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Why does the Drake 2B run an 8BN8 filament at 6.3V?

Discussion in '"Boat Anchor" & Classic Equipment' started by W0RIO, Jul 12, 2019.

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  1. W0RIO

    W0RIO Ham Member QRZ Page

    The recent thread on running tubes at lower than normal HV got me wondering about the
    use of an 8BN8 triode/dual diode tube in the popular Drake 2B receiver.
    The 8BN8 is specified to use an 8.4V filament supply and the 2B filament circuit is
    nominally 6.3V.

    There is such a thing as a 6BN8 tube.

    I'm guessing that the people at Drake got a good deal on a lot of cheap 8BN8 tubes and that's
    why they used it. I would bet that it is the tube with the longest life in those rigs.
  2. N2EY

    N2EY Premium Subscriber QRZ Page

    Why not do research first?

    I googled "Drake 2B 8BN8" and this came up:
    KA0HCP, W7UUU and K3XR like this.
  3. K3XR

    K3XR Ham Member QRZ Page

    "Why not do research first?"

    You're not suggesting that a person actually take the time to use the resources available to them and see what answer(s) they discover? Those resources, by the way, include the computer used to ask the question. Exhausted those resources and still can't find the all means ask the question.

    Ron, WB4HFN has a great web site. It's mainly Drake but has other useful information.
    N2EY likes this.
  4. AC0OB

    AC0OB Subscriber QRZ Page

    Many Manf. used the lower filament voltage in detectors and low-level audio stages to avoid the parasitic diode effect. One of those was Hallicrafters in their S-108 and SX-110 and other sets.

    It was also cheaper than using a separate diode to derive a DC filament voltage. :):cool:

  5. W0RIO

    W0RIO Ham Member QRZ Page

    Sorry, It's just laziness on my part and I'm hearing the crickets chirping around the forums during the
    northern hemisphere's dog days of summer.

    The K4OAH explanation is interesting, I still think it's strange to use an 8.4V tube at 6.3V.
    Commercial manufacturing often requires thinking outside of the box.
  6. W7UUU

    W7UUU Super Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    I think it was very clever on Drake's part - they were probably able to source the 8v tubes for a fraction of 6v tubes, due to the millions of TV sets being manufactured at the time - helped to keep the cost of the 2B down by a little bit, and eliminate the need to come up with a more elaborate method to avoid the pitfalls described in the article.

    Pretty clever stuff!

  7. WD4IGX

    WD4IGX Premium Subscriber QRZ Page

    [oh never mind]
  8. N2EY

    N2EY Premium Subscriber QRZ Page

    Interesting speculation, but....I think not. Sorry.

    Yes, there were millions of TVs being manufactured at the time, and hundreds of millions of receiving tubes. But the odd-heater-voltage tubes, made for series-string transformerless TVs, were not any less expensive than their 6.3 volt counterparts.

    The Drake 2-B is the successor to the Drake 2-A. I downloaded the 2-A and 2-B manuals, and found something interesting:

    In the 2-A manual, most of the references show a 6BN8. But there is an errata/update sheet which says that all references to 6BN8 should be 8BN8. In the 2-B manual, all references are to 8BN8.

    So it seems pretty clear that the 2-A was designed to use the 6BN8. but Drake found that using the 8BN8 solved the parasitic-diode problem, and so they made a last minute change which carried over to the 2-B.

    If there were any real cost advantage to using odd-heater-voltage tubes in transformer-powered sets, you can bet the rigmakers would have done so.

    73 de Jim, N2EY
    Last edited: Jul 13, 2019
    WD4IGX and W7UUU like this.
  9. KM1H

    KM1H Ham Member QRZ Page

    Hallicrafters and National reduced the filament voltage to several of their 6AL5 and 6H6 radios; I dont know who came first:rolleyes: Drake simply copied the idea and saved the cost of a 2W resistor.

    A couple of 1N5711 diodes work well also.

  10. W9WQA

    W9WQA Ham Member QRZ Page

    glad you were lazy and posted this interesting piece of info for all of us to learn from....
    W5INC likes this.

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