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The Globe King 400 and the elusive V70-D

Discussion in 'Amplitude Modulation - AM Fans' started by K5UJ, Jun 13, 2016.

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  1. K5UJ

    K5UJ Ham Member QRZ Page

    The topic of what to do with a GK400 if you do not have a pair of the V70-D triode final PA tubes came up on Saturday's Midwest Classic Radio Net (3885 12:30 UTC) when Liz K4GHT put out the word that she is searching for a pair.

    Since the chances of finding two are pretty much nil, I started thinking about suitable replacements and my conclusion which I mentioned to her in an email, was to change the filament supply to 6.3 v. from 7.5 and put in a pair of 812As. I think the sockets and pins are the same; the rig would run a maybe 350 watts input and the Cn caps would have to be readjusted because the g to p capacitance is a bit different, but apart from that, I think the easier to find 812As would be drop in replacements.

    Any other ideas? 572Bs? 165 watts plate dissipation on those, but higher mu I think.

    Thought it would be an interesting topic.

    73

    Rob
    K5UJ
     
  2. K4KYV

    K4KYV Subscriber QRZ Page

    8005s would also work. They are a little huskier than the V70-D or the 812A, but probably not all that easy to find, since audiophools have taken a liking to them. They have a 10v 3.25 amp filament, and the envelope is the same size and shape as the 811A and 812A. I use a pair of those in the final of my 40m rig and they run at 420 watts DC input with just a barely perceptible red glow on the plates.

    http://scottbecker.net/tube/sheets/049/8/8005.pdf
     
  3. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    Try 572B's. Ask K6LGL. his sounds very good.
     
  4. K5UJ

    K5UJ Ham Member QRZ Page

    What I don't understand is that I'd think the 572Bs would be okay, but somewhere, can't remember if it was here or some other place, Don mentioned that the high gain triodes don't do as well being modulated, because linearity suffers. For example I think that's why the 812A is preferable as a RF final over an 811A, but 811As are great as modulators.

    I'd think that with 165 watts plate dissipation, 572Bs would be great and as long as they are driven into class C deeply enough, there would not be a problem so the issue with the high gain (high mu?) triodes is something of a mystery to me.
     
  5. WZ5Q

    WZ5Q Ham Member QRZ Page

    The Low to Medium Mu Triodes have a much better Square Law Power Response in relation to the changing plate voltages during modulation then the High Mu Triodes. If you compare the Constant Current Characteristic charts between the Low/Medium Mu and High Mu Triodes, you will see that the High Mu tubes have flatter curves and there is usually not a loadline available that will double the Plate Current with the doubling of the Plate Voltage.

    Normally when you use a High Mu Triode as a Class C Plate Modulated RF Final, you would need to augment the applied Plate Modulation with either some Grid Modulation or Plate Modulation on the preceding RF Driver stage to fully modulate the RF Final to 100% Sinusoidal or to have any positive peaks over 100%.
     
    W2VW likes this.
  6. K5UJ

    K5UJ Ham Member QRZ Page

    Thanks Mike. If you have a high mu triode but its plate v. is well below its data sheet maximum, 1/2 the specified maximum, can that result in the high mu triode being made useable without augmented plate modulation?
     
  7. WZ5Q

    WZ5Q Ham Member QRZ Page

    Normally when used in Plate Modulated service, most tubes will list there Maximum Plate Modulated Anode Voltage about 20-25% (or thereabouts) under their data sheet absolute Maximum Anode Voltage. Unfortunately, I think that no matter what voltage you use with the High Mu Triode, the modulation linearity will be poor unless you augment the Plate Modulation.

    I have never tried to Plate Modulate a pair of 572B's, nor do I know of any who have. It would be interesting to see how the individual that Billy mentioned, K6LGL, built his.

    I did find a datasheet that actually listed the Maximum Ratings for Plate Modulation Service on the Cetron 572B (attached below), but it did not have any Typical Operation data. This datasheet also shows a Constant Current Characteristic Chart. Some High Mu Triode datasheets that list Plate Modulation service data will even have a footnote stating that the Plate Modulation needs to be augmented with Driver Modulation, the above mentioned Cetron datasheet does not.
     

    Attached Files:

  8. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    This is an interesting discussion. For other reasons not known to me, Skip K6LGL calls his Globe King 400 the "E" model
    due to the fact that he needed to use different tubes due to obvious availability problems. His signal is very good and the
    audio quality is quite nice, and sounds natural without any noticeable artifacts that affect performance.
    It would be interesting to hear from him. I will let him know tonight on the AMI net and see if he could chime in here
    and fill us in on how he managed his modifications.
     
  9. K5UJ

    K5UJ Ham Member QRZ Page

    Thanks Mike and Billy. I need to sort of come clean and mention that I run a rig on 75 m. that uses a pair of 3-400Z in the final running at 1900 v. with no drive modulation, just plate, and it seems to be working okay. I see positive peaks that look rounded but I need to set up a trapezoid to be sure. One of my To-Dos is to build up a voltage divider string to tap into the modulated plate v. and put that on one pair of plates of a CRT and an RF tap on the other set. It takes some trial and error to get it right so there is no phase shift on the scope display. My rig uses 572Bs as modulators. I wondered if the high mu tubes work as RF finals if they are run at low plate v. and deeply into class C. The 3-400s have around -100 v. grid bias. They are neutralized and the grid input network is just like what is found on the 833As in a BC-1T. Driver is a 6146B and only bias is grid leak.
     
  10. W2VW

    W2VW Ham Member QRZ Page

    One plus with the 3-400 is no need for safety bias.
     

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