Schematic of GI-7bT 160-meter amplifier

Discussion in 'Homebrew and Kit Projects' started by K9STH, Apr 29, 2011.

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  1. K9STH

    K9STH Ham Member QRZ Page

    I have been asked by several people for the schematic for my 160-meter amplifier using a pair of Russian GI-7bT tubes. So, I finally got around to drawing it!

    The schematic for both the amplifier and for the power supply are attached. They both were hand drawn and the amplifier schematic is a bit "confused" in a couple of places! :rolleyes:

    Glen, K9STH 160 meter amp schematic-1.jpg 160 meter amp power supply schematic-1.jpg
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Nice job, Glen!

    My 160m amp is a homebrew 4-1000A built in 1984, using a "pull" tube from an FM station that cost me $100. It's been 26 years and it still runs 1500W.:)

    I use a common power supply for that, and my 80-10 amp, and my 6m amp. I have it set up so DC power is applied to all of them at the same time (each deck has its own filament transformer) and I only switch the RF drive. Even the "keying" lines are all in parallel, via a 3-diode OR gate.

    Power on 160 definitely helps. I've never had a "great" antenna on 160, so with smallish antennas, it helps even more. At least now, if I can hear them, I can usually work them, with some exceptions.

    Hey, for me, it's already past "160 season." I work 160 from about Thanksgiving to Easter and then for the next 7-8 months don't touch it. But during the winter months it's a fun band, and if you can stay up really late, more fun!
     
  3. K9STH

    K9STH Ham Member QRZ Page

    The transformer for the HV supply came from my junque box and had only a 120 VAC primary. It weighs less than 15 pounds. The transformer that I am going to use in my GS-31b (probably a pair) is going to use a "slightly" heavier transformer that has only a 200 to 250 VAC (tapped) primary. It is a "bit" heavier weighing in at just about 70 pounds!

    Glen, K9STH
     
  4. K9FV

    K9FV Ham Member QRZ Page

    Nice job of hand drawing Glen - I've spent so many years drawing with computer (AutoCAD) that my hand drawing is too messy.

    BOY, 70 lb!!! you'd better find a place to mount that power supply and leave it!

    Thank you for taking the time to draw up the schematic and sharing with us.

    73 de Ken H>
     
  5. K9STH

    K9STH Ham Member QRZ Page

    I have pretty much decided to make a floor mount linear (i.e. Collins 30S-1) with the power supply in the bottom. I don't have any room remaining on my main console ( http://k9sth.com/uploads/K9STH_Shack_Photos.pdf ) and I will also probably move the equipment in the 4th photo to make room for the floor mounted linear. I do have several aluminum 19 inch rack panels up to 14 inches high that I have pretty much decided on using, 1 for the r.f. section and 1 for the power supply. If I can't locate a suitable rack then I will have to make one!

    Glen, K9STH
     
  6. W8JI

    W8JI Ham Member QRZ Page

    A few comments Glen, maybe drawing errors.

    The plate current metering will not work as drawn. You are trying to measure pulsing negative DC! :)
    You should have the caps back to the rectifier negative. The negative should be safety clamped with a diode to chassis. Right now it can be lethal, metering problems aside.

    You have no safety choke shown on the output. That also can be lethal. ALL tube amps with pi nets need a safety choke.

    The standby bias resistor is needlessly large for dissipation. Even a 1/2 watt will work there. Maybe that is what you had.

    The real concerns are the safety issues.

    73 Tom
     
  7. K9STH

    K9STH Ham Member QRZ Page

    JI:

    The plate meter works very well. I did forget to draw in the bypass capacitors. Measuring the plate current across a resistor to ground from the low side of the power supply to ground is exactly the same thing that Heath did in the SB-220 and SB-200. It doesn't matter if the meter is in the high side or the low side, both are DC.

    Basically, I am reading the voltage drop across a 1 ohm resistor with a 1 mA meter calibrated with a full scale of 1.5 volts. A voltage drop of 1 volt equals a current of 1 amp. Using a 1 ohm resistor makes calibration of the meter scale easy.

