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Thread: Issue with a Clone Hybrid Hiwatt/Ampeg

  1. #1
    Junior Member blackwhaleamp's Avatar
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    Issue with a Clone Hybrid Hiwatt/Ampeg

    Hi,
    I am building an sort of hybrid 200W Hiwatt amplifier including the midrange circuit from Ampeg SVT. The amp has two channels with Hi/Low. You can see the circuit attached
    I just assembled the whole amplifier, and for some reason I have two issues
    .- There is an oscillation, like something like a chopper sound, which appears when the volume/treble/master increases. Also, when I play with the midrange rocker switch, the same chopper sound appears. If I remove the midrange circuit, the chopper is gone
    .- There is a dependency between the volume at each channel. I was expecting as I introduced a valve V3 to isolate both channel.
    .- The Presence does not affect the audio signal

    I used this circuit for the midrange. I found it in an old thread from this forum (Bass preamp - inductor driven midrange - help!)

    369d1171584035-ampeg-v9-mid-control.bmp

    Apart from that, the sound is very clean and there is almost no hum.
    Does anyone has an idea about how to solve this?

    output.png
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  2. #2
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    I don't know this version of the mid control. The one that I know use 3 triodes. There is always a problem with the inductor. Where did you get it from? Apart from this, on your schematic there is one connection missing between the pins of the switch (which makes that the inductor sometimes is not connected to the circuit). Is it just a mistake on the schematic, or you forgot about it also in the amp?

    Mark
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  3. #3
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    I am wondering if the volume control interaction and the oscillation might be part of the same problem. If you keep the master low to avoid the oscillation, do you still have the volume interaction?

  4. #4
    g1
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    Quote Originally Posted by MarkusBass View Post
    I don't know this version of the mid control. The one that I know use 3 triodes. There is always a problem with the inductor. Where did you get it from? Apart from this, on your schematic there is one connection missing between the pins of the switch (which makes that the inductor sometimes is not connected to the circuit). Is it just a mistake on the schematic, or you forgot about it also in the amp?
    Mark, there is a jumper across 2 pins of the switch so I don't think there is any error. I've attached the actual Ampeg schematic.
    Attached Files Attached Files
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  5. #5
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    I see 2 problems from looking at the full schematic (2 of 2) on your link.

    1) HT3 has WAY too many cascaded gain stages off it. You should NEVER run more than 2 cascaded gain stages from one power supply node and differential stages should have their own power supply node. So you need more power supply decoupling. This will cause a motorboating sort of instability which is what I suspect is your problem (from your description).

    2) Most of the triode stages should have grid stops. I would put 10K grid stops on all of them including those 12AU7 triodes driving the mid circuits. The instability in this case would be high pitched squeals which does not seem to be what you are describing although if you have this high frequency instability it can trigger the motor boating problem described above.

    Fix the power supply decoupling problem first and then check to see if you have other issues. Add decoupling between V4 and V5 and between V3 and V4.

    Cheers,
    Ian
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  6. #6
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    Quote Originally Posted by g1 View Post
    Mark, there is a jumper across 2 pins of the switch so I don't think there is any error. I've attached the actual Ampeg schematic.
    Hmm, I was not refering to the actual Ampeg schematic. I was refering to the actual-actual schematic in the post #1 (attachment 2 of 2). I assume that this is the real schematic of the amp. And there the link is missing.

    Mark
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  7. #7
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    To save duplication of effort, see responses to the same thing on another site https://www.thegearpage.net/board/in...ampeg.1890930/

  8. #8
    g1
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    Quote Originally Posted by MarkusBass View Post
    Hmm, I was not refering to the actual Ampeg schematic. I was refering to the actual-actual schematic in the post #1 (attachment 2 of 2). I assume that this is the real schematic of the amp. And there the link is missing.

    Mark
    Sorry Mark, I didn't even notice that second attachment in the first post.
    Indeed he is missing the jumper on the switch.
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  9. #9
    Junior Member blackwhaleamp's Avatar
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    Thanks guys for the tips. I was busy during the weekend, but I'll have a look tomorrow
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  10. #10
    Junior Member blackwhaleamp's Avatar
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    I reworked on the circuit

    output.png

    I plan to make this changes and see how it goes.
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  11. #11
    Junior Member blackwhaleamp's Avatar
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    I made the inductors following this link Bass preamplifier in the Ampeg SVT tradition
    Indeed, I didn't connect properly the rocker switch
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  12. #12
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    V3 still needs grid leaks.

