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Home Brew Amplifier First Start

Discussion in 'Amateur Radio Amplifiers' started by KC0VVB, Nov 5, 2019.

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  1. KC0VVB

    KC0VVB QRZ Member

    QRZ i just finished assembly of my home brew amplifier and i am having some trouble diagnosing some issues on initial start up. I have a pair eimac 4-400a's grounded grid running 3400 volts on the plate 120 ma plate idle. and 225 ma plate curent drive. 4 ma on the cathode, with 50 watts drive from the viking ranger.

    One of the tubes turns bright orange with in 5 seconds of keying the rf the other tube shows no color or heat.

    I am attempting to use the original viking thunderbolt filament transformer which had a secondary winding for each filament and was fed 180 deg out of phase. I get positive 5.5 v ac across each cathode and 11v ac across both together. With the inner most filament taps connected together and floating above ground as the neutral.

    I also used a trifillar choke with a 3rd winding for the neutral wrapped over the standard bifillar choke sold by rf parts. T-Bolt GG Schematic Diagram  V3.jpg
  2. KC0VVB

    KC0VVB QRZ Member

    IMG_0785.JPG IMG_0786.JPG IMG_0788.JPG IMG_0787.JPG Pic of my actual tube sockets and choke
  3. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Swap the tubes and see if the trouble follows the tube.
  4. KC0VVB

    KC0VVB QRZ Member

    Thank you for your swift response. I switched the tubes and the inboard socket is hot and the outboard socket is cold just like before.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm portable and viewing on a small screen, but...

    What do you mean 4mA on the cathode? That makes no sense. Cathode current = plate current + screen and grid current, so will always be higher than plate current. Did you mean "4 volts?" If you meant that, what were you measuring?

    Although I can't view your images terribly well on this little 10" screen, I see some stuff that looks wrong:

    No plate parasitic suppressors? If there are any, I didn't see them. Without parasitic suppressors (coils in parallel with the resistors), as soon as you start drawing any current, the resistors should burn up and that would open the plate circuit. I also don't see exactly how the B+ is feeding both tubes. How are those connections arranged?

    I didn't check your under-chassis wiring, but the "grid and screen grounding" appears to be a daisy-chain wiring from pin to pin and then to ground, using ordinary wire. That rarely works; grid and screen pins need to have very short, direct, wide strap connections from pins to chassis, with a separate short connection to ground from each pin in order to allow stability and provide serious gain at higher frequencies.

    It also "looks like" you used LMR240 or LMR195 or something for the coaxial cable wiring; IMO that's not a great choice, as it's too easy to create short circuits and this stuff is not intended for use in very hot environments, like near stuff that gets hot (such as the tube socket contacts -- the filament pins can get very hot, and they should).

    Have you built amplifiers before?
  6. W1BR

    W1BR Premium Subscriber QRZ Page

    The resistor wattages in your parasitic suppressors look to be greatly undersized. 1 watt parts? Also, it looks like you used loops for the inductors on those suppressors. I'd follow a more conventional design and use OY or OY metal composition resistors.

    If one tube is not showing plate color with RF drive (and I assume the filament is glowing with power on?) then you must have a wiring error. It is hard to see in the photo, but does the center conductor from coax feeding the cathodes have a coupling capacitor to both tubes??? One socket not working must be a wiring or design error. I'm seeing caps across the filaments, and what appears to be only one RF coupling cap from the coax to one tube. Again, hard to see the details.

    That grid grounding has far too much inductance. You should use straps or wide braid brought directly to the chassis using the shortest lengths.

    Also, you really need to have some decent airflow across the bottom of the socket to cool the tube pin seals. Same for the plate cap on the tube. Those tubes need airflow in the proper amount and in the correct locations.

    N2EY likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I don't even see "parasitic suppressors," all I can see are resistors. Is there a coil across each resistor somewhere?
  8. W1BR

    W1BR Premium Subscriber QRZ Page

    Looks like they are 1/2 loops of copper wire below the carbon film 47 ohm resistors. Hard to see, but I suspect that is what the resistors leads are soldered too.

    Was this a Valiant in a past life?
  9. KC0VVB

    KC0VVB QRZ Member

    Hi steve and pete thanks for getting back to me first thing in the am. The xyl found out i have the day off so we had to do what she wanted to do before i was free to ham around.
    You are correct steve this is my first attempt at a home brew amp. You and w8ji and K5AN helped me out with a few repairs over the years with my viking invader 2000 and Swan mark II.