    There is an r.f. choke across the antenna connection, I forgot to draw it. Basically, I built the amplifier "from the seat of my pants" and only drew the schematic when several people asked me to draw it.

    The bias resistor was "handy" so I used it!

    Glen, K9STH
     
  8. W8JI

    W8JI Ham Member QRZ Page

    Glen,

    You better look again at your schematic. I don't care if your meter works in real life, the schematic you posted has the meter in the line carrying pulsing rectifier current. If you have it wired the way you have it drawn, the meter will certainly move BUT it will not read the correct current. It will read the pulsing charging current of the electrolytics. Note you have the electrolytics drawn going to ground.

    I can 100% assure everyone that is NOT the way Heath or anyone else in the world measures plate current. :) No one measures the pulsing dc from the rectifier system to ground. :) Maybe this is just a schematic error, and you wired it properly.

    http://forums.qrz.com/attachment.php?attachmentid=63583&d=1304038390


    The schematic shows you reading the rectifier pulsing AC current into the filter caps, not the cathode or plate current. Look at it.

    Then look at the Heath. Heath and everyone else in the world ties the negative of the electrolytics back to the rectifier negative. You have it tied to chassis. You have the meter between the rectifier - and the chassis. Heat has it between the - filter cap lead to the cathode of the tube, and the grid meter to the chassis.

    I know you are trying to save parts, but unless you float the negative rail up off the chassis and use a system like Heath, Dentron, Ameritron, Drake, Collins, or others do you are not measuring what you think you are measuring. How close you get to the real plate current would depend on the meter response for pulsing DC, unless your schematic is wrong.

    We should always show critical safety components. I hate to see someone get hurt duplicating something that is missing but critical. Once these schematics get up on the net they can't be called back in.

    Nothing wrong with that, but anyone trying to replicate what you drew really might get into trouble. It's just baffling that many or most of the schematics for amps like this are missing parts, have wrong wiring shown, and never seem to be proof read or corrected. For example FV was asking about bias, and out of two schematics that got posted both would not work properly. One would put the tube into operation if the amp was on standby or not, and would never go above zener voltage plus relay voltage when the radio was unkeyed. The other one had a ground from a filament pin and would bias the cathode with AC.

    It's great to see people build things but it can be frustrating for builders when drawings have errors that would make the system not work.

    73 Tom
     
  9. K9STH

    K9STH Ham Member QRZ Page

    Moving the negative connection of the filter capacitors is a one wire change! I'll try that.

    Glen, K9STH
     
  10. W9GB

    W9GB Ham Member QRZ Page

    Tom and Glen -

    It is always good to check our work. Measure at least twice and cut correctly once. :)

    I went to Tom Hammond's, N0SS re-drawn Heathkit SB-220 schematic (2006) to see where the confusion is coming from.
    http://www.n0ss.net/sb-220_schematic_2-pg_v1r4_21oct2006.pdf

    Tom, N0SS has been diligent in reviewing ANY mistakes in his re-drawn schematic based on original Heathkit documents,
    and accepting corrections from Heathkit owners and SB-220 restorers.
    That work revealed the mislabeling, by Heathkit, of C18 incorrectly as C16 across RFC-4.

    The N0SS schematic (as I read it) agrees with Tom, W8JI description for the plate metering ...
    BUT I can also see how this schematic could EASILY be misread (it happens to all of us at times) by the routing of the schematic layout and circuit traces.

    Point H and Point B on the SB-220 schematic is where the rectified (pulsed) DC is routed to the HV Capacitor bank.

    The Plate Meter is connected after the HV Capacitor bank and resistors at Point C (negative HV electrolytic) and Point A (positive HV DC)
    The Grid Current (multi-meter selection) is read between Point F and Point K (marked as ground)

    w9gb
     
    Last edited: May 2, 2011
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