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. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think there's an error in the schematic. Although I don't own one, I've used and worked on GLA-1000Bs and the tubes did not draw current in standby.

    The cathodes were floated way above ground in standby, using a high-value resistor (possibly 47K, I really don't remember) and that was switched to a bias zener on XMIT.

    It's very possible they made undocumented production changes, though. Almost everyone did! Henry was also guilty of that, although usually the changes made something better, not worse.:p
  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    It's all wild to me!
    Seems like the OP's may actually be like the B schematic and a candidate for a wiring nip and tuck.

    He could tell in a few seconds by following the cathode wire from the tube sockets (it would take longer to open the case then figure out what's going on).
    Last edited: Oct 8, 2019
  3. KC5DFP

    KC5DFP Ham Member QRZ Page

    Hey All.

    Been busy and have a moment to post.

    I’ve checked a few times and there is no R1 (47K) on the amp as shown on the GLA1000 schematic, so for that part of the schematic (that I posted previously) is correct. As I started to go through the amp and compare it to the GLA-1000B schematic I see there are a few differences between the two so who knows what rev I have. If’ve only gone through about 30% so I have more to go and I’ll post what I find.

    I did check the path from tube pin 3 as suggested and it all looks like the schematic I posted.

    Also I pulled the Zener Diode and measures 0.589v forward and open reversed bias, so it shows good.

    As for the tuned input, I did buy some cores so I’ll be winding some toroids to get the 10M input going. I’m hoping to knock this portion out this weekend, then focus on the plate current issue.

    Thangs again!
    KD2ACO likes this.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You can't measure a zener diode like that. But since it's not shorted, it's probably good.:)

    If your amp actually leaves the zener in line all the time both on standby and transmit, then it would create operating bias all the time which would explain the 100 mA idling current all the time.

    That's a really dopey idea. Not only wastes power and generates heat for no reason, but this condition can generate noise (RFI) that you'd hear in your receiver as an increased noise level; that's one of the main reasons we want to "cut off" PA tubes entirely in standby.

    I'd change that.
    WA7PRC likes this.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    The parts list in the manual (link) sez D1 is a 1N3308 (9.1V/50W) Zener.
    The schematic shows K1 operates from a ≈ 12VDC source. 12 VDC / 3PDT relays abound.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, they sure do.

    The Ameritron amps use them and Ameritron sells them as "new spare parts" very inexpensively. Same thing from DigiKey or Mouser might be even cheaper, but then you have to look through pages and pages of their catalogs to find one.:p Ameritron makes it really easy to find, they list all the "spare parts" for the amps right on the amp web pages.
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    Simplest is bestest... add a SPDT relay (with 12 VDC coil) to switch the bias. :)
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    That's what I'd do too... 3rd the motion to switch the tubes off in receive. Even that 47k resistor in the A version leaves the tubes on.

    Anyone have an opinion on wiring the amp up like the A version but leaving the 47k resistor (R1) out of the picture?

    If it were mine, I'd wire it similar to "A" and build a divider to get 150 volts or so and tie the 47k resistor to that instead of ground. Those tubes would be off for sure.
    Last edited: Oct 10, 2019
  9. KC5DFP

    KC5DFP Ham Member QRZ Page

    Yeah, I know. Was testing for opens/shorts. when I have more time I’ll unsolder the slide and test it in a circuit.
  10. KC5DFP

    KC5DFP Ham Member QRZ Page

    Good morning All.

    I was able to swap out the toroid on the tuned input board, but still have a high SWR at 10M. I also tried the 68 µF caps noted in the schematic but still no luck.

    I’m now suspect either the board itself or perhaps the switch, so I plan to disassemble that part of the amp this weekend and take a detailed.

    Also, I’m not convinced that the cap values that were in the amp or on the schematic are correct. If anyone have the same amp and can take a look at the cap value and the toroid color and let me know this would be great!

    Any insight is greatly appreciated.


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