    I came across a viking thunderbolt chassis with a couple of the original bread slicers and the roller inductor, tube sockets, filament transformer, and a brad thompson industries plate transformer 1250v ac 1.5 amp ccs slightly smaller than the one in the bti lk 2000. It also came with a large potter brumfield coax relay, a smaller auxillary power transformer and rectifier for 12v dc. and a small bias 12v relay. The original owner got a good start on it but it did not appear to me that it would have worked well.

    I came across a post of yours where you were helping K5HU finalize the resistors on his meters. and saw the link to his schematic for his solid state thunderbolt with the bad screen transformer and updated power supply he grounded the grids and made a lot of the parts un necessary that were completly missing from my thunderbolt. I have a file with the 3 pages of instructions i have been following for the past 3 months. I just pasted below. I suppose i just need some help verifying i did it right.
    I read the instructions looked at the schematics and assembled the amp and its not doing what its supposed to do.
    It may need some fine tuning in the design or a change in some of the component values. I can post more pictures of the areas you asked about. I am just trying to get 200w carrier am out when using it with my viking ranger on 75 meters AM.
    Erik Kc0vvb

    The author successfully converted a Johnson Thunderbolt Linear Amplifier to grounded grid mode of operation in the mid 1970's. That project gave second wind to a trusty Ole boat anchor, which had a failed screen grid power transformer. The original scheme floated RF Amplifier tube grids above chassis ground with a meter shunt and RF bypass capacitors.

    However in modern designs, tube grids are usually tied directly to chassis ground which gives greater stability and safety. Consequently an extra buss is required, which the author has named B1- to distinguish it from B-. Please reference the included conversion schematic diagram below. For your assistance and understanding, a Thunderbolt technical manual is available for download.

    The Johnson Thunderbolt B+ power supply operates in excess of 2,500 Volts DC. Consequently, this project is only for knowledgeable and experienced electronic technicians.

    Before starting any work, turn Thunderbolt power switches off and unplug it from the power source.

    To permit the capacitor bank potential to discharge, DELAY ONE (1) MINUTE before opening the cabinet. Then touch the V1 and V2 amplifier tube plates with a high-voltage grounding probe, AKA “Chicken-Stick”, to ensure the capacitor bank is safe.

    Disconnect all 120/240 VAC primary connections from high voltage transformer T101 and insulate these wire ends. This action ensures safety and permits the chassis to be powered up for relatively low voltage testing of the tube filaments, pilot lights and other circuits. Reconnect these wires only after all other wiring checks and active testing has been completed.


    Screen Power Supply

    Because tetrode tube grounded-grid applications don’t require a high voltage screen supply, all such related parts should be removed. A short list of major parts to be removed includes T103, L102,
    SL103, V103, V104, V105, V106, V107, V108 and V109. Any other small components attached to these devices should be removed too.

    Mode Switch

    Remove all connections to Mode Switch SW103 and remove it from the chassis because future modes of operation involve only CW or SSB. One should plan on filling the hole with a toggle switch for testing tube idling current or another necessary function.

    RF Chokes

    Remove coil E3 with 50 Ohm shunt and both coils E4 & E5 with 100 Ohm shunts.


    Remove caps C12, C13, C14, C22, C23, C24 & C25, rated at .001uf each.
    Ground Bond
    Disconnect the chassis ground connection located on or between Tube V1 pin 5 and V2 pin 1. The amplifier tube filaments must be floated above chassis ground for application of RF drive.


    Accessory Plug
    Keep J102 on the chassis rear with associated capacitors and inductors. These parts include chokes L106, L107, L108 and bypass capacitors C115, C116, C117, C118, C119 and C120. This rear service port may come in handy for connecting accessory devices in the future.

    Plate Current Meter

    Keep meter M2 with its internal shunt for monitoring 0-750 mA of plate current. Because of the additional B1- buss, the positive (+) meter terminal must be disconnected from chassis ground.

    Grid Current meter

    Retain meter M1, which has an internal movement of 50 Ohms. Original factory metering provided the following grid scales; Control Grid (G1) 0-30 Ma and screen Grid (G2) 0-150 Ma. This same meter also measures 0-3 kV Plate B+ voltage.

    Meter Function Switch SW104

    Keep this switch for selecting grid current and plate voltage functions.

