5F4 Super questions

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jh45gun, Sep 5, 2008.

  1. jh45gun

    jh45gun Member

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    Ok some back ground I built a 5f4 Super Clone out of a Hammond Tone Cabinet Amp chassis. Got it built, but had some issues I could not figure out and gave it to my tech friend to help get out the bugs. He got the amp up and running after fixing some issues one major one was the negative voltage for the bias since the Power tranny did not have a negative voltage tap for the bias circuit so I made up a circuit I found on the net but it still was not right so he fixed that with an other circuit he designed. Some tubes were iffy even after being checked so they were replaced and now it all sounds good except for the following:

    If the treble pot is cranked all the way it seems like at the last bit squeals when I play the guitar its ok but as soon as I hit a string it gives me like a high pitched feedback sound. Say if the knob goes to ten we are talking in the 8.75 to Ten range. Other wise its fine. Other pots are fine just the treble one. Tech said I should maybe try a 12ay7 instead of the 12ax7 I have in the first position now he wondering if that 12ax7 could be overdriving the circuit to give me that sound? Do you guys agree or is that squeal coming from some other issue? Tech said he would look for a 12ay7 in his stash of tubes this weekend and grab a couple for me to try. I asked about the negative feed back circuit and he assured me they checked that to make sure it was wired right coming off the OT to the 6l6s. I know they had it on a scope and if the feed back was positive that would show anyhow right? Just thought you guys could maybe give some insight on that squeal or high pitched feed back sound. I am hopeing the tube change will take care of it and the 12ax7 is over driving the circut in that VI position.
     
  2. brad347

    brad347 Member

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    Have you tried moving signal wires in the amp?

    I bet it's a parasitic oscillation of some sort. Try shortening wires connecting to the grids of the preamp tubes as much as possible.
     
  3. jh45gun

    jh45gun Member

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    Thanks Trout I will check that out. Thanks Brad
     
  4. brad347

    brad347 Member

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    If that doesn't work, try reversing the OT leads. Sometimes this works to stop oscillations in that circuit.
     
  5. bmcmusic

    bmcmusic Member

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    I have built many of these amps, both the 5e7 and the 5f4. First if disconnecting the negative feedback eliminates the problem then reverse the OT leads to the power tubes, then reconnect your negative feedback. Also, just for the sound I would go with a 12AY7 as the first preamp tube. It has less gain and allows you to turn the volume up more and get the power tubes working with a little more warmth. It is a great sounding amp when tweaked just right. I also put a PPIMV on mine and you can really get the amp to "sing" at a moderate level. I have a couple of sound samples you can check out.

    Bob
    http://www.mcmusicsound.com/ampscabs.html
     
  6. jh45gun

    jh45gun Member

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    Thanks Bob What does PPIMV stand for?
     
  7. brad347

    brad347 Member

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    post-phase inverter master volume
     
  8. jh45gun

    jh45gun Member

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  9. bmcmusic

    bmcmusic Member

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    A double thanks to Brad, I didn't get back in time to answer my own post. Aaaaaaaah, but that is the wonderful thing about the Gear Page, there are a lot of really smart guys around always willing to help.

    Bob
     
  10. Trout

    Trout Member

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    Hey Brad,

    Got any experience with this sceneraio?

    Reversing the grid wires on pin 6 of the 6L6's?

    I have had a good bit of success with reversing the primary leads on the OT, but could it also be a by-product of reversed grids causing a phase issue?
    Just curious if you ever tried it.
     
  11. jh45gun

    jh45gun Member

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    A note about this amp that I also added on the other thread The first channel is the one giving me fits with the volume and treble controls. The volume control seems to cut out at about 9 then when you turn it up that last bit it has a motorboating sound. On the treble it is a high pitched and some times warbly. Now on the second channel it works just fine both the vol and the treble settings. Of course the treble and bass are for both channels so why one works and the other gives me fits? Also if one channel works fine that would eliminate the Output section and lead to issues with the preamp section right? I still also have to get a 12aY7 to try in VI could the 12ax7 in there now be overdriving the first channel?
     
  12. brad347

    brad347 Member

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    I have no idea about reversing grids!

    Sorry. :)
     
  13. Trout

    Trout Member

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    I just did some tinkering on my bench today with a similar circuit, the Pro 5E5A.

    Now, the 5E5A, 5F4, 5E7, 5E6A all being the same basic topology and circuit, most tweaks and fixes will be common of each other.

    A few things,

    I purposely induced a ultra high frequency oscillation symptom by flipping the grid wires at pin 6 of the output tubes as a test.

    It produced a very similar sound/symptom/oscillation like having the OT primary flipped but a much higher in frequency, almost shrill.
    It had most of its effect when volume was midway to high, and the treble and presence max'd
    I found that especially interesting.

    It also caused the treble control to behave oddly especially when excess presence was used. It was very close to going into cutoff using a 12AY7 in V1.

    Cutting the NFB wire at the jack did indeed removed the whistle just like when the OT is wired incorrectly.

    Next thing,
    The 4.7meg resistor might be to small, Look at the other schematics, 5E7 uses a 10meg. I tried a 4.7 meg and it sounded sort of spikey and cold.

    I am not sure who's layout you are following, but personally I find that Webers 5F4 layout can be very handy because it is color and drawn much neater than the B&W fender layout.

    So, I answered my own previous question about flipping the grid wires at the power tubes, it can indeed produce an oscillation which is more commonly diagnosed as reversed primary wires at the OT. In both cases lifting the NFB wire at the output jack killed the oscillation.

    That might be something you want to look at.
     
  14. brad347

    brad347 Member

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    that's good info Trout. Thanks for putting into the public domain.

    I'm starting on a 5E7 later this week and that will certainly be good to remember.
     
  15. jh45gun

    jh45gun Member

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    Thank You Trout I do have to ask though when you did this was it both channels or just one? As with my amp it is only the first channel so if it was a symtom of both channels it would be worth considering and trying. If not than I am wondering about first channel instead??
     
  16. jh45gun

    jh45gun Member

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    Trout thanks for posting the weber layout. Before I used either the Fender Field Guide or Schematic Heaven's
     
  17. jh45gun

    jh45gun Member

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    So I see Weber uses a 12aX7 instead of the 12AY7 any reason except for the fact that the 12ax7 is easier to get for a new build?
     
  18. jh45gun

    jh45gun Member

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  19. brad347

    brad347 Member

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    no reason other than the one you stated.
     
  20. jh45gun

    jh45gun Member

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    OK but the 12ay does have some benifits over the 12ax right? I still want to hear Trouts comments on wether his expirements were for both channels or just one?
     

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