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 ( ) 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


    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


    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.

    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.

    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)

    Last edited: May 2, 2011
  11. W8JI

    W8JI Ham Member QRZ Page

    Right, I'm not just saying things to say them. This homebrew amp stuff is a time sink. It shouldn't take this many posts to correct an obvious error.

    Unfortunately sometimes meters move and RF comes out, but it isn't doing what we think it is. In this case as drawn in the schematic, the meter is between the run filtered rectifier output and the filter caps, so the meter is reading unfiltered pulsing dc that includes bleeder current as well as an extremely high content of ripple currents. How close that is to the real current depends entirely on luck. It might be close if the meter luckily converts rippled DC to average DC through inertia, and it might be far apart if the meter does not. That would depend on meter characteristics, and how it averages pulses and the portion of current the bleeders draw.

    The meter should never be in the path of rectified unfiltered DC, between the supply rectifiers and the filter cap.

    The negative lead of the filter caps must tie to the rectifier - terminal and the meter then goes from that junction to the cathode if we want to read plate current.

    Why are there so few schematics for these homebrew amps that are correct and safe? (two out of two bias schematics posted would not work correctly.) Because once the amp makes RF and the meters move the work is correct. :) Unfortunately lots of incorrect things give the appearance of working.

    73 Tom
  12. K9STH

    K9STH Ham Member QRZ Page


    I tried changing the position of the filter bank, etc., in the "plate meter" circuit. It didn't work! I also tried connecting the W4ZT bias board to the B- (which is 1 ohm above chassis ground) along with the filter bank changes and that didn't work! Basically, I had to go back to original configuration. There is probably something in how the bias board works that is causing this!

    There is no "hum", etc., on the linear with the original configuration so I am going to play "fat, dumb, and happy". The meter across the 1 ohm resistor is reading only plate current. When the linear is in standby and the tubes biased to "cut off" there is basically no current being registered on the meter. I checked the voltage drop across the 1 ohm resistor with one of my DMMs and the voltage showing is in the millivolt range. When the amplifier is keyed, the reading on the DMM is pretty darn close to what it reads on the analog meter.

    Glen, K9STH
  13. W8JI

    W8JI Ham Member QRZ Page


    The time this takes just utterly amazes me. I'm surprised no one else is saying anything. Surely I'm not the only one on QRZ who can read a schematic!

    First, the circuit is so simple almost anyone should be able to see you are running pulsing DC through the meter the way you have it drawn. There is absolutely no doubt about that at all. None.

    It's not even up for debate, as drawn you would run unfiltered DC plus all the bleeder and charging currents through the meter.

    So if it is working the way you drew it on paper and does NOT work the way Heath and everyone else in the world does it, you MUST have a wiring error compared to your schematic. You MUST have an actual physical circuit that does it the correct way, and just have drawn it wrong.

    You can justify the drawing error all you want, but it absolutely is WRONG. You have the negative terminal of the rectifier going THROUGH the meter shunt to the chassis, and the negative lead of the electrolytics shown going to the chassis.

    You absolutely have, no question about it, a wrong schematic up here. Anyone who can read a schematic can see the rectifier negative goes THROUGH the meter shunt to chassis, and the filter cap charging current and bleed current is through the shunt. It should take about ten seconds to see something that simple and obvious, not a bunch of experimentation. My bet is your wiring is NOT like your drawing.

    73 Tom
  14. K9STH

    K9STH Ham Member QRZ Page

    I have checked and rechecked my schematic against what is in the linear and the power supply, the drawing is correct. The only things in the power supply, other than the relay to control the AC power, are the transformer, a full-wave bridge rectifier, the series capacitor bank, the bleeder resistors across the capacitors, and the 1 ohm resistor to ground from the low side of the rectifier.

    The bias board is connected directly to the chassis in the linear. When I connect the capacitor bank to the 1 ohm resistor instead of ground the meter across the 1 ohm resistor goes high. I checked this with my DMM and get the same voltage reading. When I connect the low side of the bias board to the B- (1 ohm above chassis ground) the meter reading drops slightly as confirmed with the DMM. The loading on the B+ basically does not change. That is, I get the same voltage reading no matter what.

