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Dentron GLA-1000B... a few things I need help with

Discussion in 'Amateur Radio Amplifiers' started by KC5DFP, Sep 26, 2019.

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  1. WA7PRC

    WA7PRC Ham Member QRZ Page

    Per the schematic shown above, the value of R1 would make no difference in WHETHER there is cathode current during idle/receive.
  2. KC5DFP

    KC5DFP Ham Member QRZ Page

    Here’s the schematic for the amp. It’s a GLA1000B.
  3. KC5DFP

    KC5DFP Ham Member QRZ Page

    With the file...

    Attached Files:

  4. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    i haven’t owned one of these amps and I suspect that if I did, I’d modify it to really ensure cutoff. 47k seems a bit low because the bias will float to a dozen milliamps or so conducted instead of completely cutting off if I understand the sit-e-atian correctly.

    What say you?
    Last edited: Oct 6, 2019
    KA0HCP likes this.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    Ideally, you want ZERO anode current in stby/receive... you want to ensure the tubes don't become RF noise generators. To do that, you drive the grids negative with respect to the cathodes. In grounded grid configuration, that's usually accomplished by forcing the cathodes positive.
    That's different than the first schematic shown. It shows no bias control -- it controls ONLY the signal flow -- and the tubes will always be conducting. In the first schematic, K1 has 3PDT contacts but,in the second schematic (posted twice), K1 has only DPDT contacts. What does the amplifier actually have? o_O
  6. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Yes, the original design does not cutoff the grid at idle. There is a user developed mod to use an unused pole on the three pole xmit relay to ground it. I suspect that is why you are seeing a low current.
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    You have it 'bassackwards'. The first schematic (GLA-1000) shows K1A + R1 in the cathode circuit. The second schematic (GLA-1000B) doesn't show K1A + R1.
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    OK! Thanks for the real schematic! I missed that post on my phone. :oops: :p

    This one won't cut off unless you do something about it (mod the thing by adding a relay). I guess that Denny and crew decided that leaving the tubes on all the time was OK (and those guys know more about linears than I ever will).

    Looking at the tube data sheet, it takes about -100 volts to ensure cutoff with 1100 volts on the plate ( a bit above the tube's maximum rating).

    Having the cathodes open in receive would also ensure no current. I'm not sure how safe that technique is with horizontal output tubes.

    The other guys can chime in on whether this is an issue to be dealt with or ignored.
  9. KE0ZU

    KE0ZU Ham Member QRZ Page

    Seems 47K was Denny's go to "stand-by" Cathode resistor value, it was used in several of his amplifiers.


    There apparently were 3 different input matching filter designs as well. The CM-1, which was for the Clipperton L,


    the CMU which I believe was a "general use" filter for any SS exciter/tube amp, below...,


    ...and the CM-2 for which I've not seen any specific documentation although mentioned in one drawing here.
    KD2ACO likes this.
  10. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I find it interesting that by the 1000B version, Dentron decided to not have any cutoff and leave the tubes on all the time.
    1000B output.png

    WA7PRC likes this.

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