Why did 2.5V tubes hang on so long?

Discussion in '"Boat Anchor" & Classic Equipment' started by K1APJ, Oct 18, 2020.

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  1. K1APJ

    K1APJ XML Subscriber QRZ Page

    I t was my understanding (perhaps erroneous) that 2.5 volt heater tubes were introduced to respond to the need for tubes suitable for AC receivers, which were not too much later replaced by the 6.3 volt tubes which were also suitable for 3-cell battery/mobile applications.

    I was curious about the continued development of 2.5 volt tubes long after 6.3 had become accepted. What was the justification for the 1619 and 1624 and the RK-41 for example, 2.5 volt (filamentary in some cases) versions of the 6L6 and 807. Yes, I have seen equipment that uses them, but I wondered why 2.5 was chosen over other voltages.

    Inquiring minds want to know. Thanks in advance.
  2. NW2K

    NW2K Ham Member QRZ Page

    According to Alan Douglas (2014), with parenthetic comments by me:

    "There are two sources of hum in such a tube (the #26, directly heated): electrostatic and magnetic fields. It turns out the two cancel pretty well at 1.5V and 1.05A, so that's what was chosen....(Others note that in directly heated cathode tubes, 2.5v gives better hum reduction than 6.3v)

    (Alan goes on to say...) RCA intended to standardize tubes at multiples of 2.5V (1.1V and 3.3V were chosen earlier, for particular reasons in each case). National Union developed 6.3V tubes around 1931 for auto radios, and Philco decided to use them in home radios. I expect it was partly to tweak RCA's nose. But since Philco sold three times as many radios as RCA, they could do it, and RCA could only splutter as their "standard" 2.5V tube line went out of use."

    So...1.5v and 2.5v for reason of hum reduction. In the end, N*2.5v are not convenient battery voltages and marketing is a strong force in product development in competitive industries.

    73 Dean NW2K
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  3. K1APJ

    K1APJ XML Subscriber QRZ Page

    That agrees with my understanding. But why did the 2.5 volt versions of the 6L6 and 807 appear? That is what puzzles me!
  4. NW2K

    NW2K Ham Member QRZ Page

    We may need a real OT to sort this out. The answer to your last question might not be technical...it could have been a business decision.

    From what I can tell from a brief search, the differences include more than heater/filament voltage and cathode structure.

    You might consider posting this to antiqueradios.com

    73 Dean
  5. KL7AJ

    KL7AJ Ham Member QRZ Page

    In a nutshell....car radios.
  6. N2EY

    N2EY XML Subscriber QRZ Page

    The only reason I can think of is fast turn-on. Directly heated tubes fire up much faster than indirectly heated ones.

    Fun fact: the data sheets of both 1619 and 1624 say vertical mounting only, base up or down. No horizontal operation.
  7. K1APJ

    K1APJ XML Subscriber QRZ Page

    That could be it - A 2.5 volt filament would be a bit stouter than a higher voltage one, which would matter given the absence of the cathode sleeve to support it. Vertical mounting would prevent sag into the grid.

    But, perhaps the exception that proves the rule, the RK-41 is indirectly heated!

    Just a random observation, some jukebox amplifiers in the 1940s and 1950s used 6.3 volt tubes except for the output stage, which used 2A3s. The 6.3 volt voltage amplifiers were always lit, but, when you dropped a nickel, 6.3 volts was put on the 2A3s for a second to get them up to temperature fast. When the record ended the 2A3 filaments were turned off to prolong tube life.
    Last edited: Oct 20, 2020
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  8. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I'm not sure that would prolong tube life. The inrush current @ 6.3 volts into the cold filament every time a record was played must have been hard on the tube filament. We tend to do just the opposite with power tubes in our transmitters, using various soft-start and inrush protection circuits to warm the filments more gradually.

    Back in the day, 2A3s weren't much more expensive than the 6.3 volt tubes used in the earlier stages, so why go to all that trouble to preserve them and not the rest of the tubes in the amplifier?

    National company kept 2.5 volt tubes in the HRO receiver throughout the 1930s up until after Pearl Harbor, long after other companies had changed over to 6.3 volt metal octal tubes. Their advertising claimed the 2.5 volt tubes produced less hum. I remember someone who had worked at National said the real reason was that the company had stockpiled a large inventory of the tubes, and wanted to use them up before switching production over to 6.3 volt ones.

    Since there are 2.5 and 6.3 volt equivalents to every tube in the early HRO, 6.3 volt filaments was an option starting shortly after the receiver first went into production. All it took to convert was to substitute 6.3 volt glass tubes and use the 6.3 volt power supply. The power supply was an external unit sold separately from the receiver.
    Last edited: Oct 20, 2020
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