    I even built a new bridge rectifier, this time using 4 each 1N5408 in each leg instead of the original 3 each in each leg. I get exactly the same results. The current drawn by the bleeder resistors is about 10 mA. The total value of the bleeders is 200K ohms (8 24K resistors in series across 8 220 mfd capacitors). At 2000 volts, using ohm's law, this equates to 10 mA. Again, using the DMM, the voltage drop across the 1 ohm resistor when the final amplifier tubes are biased to "cut off", I get a reading of just over 9 millivolts which translates to just over 9 mA using ohm's law. Considering the tolerance of the bleeder resistors, this reading is "spot on".

    The "key down" current drawn by the final amplifier tubes (according to the meter) is about 700 mA. Times the plate voltage of 1800 this equates to an input power of 1260 watts. The output power of the amplifier into a dummy load is, according to both my Collins wattmeter and my Swan wattmeter, right at 825 watts. This equates to an efficiency of 65 percent. The idling current (when keyed) is right at 100 mA. Since the amplifier is operating on 1800 kHz the efficiency is pretty much what I would expect. Therefore, the reading on the "plate meter" is in line with what I should be reading.

    I agree, the schematic as drawn does not "make sense" when compared to conventional logic. However, when compared to the actual construction the schematic is correct as drawn and the readings on the "plate meter" are basically correct when comparing input power to output power. As such, since the amplifier is working fine I am going to leave the circuit as is and as drawn!

    Of course, there is probably a logical answer as to why things are working as they are. But, I cannot see what is going on and why things work out like they do.

    Glen, K9STH
  15. W8JI

    W8JI Ham Member QRZ Page

    The problem with this Glen, and it is always difficult to convince people of this, is you probably have built a circuit that will not repeat with every duplication. Meters are not generally designed to intentionally average the current. What you are relying on is the meter doing just that, averaging or smoothing the short pulses from the negative end of the bridge.

    It looks like QRZ won't properly pass the schematics in gif format. They are on this page:

    Here is the link for a proper metering system that includes grid current:


    and here is a proper circuit without a grid current meter:


    Note in either case, the negative end of the caps are tied to the bridge, and the meter reads filtered steady DC drawn only by the tube.

    As for it not working when wired correctly, that's a big warning something is being missed. It has to work when wired correctly, and it should be a major worry if it does not because you could have a major safety issue someplace from an overlooked wiring error.

    I really have very little experience looking at large samples of meters when they are calibrated in DC and fed a pulsing waveform of varying harmonic content and pulse width, and that is exactly what you have (if your circuit really is as you have drawn).

    As a rule it is always better to read what we really want to read, and not depend on luck or some unspecified performance parameter to have things work. You can be pretty sure it won't work in many cases when we depend on random unspecified performance to get a good reading. The real red flag is when you say connected properly it does not work!!!

    73 Tom
  16. K9STH

    K9STH Ham Member QRZ Page

    I am definitely not arguing with you!

    I have checked, rechecked, and rechecked several times again the wiring versus the schematic that I drew. I also tried, several times, to wire the amplifier basically the way you are saying to wire it. That is, the capacitor bank connected to the 1 ohm resistor and not to ground. I have also tried to wire the bias board to the junction of the 1 ohm resistor, the full wave rectifier, and the capacitor bank and not directly to ground. The only way the meter reads correctly, and I have verified this reading with my DMM, is to connect it in the way shown on the schematics.

    The only thing that I can think of is something in the W4ZT bias board is causing this. W4ZT, before his death, and now W4EDS, have sold quite a number of these boards and people are quite happy with them.

