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805 Modulator Issues

Discussion in 'Amplitude Modulation' started by W3SLK, Oct 14, 2021.

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  1. W3SLK

    W3SLK Ham Member QRZ Page

    I posted about this problem over on FB but I think there are more resources here that can give me a little more direction. I have a home-brew pair of 805 modulators that modulate a British MK-214D (2X 813's in shove-yank). It worked quite well and was working ok back in March when I quit operations for a while, (personal reasons). Anyway I fired everything up and I wanted to look at my signal with a scope before going on the air with it. What I found was the modulators had -25VDC on the grids, and the idling plate current was 35~40mA. This was built by my friend Ray, WD2AFJ and I can't remember if there was 1500VDC but I thought -25VDC was too high for the grids. It utilizes a Bogen MX-60A as a speech amp, (Paul WA3VJB thinks this is where the problem lies). Modulation is only ~50% and the wave form looks like a shark's fin. When I set the grid bias to -16VDC, it, (the voltage!) swings with modulation peaks! Also when the modulator HV is off, both the plate current meter and the grid bias meter swing when the transmitter is not keyed! This one kind of has me scratching my head. Any help is greatly appreciated.
  2. K5UJ

    K5UJ Ham Member QRZ Page

    I don't know anything about a Bogen, but it seems to me that you need a brute force modulator bias supply.
  3. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    My 8005 rig uses a pair of 838s in the modulator. Those are electrically equivalent to the 805; in fact when I first built it I used 805s, but after decades of use mine all tested weak, so I replaced them some like-new or n.o.s. 838s, of which I have plenty of spares. All I had to do was move the plate connection from the cap on top of the envelope down to the plate pin at the base. The plate cap connection at the top allows the 805 to safely run higher maximum plate voltage; otherwise the internal structure of the two tube types is identical. The characteristic curves for both types are identical.

    I run around 1350v on the plates (even though RCA lists the maximum Vp for the 838 as 1250). As I recall, the grid bias, when adjusted to give a little over 100 mA of idling plate current, is somewhere between -5 and -8 volts. -25 VDC seems high to me, unless you are running well above 1500 volts on the plates. According to RCA, the 805 operates at zero bias at 1250 plate volts, and -10v bias at 1500 plate volts.

    At 1250 Vp, the idling plate current (for both tubes) looks rather high, 148 mA. At 1500v, it is 84 mA. In both cases this is well within the rated plate dissipation of the tubes.

    The bias supply is brute-force, using a filament transformer, diode rectifiers, a "computer-grade" electrolytic filter cap a couple hundred mfd @ circa 50v, and a low-resistance wirewound rheostat as a bleeder and to pick off the bias voltage. The bias is adjusted to allow the appropriate resting plate current.
    N2EY likes this.
  4. W3SLK

    W3SLK Ham Member QRZ Page

    I'm pretty sure its 1500VDC. I will have to breakout the HV probe and measure it to make sure. But you are right Don, -25VDC is too high for the grids. At 1500 it should be -16VDC. He is coming off the B- supply and rectifying it with a couple of diodes and a slide resistor to set the bias. I need to investigate this more.
  5. W3SLK

    W3SLK Ham Member QRZ Page

    So a little bit of an update. I looked at the output of the Bogen MX-60A amp last night. In my desire to obtain 100% modulation, I was over driving the PA into distortion. It looks pretty solid to me now with a clean output when I whistle into the microphone. I have since readjusted the grid voltage to -16VDC. I am still unable to get anywhere close to 100% modulation. The thing that bothers me is that I'm seeing my grid voltage swing with AF peaks. I guess I should throw another speech amp/PA amp on just to eliminate that. The only other fly in the ointment is the builder place a 'audio output transformer' in reverse. Whereas the 8 ohm output is driving another 8 ohm 'output' with a center-tap 'secondary' going to the grids of the 805's and the power supply for the grid bias is feeding the center-tap. Next will be the 805's. Both filaments light so I'm not so sure about that being the cause. But I remember the actions of PFM from my Vo-Tech days!
  6. N2DTS

    N2DTS Ham Member QRZ Page

    What grid voltage is swinging, the modulator?
    What is the modulator plate current doing?

    If you are driving the grids with an 8 ohm PA amp, you want the 8 ohm to something like 5000 ohm transformer with the center tap to the bias supply. Why have two back to back transformers??
  7. KL7AJ

    KL7AJ Ham Member QRZ Page

    I would run them with very low audio drive and kadjust the bias until the crossover distortion JUST goes away. Be sure to disconnect any negative feedback, because it can mask crossover distortion pretty effectively.
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Does the design use the Bogen Bias voltage or does it have a separate bias supply for the 805's?

    In either case, scope the bias supply. You might see a sawtooth waveform appear when the audio peaks. If this is the case, the bias supply does not have a low enough impedance; you may need more filtering e.g, a new and or larger filter cap.

  9. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Example: use the "brute-force" bias method, i.e. a low-voltage high current DC supply, such as a filament transformer driving a bridge rectifier with ss diodes, with an adequate filter capacitor, and a low-resistance rheostat to pick off the bias voltage.
  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I am trying to visualize your circuit. Does it look similar to this?


    Attached Files:

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