New Modulator

Discussion in 'Technical' started by K4TQF, Oct 15, 2017.

  1. W5HRO

    W5HRO Administrator

    It's possible though they were running them below 1500V and if so the plate-to-plate Z would drop quick and they could be 5.5K instead under those circumstances.

    You know, what you might do is connect the primary to 110V AC and look on the secondary with a scope to see if the voltage goes down or stays about the same. Just ground the primary's center-tap to the 3rd ground prong of your wall outlet. If the peak-to-peak voltage stays about the same on the secondary then it's a 1:1 ratio. If it goes down then it’s a step down. That bit of information would be the most helpful right now I think.
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  2. K4TQF

    K4TQF Member

    Those measurements sort of follow that hand drawn graph ... Pay no attention to the baseline... my oscillator has a variable output. I'm just looking at the trend. Just a quickie... I'm not taking the time to re-cal this setup every time I change it.

    RCA hand drawn.jpg
     
  3. W5HRO

    W5HRO Administrator

    I'll quote myself here, have you tried it yet? If you just stick 110Vac on the primary from your wall outlet then look at the peak-to-peak output with no loads on the transformer you will be able to determine if it’s a 1:1 or a step down. Do you not have a scope? If not, you could use an RMS AC voltmeter too I guess.

    If it's a step down the peak-to-peak voltage will drop a bunch on the secondary. If it's not then it should pretty much stay what it is across the two outer primary windings with just a very slight drop due to the coupling loss and frequency response of the transformer, but the test should work.

    If it turns out that the 5.5K on the primary is just from one end to the center-tap then that would be good news as long as it’s a step down to the secondary. All that would matter then is using tubes on each side with the correct impedance ratio. The only thing that really matters is the ratio.
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  4. K4TQF

    K4TQF Member

    It reads about 7K ohms, in both directions, on my impedance meter with a 5600 ohm resistor for load. IIRC, that impedance meter puts out a tone around 1KHZ.

    I'll need to find a suicide cord for my variac to do a voltage reading.
     
  5. W5HRO

    W5HRO Administrator

    Do the AC voltage test with NO LOADS.
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  6. K4TQF

    K4TQF Member

    Yes, basically 1:1
    IMG_1476.JPG

    Input:
    IMG_1478.JPG
    output
    IMG_1477.JPG
    no load on either side...
     
  7. W5HRO

    W5HRO Administrator

    K, that's means they were more than likely running the 813's below 1500V. Probably not too far below it, but below it. That would explain the lower 5.5k primary impedance.
     
  8. K4TQF

    K4TQF Member

    OK... re-thinking this a little. Less than 1500V isn't going to do much for the 4-400 or the 250TH... but wait ! I have a good supply of 211/VT-4C. I think they will run up to 1250VDC on the plate. The Norton amp has 8 X 211 4 pin jumbo sockets and a 60Amp filament xfmr.