    Plate HV Power Supply

    Keep in service most B+ power supply parts which include, T101, L101, C101, C105, C110, R101, R102, R103, R104, R105, R106, R107, R108, and R109.

    Filament Power Supply

    Keep transformer T102 in service for amplifier tubes V1& V2.

    Tuned Input Circuitry
    Keep the Grid Band Switch with wafers SW1A, SW1B and SW1C, subject to the following necessary modifications. Point “T” is referenced on the schematic as the Drive Tank junction between C1 and L1.

    (1) Remove the following capacitors; C21(.0005), C26(.002), C10(.001), C11(.005) & variable capacitor C2(11 MMF). Neutralizing capacitor C2 isn’t necessary because a grounded grid (GG) amplifier should be stable and free of parasitic oscillations.

    (2) Remove inductors L8 and L10.

    (3) Remove resistors R1(15K) and R2(15K) Ohms.

    (4) Remove Inductor E3 with its 50 Ohm shunt from SW1C-8.

    (5) Remove all other connections from band switch wafers SW1A and SW1C. Clear these switch lugs completely off.

    (6) Connect switch SW1B-11 to chassis ground.

    (7) Connect the resistor combination of R3, R4 and R5 to point “T”.

    (8) Utilizing a shielded cable, reconnect RF Input J1 to point “T”.
    (9) Also utilizing a shielded cable, connect point “T” to the cathodes of tubes V1 and V2. Pass this RF signal through a 0.02 uf, 1 kV disk ceramic capacitor.

    Since grid-driven tube amplifiers have very high impedance input, Johnson utilized step-up transformer link coupling into the resonant grid tank. However this scheme won’t work anymore, because the cathode drive impedance is relatively low. Consequently the resonance effect should be kept, but without making any impedance transformation.

    Switch SW1B changes band taps on the secondary side of inductors L1 and L2 through L3. These inductors form a parallel resonant tank circuit with variable capacitor C1 (75 MMF). However, the “Resistive” position on SW1B won’t have any future purpose.

    The non-inductive 333 Ohm parallel resistance of R3(1K), R4(1K) and R5(1K) wastes about 15 watts of RF power on all bands, but solid state transmitters may load better. However one or more of these resistors may be disconnected, if insufficient drive is encountered.

    During amplifier setup, place an SWR bridge between the transmitter output and amplifier input. Preset the drive band switch and adjust transmitter excitation to about 40%. Adjust PA tuning by standard “dip” and “load” method. Adjust drive tank capacitor C1 for minimum reflected power. Try different combinations of the drive band switch and C1 settings to obtain the best exciter load match.

    Grid Current Metering

    Beforehand, remove all attached components and associated wiring from SW104. For proper switch reconnection, follow the included new schematic diagram. Keep all associated meter inductors L111, L112 and capacitors C106 and C108.

    The lower resistance values for meter shunt Rs are better choices. However don’t exceed 0.30 Ohms, which risks approaching the forward bias threshold of the DP2 protection diodes. Within a 1% tolerance, select paired resistor values from the following list. The power rating of Rs should be 1 Watt and Rm should be 0.5 Watts.

    Carefully solder the twisted wire pair about a quarter inch from the Rs shunt resistor body, but don’t overheat it. This method prevents ground loop currents and gives more accurate readings. All resistor pairs yield the same 0-300 mA grid current scale.

    Rs 0.20
    Ohms, Rm 9.80 Ohms
    Rs 0.22 Ohms, Rm 15.78 Ohms
    Rs 0.25 Ohms, Rm 24.75 Ohms
    Rs 0.27 Ohms, Rm 30.73 Ohms
    Rs, 0.30 Ohms, Rm 39.70 Ohms

    By an x10 multiplier, meter function switch SW104 position P1 now reads combined (G1+G2) currents with a 0–300 mA scale instead. However the original screen grid (G2) function has been abandoned on position P2.

    Filament Transformer T102

    Reference the new schematic for modifications to the filament wiring of tubes V1 and V2. However, the T102 secondary windings must be phased correctly. Voltage between points x-y must equal 10 VAC.

    But if near zero volts instead, then reverse one filament winding. Alternative test points between V1-1 and V2-5 can also be utilized. Ideally for balanced V1 and V2 filament loads, the neutral current is zero Amps. However, incorrect phasing places double current in the W3 neutral, although both tube filaments will light.

    Because of the solid state rectifier modification, disconnect and insulate the 2.5 VAC winding for the 866B tube filaments.

