'68 Super Reverb Issues

Discussion in 'Shock Brother's DIY Amps' started by CK5150, Oct 4, 2019.

  1. CK5150

    CK5150 Tele-Meister

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    Hi All,

    I have a 1968 Super Reverb (original not re-issue) and after about 15-20 mins of operation it slowly builds up a howl and then crackles out, then after a few more minutes builds up a howl and crackles out, and repeats this cycle a few times and eventually just emits this bonkers howling noise that is consistent until I power off the amp.

    A few facts about the amp / issue:
    - It is originally a silverface but had been "blackfaced" by an amp tech in Denver by the previous owner
    - I have it separated out into a head and cabinet but the issue happened when it was in the original cabinet as well
    - My new speaker cabinet uses the same speakers as when it was a combo but I have it wired to yield 8 ohms so I can use the cabinet with other amps. I run the Super Reverb head into a Weber Z-Matcher where the head runs into a 2.6 ohm setting on the Z-Matcher and the out to the cabinet is set to 8 ohms
    - On the Normal channel, with volume up but treble and bass turned all the way down, I can hear my guitar which is expected. On the Vibrato channel, when I do the same thing (turn volume up and treble/middle/bass off) I do not hear anything when playing which seems odd to me. Not sure if this is normal SR behavior.
    - The howling mentioned above happens regardless of what position any knobs are in or what channel I'm plugged into.
    - I have completely changed out the entire tube set to new tubes and the problem still occurs.

    Sorry I don't have pictures/audio/video right now I will try to get some up here but just wanted to see if anyone had any ideas as to where to start troubleshooting. It seems like since it takes so long for the problem to happen it might be an issue where something is heating up too much and causing problems.

    Thanks in advance for any ideas you have to help. If I can avoid taking it to a tech that would be great. I have built a champ, a deluxe, and an 18 watt clone kits so I am at least capable of doing some fairly complicated work to spec but in a bit over my head when troubleshooting or dealing with more complicated stuff so I'm kind of at a loss as to what to look at to figure out the problem.

    Regards, Craig
     
    Wally likes this.
  2. dan40

    dan40 Tele-Afflicted

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    Has the amp been recapped or serviced in the past few years? The electrolytic filter caps in the power supply do dry up over time and fail. When this happens, you can experience weird symptoms such as yours or possibly transformer damage if they decide to blow.
     
    rangercaster likes this.
  3. Mexitele Blues

    Mexitele Blues Tele-Holic

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    Were your speakers wired at 8 ohms prior to splitting the head out? 8 ohms is 2 steps up from the super's 2 ohm OT which is beyond regular recommendations.

    The vibrato channel's mid knob should cut the sound when bass and treble are already at 0. You are working fine there.
     
  4. corliss1

    corliss1 Friend of Leo's

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    More clarification on the wiring of the speakers/output transformer would be good. It's possible there could be damage there causing your issue.

    However, if it hasn't been serviced since you had it, it's at least time for a good checkup. If you're not familiar with poking around inside amps with high voltages, it's tech time.
     
  5. flyswatter

    flyswatter Friend of Leo's

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    Based on this quote alone, you need a professional tech. I also have a 68 SR that I refurbished. When I got it the filter caps were all blown out and the bias cap had also dried up. No surprises -- as replacing those is standard procedure on an amp that old.

    If you noticed any redplating on the tubes or excessive heat when the crazy stuff was happening, that's a good sign that one or more components on the bias board is toast. You're risking both your power tubes and transformers by messing around with it anymore, if you don't have the knowledge to troubleshoot the amp properly.
     
  6. CK5150

    CK5150 Tele-Meister

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    Thanks for the responses. I am going to pull the chassis and check out the filter caps to see how they look. I've worked on plenty of amps so know how to discharge caps, be safe when working, etc. but I appreciate the warnings.

    To be clear re: the OT and speakers - I am using a Weber Z-Matcher between them that balances the 2 ohm OT with the 8 ohm speaker load so I don't believe it is causing any issues. Not sure why I included that in the original post sorry for any confusion. It's a cool bit of kit and lets me use a variety of heads with a variety of cabs w/o worry.

    https://www.tedweber.com/z-matcher
     
  7. corliss1

    corliss1 Friend of Leo's

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    The caps might not look bad, but if you can find any date codes and they are 15ish years old, they should just go anyway. That way you know you're starting fresh. That still may not mean there's other issues, but if the power supply isn't happy then nothing else will be happy either.
     
    dan40 likes this.
  8. CK5150

    CK5150 Tele-Meister

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  9. CK5150

    CK5150 Tele-Meister

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  10. CK5150

    CK5150 Tele-Meister

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  11. CK5150

    CK5150 Tele-Meister

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  12. CK5150

    CK5150 Tele-Meister

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    The filter caps under the dog house look pretty new actually and the barrel style caps on the board also look pretty new. The thing I noticed is the disk style capacitors in the middle of the board look original. Pardon my ignorance but will these wear out like electrolytic filter caps will or is this not something to worry about?

    Plenty of pics for those interested. If anyone spots anything they think would cause problems a comment would be much appreciated.

    Regards, Craig
     
    sds1 likes this.
  13. sds1

    sds1 Tele-Afflicted

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    Looks can be deceiving. Check the date codes.

    Those filter caps are over 20 years old.

    The bypass caps are 19 years old.
     
    corliss1 likes this.
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