Smaller tubes with a lot of glow?

Discussion in 'Amateur Radio Amplifiers' started by KC8QVO, Dec 26, 2019.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    Makes sense.
  2. K7JOE

    K7JOE Ham Member QRZ Page

    heard him call cq 3 or 4 times..he didn't work a soul... funny.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Actually, it's not even a typo.

    The "single phase full-wave" shows "per tube" and indicates a FW CT circuit. Per tube .25A = .50A total for the circuit.

    The "full wave bridge" shows "for four tubes" and .50A load current.

    They are exactly equivalent. But a very screwy way to rate anything.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I recommended the 4-125 instead of a 4-65 mostly because the original post discussed "150W output," which the 125 can easily do, but a 4-65 would be straining to hit that.

    Also, the 4-125 is very common and inexpensive.

    The "glow" would come primarily from the filament, which is bright.

    The heaters in indirectly-heated cathode tubes that run the heaters at lower temperatures: Not so much. Also, most of those require a minute or two "warm up," whereas the 4-125 is "instant on" and can be used within a second or two after powering it on.
  5. N2EY

    N2EY Premium Subscriber QRZ Page

    Yes, it is.


    The 866Jr is rated 0.125 amps per tube load current. Two in a FWCT circuit gives 0.25 amps total.


    In a full wave bridge, two rectifiers are in series. You get more voltage but the same current.

    The 0.5 amp load current in the last column is a typo.
    W1BR likes this.
  6. N8FVJ

    N8FVJ Ham Member QRZ Page

    That is what I said, typo
  7. N2EY

    N2EY Premium Subscriber QRZ Page

    I know. I was replying to @WB2WIK
  8. N2EY

    N2EY Premium Subscriber QRZ Page

    While it is possible to parallel four 807s/1625s in grounded-cathode, it's not easy to do so and have the amp be stable. And the higher you go in frequency, the harder it gets. Neutralization will be a must, as will getting tubes that are reasonably close in characteristics.

    The OP presents us with a bit of a challenge in that there's only a few watts of drive power available. This rules out most grounded-grid designs because they typically don't have enough gain to provide 150-200 watts output with 5 or 10 watts of drive. In fact one reason grounded-grid amplifiers became so popular in the late 1950s/early 1960s was that they didn't waste the 50 to 100 watts of available driver power in an attenuator.

    IMHO, the best solution would be the following design:

    - Single 811A in grounded cathode (grid driven) A 572B could also be used.
    - HV of 1250 volts.
    - Zero bias Class AB2 operation
    - Tuned grid circuit, link coupled, with neutralization
    - Start out with a single-band design, probably 80 or 40 meters, but leave plenty of room on the chassis for bandswitches and tapped coil arrangement.
    - External power supply. Depending on parts availability, either the classic pair-of-866As-full-wave-center-tap-choke-input-filter or the conventional silicon-diode-capacitor-filter-voltage-doubler setup for HV.

    The result should be an amp that will produce about 150 watts output for about 8-10 watts of drive, and can do so on all modes.

    73 de Jim, N2EY
  9. W1BR

    W1BR Premium Subscriber QRZ Page

    150-250 watts, glow, big inexpensive tube? Seems to be ideal for single ended 813 amp. I just picked this home-brew amp from the estate
    of Norm, N1PF. When I get a chance I'll be adding a second 813 and upgrading the PS to an AnTek transformer.

    W9WQA, N2EY and AH7I like this.
  10. AH7I

    AH7I Ham Member QRZ Page

    I agree that single tube is a bit easier. I'd recommend considering the 813 for a single tube for the additional power capability, lower drive requirement, and ease in adjusting power output via the screen. If not already on hand, $ will need spending on a transformer either way.

    But, neither meets the four tube spec and both require more of a power supply.
    This, at least, was my thinking in suggesting single band with four 807/1625.
    Size is specified as NOT an issue so if, say a 40 m provides a lot of fun, then another band could follow.
    Separate RF decks, leave room for plug in coils, or look for some switches. I'd keep it simple with separate decks.
    Perhaps a push-pull of paralleled pair if higher bands are of interest?
    Put the supply on it's own and use the same supply for each RF deck or other projects.
    807/1625 requires a tad ~1/2 W drive/each.

    I'm working on a single band 4-65 TX and also on a single band TX with a pair of 1625. Neither is finished. But, they are coming along.

    The 4-65, while very attractive performance wise, requires HV.
    I've assembled a 500 W microwave oven transformer based HV supply that works well.
    Attendant HV tune cap is a PITA to find. Also, it's hard to find good 4-65s.

    The 1625 TX power supply uses an old TV transformer I had on hand. It can easily power four 807/1625 tubes.
    These transformers still turn up cheap at hamfests.
    If I had 807s and sockets, it would not need the additional transformer to boost heater voltage.

    The globe chief uses a pair of 807.

    73, -Bob
    Last edited: Dec 29, 2019

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