  13. #13
    Junior Member blackwhaleamp's Avatar
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    I rearranged the circuit like this:

    secondtrial.png

    The motorboating (aka put-put-put) is gone. The midrange circuit works (with some noise). However, I still have some issues

    1.- The amplifier is not as loud as it should. I am not sure if I am missing one additional pre-amp stage, or simple there is another thing wrong with the amp
    2.- After the modifications, the pre-amp was not working. So, I connected the scope to C28 and C30 and suddenly it works.
    3.- The PRESENCE is subtle, almost unnoticeable.

    Someone recommended me to add a 1M resistors to V3 grid to ground. I will do that tomorrow, and see the effect

    Thank for the help
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  14. #14
    Junior Member blackwhaleamp's Avatar
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    Hi,
    I managed to solve almost every issue in the amp. However, there still one thing that I couldn't solve so far. When the amp is switch ON for at least 10min, there is an oscillation PLUK-PLUK-PLUK coming up. I changed the position of every knob to see if there is a correlation between that, and the oscillation. However, there is not.
    I also add another filter stage at the power supply, just to check if that could help somehow. No luck so far.
    I attached the latest version of the circuit to this post
    circuit.png
    I appreciate any help that you could get me
    Thanks
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  15. #15
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    Just checking, is the LTP tail resistor R43 really 220k? Even if it's 22k, are you sure that the LTP can put out enough signal to fully drive the KT88 (ie to Vg1-k = 0, on peaks)?
    And there's no global NFB loop around the power amp?
    Does removing V4 stop the oscillation?
    If not, then V3?
    Last edited by pdf64; 12-30-2017 at 02:14 PM.

  16. #16
    Junior Member blackwhaleamp's Avatar
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    The tail resistor is indeed 220k. I need to measure with the scope, but I think the LTP is enough to drive the KT88. There is no NFB loop around the power amp. Should I put it?
    I removed V1 and V2, and the oscillation still there. If I remove V3 or V4 it dissapears. Then, I decided to leave all the tubes and start to remove the plate voltage. So, if I also removed the power for V3 at R31, the oscillation still there. Same case if I remove the power for V4 at R33. Another thing about V4, only half tube is used. The other half is no connected, but the fillament at PIN 5 is connected.
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  17. #17
    Senior Member nevetslab's Avatar
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    I would put the NFB loop back in.. I was wondering if you lifted the tube connections for C6 and C17 briefly from the Presence Control, does the oscillation go away? Removing either V3 OR V4, the oscillation goes away, you said. Still makes me wonder about the Presence circuit and the absence of the NFB in the most recent circuit diagram.
    Last edited by nevetslab; 01-03-2018 at 08:07 PM.
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  18. #18
    Junior Member blackwhaleamp's Avatar
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    Quote Originally Posted by pdf64 View Post
    Just checking, is the LTP tail resistor R43 really 220k? Even if it's 22k, are you sure that the LTP can put out enough signal to fully drive the KT88 (ie to Vg1-k = 0, on peaks)?
    And there's no global NFB loop around the power amp?
    Does removing V4 stop the oscillation?
    If not, then V3?
    I think you are right. I measured the signal after the OT, and I saw 15V with 8 Ohm. That's only 28W. I was expecting to see at least 35V.

    Do you think that I should put another tube right after the LTP, or should I change the LTP configuration?
    Black Whale Amplification creates custom all-tube guitar amplifiers (heads and combos) and cabs from Brussels Belgium
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  19. #19
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    I think that a smaller value for R43 would allow a greater potential signal swing at the LTP plates.
    Why is it such a high value?

  20. #20
    Senior Member mhuss's Avatar
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    Agreed, you're losing a lot of your potential voltage swing across that 220k resistor. And is a 12AY7 the best choice for driving a quad of KT88s? Perhaps a 12AT7 would be better?

  21. #21
    Senior Member nevetslab's Avatar
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    I think the 12AU7 would be a better choice...those are used in the Ampeg SVT-CL drive circuit off of the cathodes, with the bias adjust circuit on the 12AU7's Grids. If you need even more current, ECC99 do that well. Taller tube, same height as an EL84. Ashley's BTA-400 uses that tube to drive 8 KT88's off of it's cathode
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