Normal operating color for a Type 80 rectifier tube?

Discussion in '"Boat Anchor" & Classic Equipment' started by N8YX, Sep 22, 2020.

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  1. N8YX

    N8YX Ham Member QRZ Page

    Restoring an oldie but goodie (Dewald 610LW) for a family member. Uses an 80 in a choke-type voltage doubler arrangement. I get no HV on the plate caps of the various tubes, and am seeing color on the rectifier tube plates after 15 or so seconds of power-on.

    Output side of the filter choke looks good with respect to resistance; new capacitors in the doubler circuit.

    Should this tube show any color at all except for filaments?

    I'm chasing any possible shorts down now.
     
  2. WA1GXC

    WA1GXC Ham Member QRZ Page



    No color, whatsoever, OM. Not in this power-class of rectifiers.

    Along with checking for the obvious--wiring errors-- check for a solder flake or small metal burr that might be trapped and grounding a
    tube socket terminal to ground. Old radios seem to be magnets for old particles finding their way to spaces where they shouldn't be.

    G L
     
    N2EY likes this.
  3. N8YX

    N8YX Ham Member QRZ Page

    I think I found my problem - sorta. ;)

    The OEM HV section used non-polarized 8uF capacitors in the filter portion. I could only find polarized electrolytics to replace the questionable parts.

    Reversing one fixed the plate glow. However, I only get about a third of the voltage I should (spec: 240; observed: 70) on the plate of the 6F6 AF amp tube. Have to chase this one down.

    The schematic shows a smoothing choke that's part of the speaker coil; one side of this is tied to ground. I connected that particular capacitor's negative lead to that point when I replaced both. Perhaps this is also in error?
     
  4. K1APJ

    K1APJ XML Subscriber QRZ Page

    That is a bit of an unusual circuit (by today's standards at least,) the choke is in the negative lead and its resistance is used as a source of back bias.

    BOTH filter capacitors should have their negative sides connected to the choke.



    upload_2020-9-22_14-26-31.png
     
  5. N8YX

    N8YX Ham Member QRZ Page

    So...both positives to one filament leg, then?
     
  6. K1APJ

    K1APJ XML Subscriber QRZ Page

    Yes.

    Hoping you haven't ruined the capacitors, but it is possible.
     
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  7. N8YX

    N8YX Ham Member QRZ Page

    Double-checked - even replaced C17 (which I originally had backwards).

    Both are correctly installed now. If I look at the junction of C17, the field inductor and R16 with power applied I see -220VDC.
     
  8. K1APJ

    K1APJ XML Subscriber QRZ Page

    That could indicate that the field coil is open or that the other end is not grounded.

    Or could be that there is a very high B+ draw, perhaps if C15 is shorted or very leaky. There is another capacitor from the plate of the audio output to ground, if shorted that could cause that symptom. Either of these things will make the field coil get very hot very fast.

    Your best bet is to leave the radio turned off and check ALL resistors and capacitors. It is almost certain that most of them are defective.
     
    Last edited: Sep 22, 2020
    N2EY likes this.
  9. N8YX

    N8YX Ham Member QRZ Page

    That junction I mentioned is actually the negative terminal of the power supply. (I referenced a power supply schematic using an 80 as an example - Dewald drew theirs a little wonky). If I use the junction as my negative reference and connect the positive side of my multimeter to the 6F6 plate supply lead I get 230-235VDC, which is right at spec.

    You may be correct about all components needing checked out or replaced. The set appears dead - though nothing smokes upon power-up - and I'm going to start at the AF amp (6F6) then work backwards with a signal tracer and see what's what.
     
  10. WA1GXC

    WA1GXC Ham Member QRZ Page



    Yes, clever. The WW II BC-348 is properly powered by a floating B- line to supply proper cathode bias to the audio output tube.

    Guys often don't observe this and burn stuff out during rebuilding process. A number of restoration articles incorrectly state this configuration

    only applies to J,N,Q models. Dynamotor negative is below-ground potential on all variants as designed thru a choke integral to the audio

    output transformer assembly.
     
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