    The meter itself is a 1 mA movement with the scale actually calibrated for 0 to 15 VDC. To achieve this calibration a 15,000 ohm resistor is in series with the meter. I changed this resistor to 1500 ohms to change the scale to 0 to 1.5 VDC. The scale has calibration marks originally at 0.5 volts which are now 0.05 volts. Thus, measuring the voltage drop across the 1 ohm resistor equates to 50 mA for each calibration mark. I have compared the meter reading with my DMM and the calibration is "close enough for government work". That is, the indication on the meter tracks well within 5 percent (usually within less than 2 percent) of the reading on the DMM. Since I am dealing with a 3 inch wide meter, I believe that kind of accuracy is excellent.

    Again, I have absolutely no idea as to why the circuit functions as it does. But, it definitely does function in the manner which I have described. The meter reading is steady (no movement can be seen under "key down" conditions), I can hear no "hum" on the signal, other stations report that the signal is "clean", and so forth.

    I have been working on the linear / power supply on my workbench with no r.f. excitation, etc. That is, just the linear and power supply. The bias voltage from the bias board remains basically constant when it is connected to ground when the capacitor bank is connected to either ground or to the "high side" of the 1 ohm resistor. It also remains basically constant and at the same voltage when it is connected to the 1 ohm resistor. The tubes are "cut off" until I activate the relay which applies the bias which does have a fuse in it "just in case" the bias fails and the tube starts drawing too much grid current.

    I agree that the circuit should not be performing in the manner in which it is performing. But, I cannot find any configuration that works besides the one shown in the schematic.

    Glen, K9STH
  17. W8JI

    W8JI Ham Member QRZ Page

    I'd try to find out why it doesn't work with the proper circuit, and does with the wrong circuit. I'm think an accidental or hidden short or open you can't see. I wouldn't expect hum on your signal, you are just driving the meter with pulsing DC. Not the tube. The tube has filtered DC.

    Just a thought....if you have a short or unknown ground from the negative side of the filter caps to the chassis you are not aware of, and you tie the filter cap negative back to the rectifier negative like it should be, the meter will stop working. There are other possibilities also.

    Anyway I am working on that page on metering and adding to it.

    That can take months, but I need to do it.
    Last edited: May 5, 2011
  18. K9STH

    K9STH Ham Member QRZ Page

    The filter capacitors are built onto a piece of "perfboard" which is isolated from the chassis by ceramic spacers over 1 inch long. There are only 3 wires going to this board, 1 from the B+ input, 1 (continuing from the B+) to the cable to the amplifier, and the final one is the negative lead. The reading across the bleeder resistors is right at the 200,000 ohms which 8 25K resistors in series should have. The connections between the capacitors are good as well as the connections to the bleeder resistors.

    I do have another bias board coming from W4EDS for my "new" (pair GS-31b tubes) linear. I may try it in the pair of GI-7bT linear to see if it makes a difference. If it does make a difference, then I will have to investigate what has gone wrong in the present board. If it doesn't make a difference, then the head scratching begins again!

    I have checked and rechecked the wiring in both the power supply and in the amplifier. There is nothing going to ground that is not supposed to go to ground. Again, I agree that there is something screwy. However, the amplifier is working fine and it won't work when the capacitor bank is not connected directly to ground!

    Glen, K9STH
  19. W9GB

    W9GB Ham Member QRZ Page

    Glen -

    I went back to the earlier thread on your 160 amplifier project.
    4 weeks ago you changed out the original HV transformer you used for the 160 meter contest with this 70 pound transformer.

    Were the secondary windings the same (no center tap, additional windings) as the earlier transformer ??
    Is the metal frame of that HV transformer connected to safety ground (AC safety ground / green wire) ?

    The bridge rectifier was connected to this HV secondary in the same manner?

  20. W8JI

    W8JI Ham Member QRZ Page

    Something is open or shorted that is not supposed to be, making the system act like it does.

    I think differences between the drawing and the REAL circuitry are a very common occurance in these HB amps, because about three out of four schematics I looked at won't result in a properly working amplifier, yet apparently the amplifiers work. :)

    The only possible explaination is what is there in hardware does not actually match the drawings. That wouldn't be a problem unless some poor souls use the wrong schematic, or worse yet an unsafe schematic, as a construction blueprint.
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