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The Case for a Grounded B+ & other Innovations in the Design of a Vacuum Tube RF Power Amplifier

Discussion in 'Homebrew and Kit Projects' started by W9JEF, Jul 6, 2021.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Does the Bandit use the four 811A's?

    If so, I'd guess the Kilovolts DC meter on the right side of the panel isn't really indicating kilovolts DC, as 3500V would blow the tubes right out of their sockets.:):p

    I love those big meters, however. Unless one is lucky enough to find meters like that on the surplus market from a seller who hasn't checked market value lately, meters like that "today" can run $100 each.
     
    W9JEF likes this.
  2. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    The note on the meter is intended to instruct the op to multiply the reading by 0.3, so the voltage on the 4 is a conservative 1080. :cool:
    [​IMG]
    The Bandit moniker is the combination of Band and dit (as in Morse). And also, the two top meters are reminiscent of a bandit's mask. The “smiling” grid current meter at the bottom is upside-down for easier reading.

    A couple decades ago, I got lucky at a second-hand store, and picked up several of those meters for cheap (don't remember the actual price). I suspect that they came from the estate sale of an area ham, or perhaps from a U of A physics instructor.

    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     
  3. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    The grounded B+ is nothing new in the history of RF power amplifiers. The Harris 5kW AM broadcast transmitter installed in the late 1980s at KUOA, where I was CE, featured a grounded B+.

    However, the 2016 ARRL Handbook (the latest edition in my library) shows only these 3 ways to feed B+ to an RF power amplifier:

    DSC02176 (2).JPG DSC02176 (3).JPG
    (A) shows the conventional circuit, but it's noted that since the choke does not present a sufficiently high impedance on the wide range of frequency bands, the heat could burn it out.

    (B) shows the choke at the low impedance end of the tank coil through which plate current flows, but this doubles the peak voltage on both the Tune and Load capacitors.
    DSC02176 (4).JPG DSC02179.JPG

    (C) has the blocking cap taking the voltage off the Load capacitor, but not the Tune capacitor.

    (D) is the omitted way, grounding the B+, eliminating the problems of (A), (B), and (C).

    Does anyone have a later edition of the Handbook; and if so, does it include option (D)?

    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     
    Last edited: Jul 27, 2021
  4. N2EY

    N2EY XML Subscriber QRZ Page

    I have a 2018 and it doesn't show D, either.

    IMHO, at amateur power levels, grounding the B+ creates more problems than it solves. But that's just me.
     
  5. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    There are the tradeoffs. Could we discuss the problems of grounding the B+ at amateur power levels? :cool:

    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     
  6. N2EY

    N2EY XML Subscriber QRZ Page

    Think about the circuit and what that would do.

    "Zero bias" means the DC voltage between the grid and filament is essentially zero. (For the 811A operating at 1500 volts, about 4.5 volts bias is needed). If the filaments are not at ground potential for DC, the grids can't be either.

    A drawback of the grounded-B+ scheme is that the input circuit and the grids have to be insulated to a level that can handle the B+. That means the entire bias and filament circuits.

    Not really. It simply exchanges one set of issues for another. Whether that's worth the trouble is a design choice.

    The Big Advantages of the grounded-B+ arrangement is that the parallel-feed RF choke doesn't have as stringent requirements because it's at a low-impedance point, and there's no plate blocking capacitor. The Big Disadvantages are the complexity of the filament and grid circuits.

    R3 and R4 are only used because the filament transformer doesn't have a center tap.

    And your Amateur Radio callsign is?

    73 de Jim, N2EY

    1967 Novice
    1968 Technician/Advanced
    1970 Extra
     
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  7. N2EY

    N2EY XML Subscriber QRZ Page

    The main problems I see are that the entire input and grid circuits have to be insulated to a level sufficient to handle the B- voltage.
     
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  8. W9JEF

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    SWL37632 said:
    W9JEF: Since 811's are zero bias tubes, besides wanting to measure the grid current, why not ground the grids directly ....this would also reduce 'parasitic' resonances ?
    With a conservative 1080 plate voltage, my 811-As at zero bias draw about 70 mA. The Handbook chart specifies zero grid volts for class B GG (with 1250 plate volts) But if you need bias, my circuit provides for it. Directly grounding the grids would preclude the capability of the convenience of standby bias, and would be of little benefit, considering that .001 disk caps are actually series resonant at VHF.

    [​IMG]
    The key solution to the complexity of the input circuit is a sufficiently-insulated filament transformer. Luckily, the 6.3 VAC transformer on DC power supply chassis I built my amp on had ceramic standoffs for the secondary, so I assumed it could take the full B-minus voltage. (In a forthcoming post, I'll show how a Variac® (variable transformer) can be fitted with a well-insulated secondary.)

    For bias, it was a simple matter to use the small transformer of a wall-wart bass-ackwards: secondary fed by the well-insulated filament transformer, and bias rectifiers off the secondary. The whole bias board is insulated by being mounted on the terminals of the plastic grid current meter. An LED/photo-resistor enclosed in a light-tight container controls a 5000 ohm relay for bias switching.

    SWL37632 said:
    5) It seems that this configuration A): Reduces/precludes the plate high voltage personal safety risk (assuming you want to experiment with the anode circuit) AND B): Reduces 'VHF' parasitics and these are the 2 advantages of this configuration. Is this correct?
    IMHO, the complexity of filament and grid circuits of a grounded B+ RF power amp are dwarfed by the problem of designing an RF choke in the plate circuit of a 1.8 through 54MHz RF amplifier. Did you see that amp project in QST that had an actual cooling fan for the plate choke? :D :eek: :p

    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     
    Last edited: Jul 27, 2021
  9. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    R3 and R4, like many other components in the diagram, serve a dual purpose. In this case, besides providing a phantom center tap for the filament transformer; effectively in parallel with plate current, also act to limit instantaneous surge current in case of a flashover in the tube.

    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     
    N2EY likes this.
  10. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    Considering component count alone, the advantage of grounding the B+ is elimination of arguably the most critical one of all: the plate RF choke. For safety, there's already the choke (with orders of magnitude less stringent requirements) at the low impedance end of the pi-network.

    The eliminated plate blocking capacitor is moved to the input side. Other than the requirement of a well-insulated filament transformer secondary, and insulating the bias circuit (with its inexpensive LED and photoresistor), it's certainly less complex than bandswitching the plate RF choke (or providing a cooling fan for it).

    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     

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