    HV Pilot Lamp X101

    Replace the B+ indication lamp with a panel-mounted 120 VAC red neon type. As advantages, neon lamps are usually brighter and more reliable than incandescent bulbs.

    Alternatively, one may prefer to remove the X101 internal parts and preserve the lamp jewel for esthetics. A red neon lamp may be able to fit within the old socket assembly, although the author hasn’t tried this. Be sure to include the necessary series resistor and insulate all 120 VAC wiring from the metal socket assembly.


    B+ Step-Start Relay
    This is necessary to protect silicon diode string Dr and transformer T101 from high cap bank charging current. Consult the ARRL Handbook for details on installing this protection scheme.

    Accessory Transformer T10X

    The dial lamps require 6 Volts AC/DC, whereas keying relay Kx requires 12 Volts DC. Obtain a 12.6 VAC center-tapped filament transformer with a 2 Amp secondary. Build a rectifier bridge from four 1N5408 diodes, which yields both +12V and +6V via the center-tap.

    Trifilar Filament RF Choke

    Obtain about 10 feet of #12 stranded PTFE Teflon® insulated wire and a bifilar wound RF choke for tube filaments. In the same rotational direction, wind a single layer with the same number of turns over the bifilar coils. To prevent untwisting of the loops, tape or place tie-wraps at the rod ends. This extra coil is the neutral return path, which is marked W3 on the schematic.

    Tube filament wiring may be better arranged, by physically rotating one tube socket so that V1-5 is directly adjacent to V2-1. But tighten the mounting screws very lightly, because it’s easy to crack the ceramic tube base.
    Grid Bias Diode Db
    Make bias diode string Db with six (6) series connected type 1N5408 silicon diodes on a terminal strip. Before initial startup, jumper around the entire string for zero-bias operation. Without RF drive, test the amplifier for idling current to be about 140-180 mA. But if this current is excessive, include one or more diodes in the string.
    Unless otherwise specified, all necessary capacitor values for the modified RF deck section are clearly marked on the schematic diagram. All such new caps are Disk Ceramic, 1 kV type devices.
    Electrical Wiring
    It’s recommended to utilize the following colors; Red for B+, Black for B- and Blue for the B1- buss. However, be consistent with the adopted wiring color plan.

    Solid State Rectifier
    The schematic diagram was modified to show a solid-state replacement for the 866B rectifier tubes. However, these tubes can be left in service if desired.
    Each leg of rectifier string Dr consists of ten (10) type 1N5408 silicon diodes. These diodes are connected in series, anode to cathode, to provide a total peak inverse voltage rating (PIV) of 10 kV. However to provide voltage balance, each diode must be shunted with a capacitor and resistor.
    Hams usually build these devices on a strip of vector board elevated about an inch above chassis on ceramic stand-off insulators. Consult the ARRL Radio Amateur’s Handbook for specific details.

    Capacitor Bank C101 Upgrade
    Do your own research and consider bypassing the L101 swinging choke. The following cap bank design changes and component upgrades are suggested. However you have ultimate responsibility for making the final design changes, if any.

    Close attention must be given to overall power supply design because of the higher B+ voltage. After removal of choke L101, operating voltage will peak about (Sqrt 2) x (2400) which equals 3,400 VDC.

    Originally a string of six (6) electrolytic capacitors rated at 80 uf, 450 WVDC, was utilized to provide a 13 uf cap bank. But due to their age, consider replacing all of these capacitors and resistors with new parts.
    Originally every cap was bypassed with a 120K Ohm resistor. These resistors are required to balance the voltage across each capacitor. The original B+ bleeder string consisted of six (6) series connected 5K Ohm resistors. Their total power dissipation is a surprising 208 Watts of heat, at 2.5 kV applied voltage.
    It’s suggested to utilize nine (9) stages of 270 uf caps at 450 WVDC and change their bypass resistors to 100K Ohms, 5 Watts each. This creates a 30uf bank, which works well for single sideband operation.
    It’s also suggested to replace bleeder resistors R101 – R106 with 20K Ohm, 25 Watt values and utilize nine (9) identical stages in series. Extra stages reduce high voltage stress and heat applied to each. This 180K Ohm bleeder modification reduces heat output to 64 Watts, provides a cooler cabinet and saves electricity.

