't have that weird capacitor after the input jack. I think that was a Canadian safety requirement kind of thing.

    If you've already swapped the 12AX7 and the noise is still the same, it probably ain't the tube. What are the odds of (2) tubes having the same crackling noise? Not too likely. Which means your original tube is probably OK.

    Don't kill yourself, etc. All the usual safety disclaimers.

    Use a 2 Watt 100K resistor for replacement, or 1 Watt if you can't get a 2 Watt. Visually inspect ALL solder joints while you're in there. Might as well replace the 2uF/50V cap too. It could be leaking.

    A Musicmaster Bass was my first amp ever, and I eventually sold it a couple years later when I needed a bigger amp to play live with. I regretted it 10 years later when I wanted a good at home jam amp. I picked up another one on eBay for like $120 a year or 2 ago. Most bass players don't like them, but I dig 'em. You don't need 1000Watts for playing while watching TV, and the sound is actually more pleasing than most of the hollow sounding crap I hear nowadays. I mean come on, tweeters on a bass amp is the most ridiculous thing I've ever seen. :D Most people with ears eventually realize that those 14 KiloWatt, 20Hz - 20kHz hi-fi setups that all the posers go for sound like doodoo in a band setting.

    Chris
     
  13. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    OK, now it works.

    I borrowed known good 12ax7s from another amp and went upstairs to test them. I turned on the amp 1st to hear the "before" static to have a comparison, but it was gone! The amp sounds as great as it ever did.

    Maybe something dried out? Maybe some of the deoxit I sprayed all over yesterday finally soaked in?

    With my luck, it's an intermittent problem. I know I won't trust it live for awhile. I had been planning on bringing it to a garage jam next month where I will play g****r (hence my thoughts about loudness mods or the 16 ohm extension speaker), but now I will use another amp. :eek: