Where would you look first?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jh45gun, Sep 13, 2008.

  1. jh45gun

    jh45gun Member

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    OK been playing with my Super 5f4 build that I got back from the tech after he got most of the issues resolved that I had. The Chassis was a Amp out of a Hammond Tone Cabinet I gutted out the chassis and used the Hammond PT and a Peavey OT. Mostly he had to redo the bias circuit for the negative feed back circuit because the PT I had did not have a negative bias tap since this amp was made from old iron. Two the filter caps had to be replaced as that negative bias issue blew one of the filter caps and weakened the rest or it could have been the voltage issue that described in a bit. I had built a circuit for the bias taken from a fender amp but it did not give a high enough bias voltage. Second issue he had to fix was the PT was made for a lower Primary voltage it said 110 instead of 120 so that made the secondary Voltage higher which also may have been an issue with the filter caps. He resolved this with a couple of heavy large wirewound resistors. Third was replace a couple of tubes that may have been iffy. Ok now he gave it back and it sounds really good except for the following issues. The treble knob if turned up to almost max squeals or feeds back. The Volume pot if turned to about 8 seems to cut out then if you go past that you get a motorboating sound through the speaker. Note this is only on channel One Channel Two seems to work fine with both controls maxed. On issue is the first Preamp tube V1 is a 12ax7 as I did not have a 12ay7 to put in that spot. The tech did not have any at the shop but was looking through his stash at an other place for one. Could the 12ax7 just be overdriving that first channel? Or any other suggestions where to look. One thing I thought of is the input jacks are just grounded to the chassis using the barrel of the jack would running a ground to each jack be a good idea? They are switchable jacks.
     
  2. SatelliteAmps

    SatelliteAmps Member

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    Could be a tube. Also could just be a build issue. Massive oscillation from poor layout.

    I don't really understand exactly what your tech is doing. Lowering the primary voltage will lower the secondary. It is usually easier to lower the secondary. It shouldn't have made a difference to the filter caps, unless they were way under rated. Those old Hammond power transformers don't put out that much voltage to blow modern filter caps. A negative bias feed shouldn't blow any filter caps unless it is wired incorrectly. And if it blew one, it shouldn't blow a second one or weaken any others. Were the tubes replaced actually bad, or just maybe iffy as you said?

    To be honest it sounds like an iffy build that was serviced by an iffy tech. I would suggest finding a different tech to take a look, or at least get better answers as to what was being done.
     
  3. nella

    nella Member

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    jh45gun,
    It might help if you posted some pictures.
    Scott
     
  4. Blue Strat

    Blue Strat Member

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    Yep. First thing I'd look at is the layout and wiring. Disconnect the NFB and see if that helps. You may have your output phase reversed.
     
  5. jh45gun

    jh45gun Member

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    Pretty sure he checked the output phase and would'nt that show up on a scope as they had it scoped. I know my wireing is off because of being using old iron chassis and not one for a 5f4 I had to extend some leads maybe on the longer leads I should use shielded wireing? My Digital camera went bad and I have an other but I do not have the cable for the computer hook up so I am waiting for a card reader a friend is sending me. As far as the PT goes I do not understand exactly what he did but he did say the primary was rated at 110 instead of 120 and what ever the issue the voltage on the secondary was over 500 Volts. If I remember right the caps were rated at 450. Anyway what ever he did it has the right voltage now. The tech has a busy shop and has been a tech for years he learned it from his dad who was a tech for years and worked at all the local music stores in the area fixing their amps so I know this guy knows his stuff. He did say however and I would have to agree fixing a factory amp that was known as working at one time is a lot more easier than trying to fix a home build that could have build issues I would have to agree with that. So if I use shielded wire on some of the longer leads I have never used it so do you just ground one end or what?
     
  6. brad347

    brad347 Member

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    Remember that many old Marshalls behaved the same way. It was a result of using the roughly the same circuits as tweed Fender amps but in much larger chasses, where the tubes were farther away from the other components. This necessitated longer grid wires, which were often just long enough to cause the 'mosquito' sounds or oscillations at extreme settings of treble and presence controls.

    It might just be a reality of the layout you were forced into with your chassis platform.

    You should start by making all grid wires as short as possible. You might even have to shield some of them.
     
  7. jh45gun

    jh45gun Member

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    Guess I should ask is there some wires that are more critical than others in the circuit for oscillation. I will agree that the build may be a bit iffy becuase of the chassis not being a standard 5f4 chassis so wireing has to be routed differently. I do however have faith that my tech is a good one. What ever he did for the voltage problem works and same with the bias situation. I also cannot spend any more on this so any wireing issues I will have to figure out on my own. Tubes have been checked and rechecked and substituted too. Only issue tube wise I could think of is not having a 12ay7 in VI
     
  8. jh45gun

    jh45gun Member

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    OK thanks for that info Brad I will study the schematic again also since this is only on the first channel for the motorboating sound I suspect maybe I should use sheilded wire in them circuits also and what about grounding the input jacks besides just relying on the chassis for ground where the barrel of the jack goes through? I know I did have to run the wires longer for the output tubes both off the board and from the OT so should I use shielded wire for both the wires coming off the OT where I made them longer and coming off the board where they are long to the output tubes?
     
  9. brad347

    brad347 Member

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    Yes, some wires are more critical than others. The grid wires. Any wire connecting to the grids of any of the tubes should be as short as possible. You have to figure out a way to route them. Tone caps should ideally be mounted ON their respective pots like on an old amp, otherwise you're adding extra wire to the grid circuit.

    Plate wires can be long. In fact, leaving the plate wires long in itself might help a little due to increased inductance.

    Basically, I'd shorten all grid wires, leave all the plate wires how they are (long), and see if that helps it. If not, I'd be looking at other ways of removing wire from the grid circuit, and failing that, I'd start shielding grid wires (make sure to ground the shielding at only one end).

    You mention that "the wiring was routed differently"... the main thing you want to look out for is signal crossing back over itself. The signal should always "flow" from input to output, and never backtrack. If it does, you are just asking for an oscillation. Pay particular attention to any area in the build where signal from a later stage might be in proximity to an earlier stage, or 'crosses back over' on itself.

    As you're figuring out, there's a LOT more to laying out/building an amp than just duplicating the circuit on the schematic. There are lots of 'parts' that are not on the schematic, like interelectrode capacitances in tubes, stray capacitances between wires in the amp, stray resistances between points on a ground plane/buss, etc.
     
  10. jh45gun

    jh45gun Member

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  11. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Are good chassis pics out of the question?
     
  12. SatelliteAmps

    SatelliteAmps Member

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    I agree about the grid wires.

    How is the grounding scheme? Is everything tied to the chassis at different spots? If you are grounding to the pots, are the pots wired to the chassis as well?

    Any idea what the B+ is now? If it was 500v at 120v on the primary, dropping to 110v on the primary will still keep the B+ over 450v.
     
  13. jh45gun

    jh45gun Member

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    I did not measure the B+ but it is within specs now for the circuit. He did not drop it to 110 that was the issue he said as the tranny was stamped 110 instead of 120. All I know is he used two large wire wound resistors to get the voltage where it should be. I do know the tranny even with out being in the circuit was too high on the secondary as I measured it with nothing hooked up to it and it was high before I ever started the amp build. Grounds are various places on the chassis. The Pots are not grounded with wires but just using the chassis as ground with the barrels of the pots for ground. Would it be a good idea to ground them with a wire? OK here is a problem and since the amp is working at least 90% I hate to start moving the board around ect. The board is in backwards so I did not have to cross wires across the board since the chassis is a different one than a standard 5f4 chassis. The amp is built into a head. I really do not want to move the board as I know some may suggest as since the amp is mostly working I do not want to maybe mess something up and I do not want to have to take it back to the tech again. I will post pics that I took as I built it as I cannot take any more until I get a card reader. So yea I know thats a problem but I felt it would be better to wire that way as either way the wires would be longer than normal. One thing that does confuse me and that is if the grid wires were screwed up because they are too long wound not I have issues on both channels? I only have issues with the first channel the second channel works fine dimed. ??? On this pic it does not show the resistors he added they are mounted on top of the chassis to keep them cool and they are sitting between both trannies.

    Note that Silver multi cap can is gone that was only temporary when I blew the filter cap and I put that one it to do more testing until I took it to the tech. He replaced all the filter caps to be on the safe side as one was blown and at least one was hot to the touch so I told him to replace all of them.

    [​IMG]

    [​IMG]

    [​IMG]
     
  14. SatelliteAmps

    SatelliteAmps Member

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    No offense, but the problem is most likely layout. There are a lot of wires crossing back and forth that shouldn't be.
     
  15. brad347

    brad347 Member

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  16. jh45gun

    jh45gun Member

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    So tell me and I sure am not trying to be smartalecked but how do you wire one of these with out crossing wires? Also it seems to me that other older vintage amps were described as rats nest in their PTP wireing so whats the dif?
     
  17. jh45gun

    jh45gun Member

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    OK one thing that is still confusing me and that is you guys keep going back to the issue of the OT wires going to the grid wires maybe reversed. Again would that show up on a scope? and two and the main thing that is confusing me right now is that the Second Channel has no issues just the first so would not that grid issue show up on both channels if it was the issue?
     
  18. SatelliteAmps

    SatelliteAmps Member

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    If it is one channel only, then it is in the preamp section of that channel, not the output section (or output transformer).

    Wiring layout is not an easy thing. Figuring out how to have a minimum of wires crossing (and those only crossing at 90 degree angles) is a large part of it. Having large current carrying wires going back and forth in parallel to each other is only asking for noise. Signal wires (and most capacitors) will act as antenna and pick up as much noise as they can.

    Look at the Fender layout for that amp, and notice how few wires cross, and when they do, they cross as close to a 90 degree as they can.

    The difference between a Super and some of the other older rats nest is that the older rats nest usually are still laid out in a manor that will minimize these issues. It looks ugly, but it is thought out a bit. Just no boards usually.
     
  19. jh45gun

    jh45gun Member

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    Thanks Adam so I guess I can eliminate the grid wire issue. Would you suspect that a 12ay7 in VI might make a difference since there is a 12ax7 in there now and that might be overdriving things. And just for my reference would that grid issue show up on a scope?
     

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