Strange pre behaviour (micro champ)

Discussion in 'Shock Brother's DIY Amps' started by allesz, May 31, 2019.

  1. allesz

    allesz Tele-Meister

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    Hallo mates, I am building a micro champ/deluxe from the projects of Mr. Robinette.

    It was my purpose to NOT buy anything, so I just used parts I had liyng around and, of course, some part is out of spech.
    For example the output transformer is a 9K single ended ot that I got from an aborted project and the volume pot (right now I did not bother with master volume and tone control) is 2,2 M instead of 1M.

    Althought this differencies the amp doesn't sound too bad but the volume pot is behaving in a strange way: at minimum it makest a strong hum and noise (just like when you leave a jack unplugged and it picks up noises). When you raise the volume the sound it's ok, even a little bit of crunch near maximum, but when the pot is maxed I get no sound at all.

    I noticed something strange: I have 100K plate resistor and a 2,2K cathode resistor on the second stage of the preamp; I tried bypassing the cathode resistor with a cap (all the other cathodes are already bypassed), but the sound does not change. I also tried another cathode resistor (I got aroun 1K) and it made no difference too.

    I tried all the pot I had (a couple of 470 K and another 2,2M) and they all exibit the same problem reported, so I think it's not the pot.

    Have you got any idea about the possible cause?
     
  2. allesz

    allesz Tele-Meister

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    Ok, I solved it by myself: 100k on the plate was too high for the second triode; I put on a 56k plate resistor and now everything works!

    I am experimenting with cathode caps. The volume jumped up quite a lot, now I can think of a tone control.

    Anyway it's strange, voltages seemed ok, compared to what Rob indicates. I will check also the first triode, just in case.
     
  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That is strange behavior and the lower plate load resistor fixing it is even stranger.
     
  4. allesz

    allesz Tele-Meister

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    Yes, there is definitely something strange: the voltage on the cathode varies between 20 and 40 V... I swapped all the parts around that second triode, caps, resistors on the plates and on the cathode to no avail.
    Of course I tried three different 12ax7, a 12ay7 and also a 12au7... always the same result.

    And The gain of the amp is not quite near to the demo you posted.

    At this point I fear that I have a bad socket... of course I tried cleaning it with no luck; what a drag having to desolder everything and then redo the thing from the start....
     
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Something is wrong from the second triode onward. Compare your circuit to the layout and inspect every joint and component value. Give us a voltage chart of all the tube pins.

    Lowering the plate resistor lowered the gain for that triode which helped hide the real problem.
     
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  6. muswell_hillbilly

    muswell_hillbilly Tele-Meister

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    Just thinking out loud, but assuming the 9K spec of the substituted OT is also with an 8 ohm load, could this ~1/3 reflected impedance compared to the expected value (25K) be contributing to your problems?
     
  7. muswell_hillbilly

    muswell_hillbilly Tele-Meister

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    Continuing along this line of thought, I believe lowering the gain of that second triode by reducing the plate resistor would have the same effect as increasing the reflected primary impedance of the output transformer. For the sake of discussion, let's assume the 9K impedance presented to the output stage is too demanding. To address this, you can either reduce the voltage presented to the 9K load (cut the gain of the final output stage) OR increase the impedance presented to the output stage (increase speaker impedance), reducing the load on the output tubes.

    So if you went back to the original plate resistor of 100K and instead connected a 16 ohm speaker (assuming you have one), do you have similar results as when you dropped the plate resistor to 56K? And then connecting 8 ohm and 16 ohm in series (27K reflected?) how is it? This would be close to the 25K from the Micro Champ design?
     
  8. allesz

    allesz Tele-Meister

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    Well, on the power tubes I get 315V and 11 or 12V on the cathode.
    The first triode gets 15O V on the plates and 1,2V on cathode.

    The second triode makes me mad with around 70-80-90 V on the plates and 20-40 v on the cathode. Also I noticed that at max volume the output drops.

    I put another tube socket, but it's the same.

    I have a doubt about the power amp: I have a diode bridge rectifier and three b+ nodes. Since here we deal with triodes I separated the nodes of v1 and v2 plates, did I do wrong? Should I use a single node? I thought to get better filtering and separation....

    The power tube grid stoppers are 47k and the grid bias resistor is 470k...
     
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    A 12BH7 likes around 10k of load so trying one would answer that question.
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

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    There's something wrong with V1B. Those voltages are way off, the voltages should be almost identical to V1A. Measure the resistance from V1 pin 8 (cathode) to ground to verify the cathode resistor because your voltages suggest a much higher value than intended. I believe your problem lies with the V1B circuit, not the power amp circuit.
     
    Last edited: Jun 5, 2019
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  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    [​IMG]
     
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If you haven't tried it already, disconnect the NFB for troubleshooting because it's connected to V1B.
     
  13. D'tar

    D'tar Tele-Afflicted

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    +1 disconnecting NFB

    a few good pictures may shed some light here.


    Like a 1M volume pot:)

    Doubble trippple check the connections to V1 p6,7,8 making sure your leads are going where you think they are.
     
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  14. shortcircuit

    shortcircuit Tele-Meister

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    My money is on this.
     
  15. allesz

    allesz Tele-Meister

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    Thank you for the support
    Mates!

    I redid the entire board and it works! To me it is the same as before.... who knows.

    The volume is terrific, I will have to put a master volume. And then I will start to trim the sound.

    Thank you Rob. What a monster!

    I will post some pictures soon.
     
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  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Congrats on getting it up and running right. These little guys do benefit from a master volume, that's why I have one on the schematic and layout.
     
  17. allesz

    allesz Tele-Meister

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    I just added a master volume, great crunch from this little beast!
    When it was badly working I put on a 12at7, because the volume was a little bit better, now I went back immediately to the 12au7; the volume is still too much! I really can't believe it.

    By now I don't plan to add a tone control, using only the one on the guitar gives a sound I like a lot (and saves some gain); since I had already drilled an extra hole in the chassis, I put on a second input: can't wait to jam with a friend with this boy (I am planning to use it in a 1x6 or 1x8 combo).

    The last question: do I post pics here or is there some "micro champ/deluxe/bassman club" around here?

    I vote for "Rob's micro crew", just in case.
     
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  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Post those pics here for all to see.
     
  19. allesz

    allesz Tele-Meister

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  20. allesz

    allesz Tele-Meister

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    The outside is not bad, the innards are terrible, I know, but remember that I did not whant to buy anything and, in particular, an expensive chassis.
    As you know I had to redo the board (once) the wiring (at least twice) and swap a tube socket (once)... that's the mess left, but it works and it's not noisy (a little bit of hum, of course, but at volumes to high for home usage).
     
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