More Fun With Magnavox: The 9300 Series

Discussion in 'Tube Audio' started by dcgillespie, Nov 7, 2015.

  1. gadget73

    gadget73 junk junkie Subscriber

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    What size resistor are you running between the output of the buffer and the input to the amp? A higher value will help with HF stability problems. Likely needs to be something in the 47k-100k range.
     

     

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  2. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    0AFB3E4C-2FC3-4858-9D4F-9196A6201D1B.jpeg
    I’m following your drawing that I’ve attached to this post. But I seem to remember Dave stating it should be 47K.
     
  3. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    This is it I’ll give it a try and report back.
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    I think Dave's comment was for the buffer added to the amp circuit with stock transformers. Increasing that resistor won't hurt anything though. It will knock down some of the HF response, which may be a help for stability.

    Ideally you need a scope and a signal generator for this since your setup is slightly different from what has already been done.
     
  5. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    Gadget you are correct changing that resister from 10 to 47K had no real effect. What I found was if I removed the 680pf cap from the NFB network it goes away and everything is stable. My current configuration is 3.9K ohm and 680pf in parallel connected to the output of the OT and going to pin 3 of the phase inverter tube. I have also tried a 2.7K in this network with no change.And from that point a 20pf cap going to the brown transformer input lead. I do have a scope and a signal generator. I’m feeding a 1Khz signal in at .3 vac.

    I think I’m going to put the 10K back in,I’m a fan of if it doesn’t work put it back the way it was. Maybe stick a pot in the NFB network and see if I can find a value for the resistor. I’m very much on the front end of learning about this it’s nice to have people who are willing to help.
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    What does a 1khz square wave look like? Output should be somewhere around 2-3v p-p.

    For the designed feedback level, 2.7k from the 8 ohm should be the right value. Its supposed to be 12db. There are ways to calculate it, I prefer to just measure output with the feedback disconnected, then connect it and measure the difference. A bit of math will turn the voltage difference into db.

    The phase cap value may be wrong for your particular transformers. Thats just what I happened to end up with for a decent compromise of stability and frequency response. Feeback is something of a balancing act, the wider the frequency response, the more likely it is to become unstable.
     

     

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  7. Moonwatcher

    Moonwatcher New Member

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    I have been through a lot of 9300 series pages and have yet to find a schematic for the original console's tuner/preamp (59-31-10). I have one that needs attention. Thanks, and sorry if this question has been answered elsewhere.
     
  8. 6DZ7

    6DZ7 Super Member

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    You won't find that info in the 9300 discussions. It's just the amp half. Try Sams site if your in a hurry, the downloads are reasonable cost. The 9300's used many tuner/TT configurations.
     
  9. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    Check page 2 post 37 of this thread.

    Quick update on mine found that I’d put the wrong voltage caps in for coupling and had 2 questionable 12AX7’s I’m still waiting on parts...
     
  10. Moonwatcher

    Moonwatcher New Member

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    Many thanks!
     
  11. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    I’m in need of some help on this one. With the power tube removed it seems to be working correctly.
    The Fluke scope meter is on the left channel input signal. The fluke 23 is on the output of the phase inverter.
    The scope is on the output of of the phase inverters of both channels, probes at x10. The last photo is of the output transformer output. The NFB is parred back to just a 2.7K resistor.
    I’ve taken a series of shots at each setup. 1) no output tubes installed. 2) output tubes installed. 3) tubes installed scope on OT is in next post. Last is OT info.
     

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  12. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    OT pics.
     

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  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    What power level is the waveform from the OT taken at? It looks like the output stage is being severely over driven.

    Dave
     
  14. gadget73

    gadget73 junk junkie Subscriber

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    If thats the output waveform at 0.4 vac, something is seriously wrong. I *think* the scope is at 1v/div so it looks like you're showing a little under 3v p-p, which is a smidge over 1vrms. Something doesn't agree here.

    either way it definitely looks like something is heavily overdriven
     
  15. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    Dave you are correct .4 volts before the the two 470K ohm resistors. That is the same voltage in all pictures. The first four are taken with no output tubes in the amp. I’m only looking at the left channel in these photos but the right is the same. Is the 10.4 and 11.6 volt at the unloaded inverter seem right? As soon as I introduce the EL84 tubes it goes into overdrive.
    The scope is 2v per division.
    I added pictures of my build here, http://audiokarma.org/forums/index.php?threads/more-fun-with-magnavox-the-9300-series.687735/page-41
    I’m just not sure what to do next. Any advice will be greatly appreciated. I’ve attach the drawing used for the build, I have made some changes based on the OT I’m using.
     

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  16. gadget73

    gadget73 junk junkie Subscriber

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    pull the feedback resistors and see what you're getting. That will eliminate possible goofiness related to that.
     

     

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  17. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    I removed the NFB from the left channel and here’s what I got driving at .4 v . 3 pics

    Then I drove it till the waveform was just shy of distorting. Next 2 pics.

    And last 1K hz square wave. Next 3 pics.

    Also note my scope meter is set to rms not pp.
     

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    Last edited: Jan 20, 2019
  18. gadget73

    gadget73 junk junkie Subscriber

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    so you're getting basically 0.45 volts rms before it starts to turn into a square? Is this with all of the tubes in? Does the amp bias reasonably and are all voltages at least in the ballpark of correct?
     
  19. Kkbubar

    Kkbubar AK Subscriber Subscriber

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    Bias is at 15ma - 17ma with all tubes in and input grounded. These tests were just output tubes in the left channel.

    So does this require NFB? If it’s required where should I go from here?
     
    Last edited: Jan 20, 2019
  20. gadget73

    gadget73 junk junkie Subscriber

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    Are the other voltages on the tubes at least close to the schematic?

    is this working into some sort of a dummy load?

    eventually it'll need feedback, just trying to eliminate possible problem areas at the moment. It doesn't seem to be a feedback-related issue though. I have a suspicion that voltage checks will turn up a problem. Absolute worst case, it'll have to be signal tracing through the amp with the scope to find the problem stage.
     

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