    Because of higher applied voltage, B+ voltmeter scale resistors R107 and R108 must be changed. It’s suggested to utilize 910K Ohm, 1% tolerance, 2 Watt values. Include four (4) such identical resistors in series to reduce voltage stress. This change rescales the plate voltmeter to 3,630 VDC with a 1.21 multiplier. For continued safety, replace R109 with a new 10K at 2 Watt, 1% tolerance resistor.

    This 30 uf cap bank stores a potent 173.4 Joules of energy. One RC time constant is (30 uf) x (180K Ohm bleeder) equals 5.4 Seconds. A capacitor is considered fully discharged in five (5) time constants, or 27 seconds in this case.


    In relation to the enhanced cap bank, assume the HV B+ power switch SW102 was turned off and the cabinet opened immediately thereafter. Such a careless act would crash discharge the 30uf cap bank C101 through the 17 Ohm fault protection resistance Rf after about (5 x 30uf x 17 Ohms) = 2.6 Milliseconds.

    Assuming a B+ of about 3.4 kV, this discharge would initially be (3,400 Volts / 17 Ohms) = 200 Amps. After such a potentially destructive fault, test all C101 caps, both Rf resistors, and the Safety Switch to ensure proper future operation.

    The cabinet safety switch circuitry must be kept operating properly for such an emergency, however it's not something to carelessly depend on. After power switches are turned off and the T-Bolt is unplugged, always wait one (1) full minute before opening the cabinet and then exercise the Chicken-Stick.


    If the B+ buss faults anywhere within the amplifier cabinet, probably the B- and/or possibly the B1- buss will tend to rise above chassis ground to full B potential. Therefore a radio operator might become at-risk because of possible exposure to such an elevated metering circuit potential. Consequently the following circuit should be installed for personal protection against such an event, however unlikely.

    For such B+ faults, crowbar safety diodes Dp1 and/or Dp2 conduct and clamp the B- and/or B1- busses to near chassis ground potential. Any B+ fault to chassis returns to B- via diode Dp1. However, any B+ fault to buss B1- goes to chassis ground via Dp2 and returns to B- via diode Dp1.

    Moreover the B+ power supply will load into an effective short-circuit, which blows primary fuses F101 and F102. As a result, the radio operator and both meters are protected.

    Consider this calculation for the M1 meter grid current, V=IR = (300 mA) x (0.22 Ohm Shunt) = 0.066 Volts. This maximum scale value should not forward bias Dp2, because diode headroom is about 0.5 volts.

    Consider this calculation for the M2 meter plate current, V=IR = (750 mA) x (0.1 Ohm Meter//Shunt) = 0.075 Volts. This maximum scale value should not forward bias Dp1, because diode headroom is about 0.5 volts.

    If a B+ fault ever occurs, the following devices should be verified for proper operation. Test each protection diode Dp1 and Dp2. Test each diode Dr, in both high voltage rectifier strings. Also test caps Cp1 and Cp2, but don’t exceed 50 working volts for replacements.

    A possible source for such a B+ fault is flash-over within a defective V1 or V2 power amplifier tube. Such an internal arc will probably fault directly to chassis ground through the grounded grids. However diode Dp1 provides the necessary protection for such an event.

    Never open an energized T-Bolt because the cabinet safety switch will fault B+ and blow primary fuses F101 & F102. As shown on the schematic diagram, carefully reconnect this safety switch to work after fault current limiting resistor Rf.

    As a working rule of thumb, Rf is established at 5 Ohms per kilo-Volt. Whereas, (2.5 kV x 5 Ohms = 12.5 Ohms) and (3.4 kV x 5 Ohms = 17 Ohms). For redundancy, the author suggests making Rf with a pair of 25 Watt resistors connected in parallel. Mount these resistors on ceramic stand-offs about one-half inch above chassis ground.

    Proper sizing of Rf protects transformer T101 and rectifier bank Dr during such a fault, although personal safety is preserved. To provide an extra safety margin, the typical 5 Ohm total resistance contribution of cap bank ESR plus other devices and wiring has been excluded from calculations herein.

    Technical Credit

    With appreciation, Charles W8JI is credited for his excellent treatise on the subjects of personal safety and equipment protection for Amateur Radio linear amplifiers.
  10. W1BR

    W1BR Premium Subscriber QRZ Page

    Those grids are not directly grounded. Too much inductance in the lead lengths. DC grounded maybe, RF grounded, no... especially at VHF parasitic frequencies. You have a tuned, resonant VHF network tying the grids to ground.

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