4-1000A Grounded Grid Amp for AM

Discussion in 'Technical' started by W5HRO, Jul 28, 2015.

  1. Doc813

    Doc813 Member

    Why not just wind one for each tube i would think that would be much better than putting rf on the trany...or maybe for one tube that takes a lot of current a good sized wire an several ferrite beads on each fil lead.....just my 2 cents
  2. WD5JKO

    WD5JKO Member

    Having two Hammond 165V7's would do the job. A pair cost $110 total which isn't too bad these days. Those Hammond's also have the split bobbin approach that I mentioned, might tolerate a "RF Hot" 1/2 bobbin. :-)


  3. KE7TRP

    KE7TRP Member

    I was at the ham fest today and saw my Friend who is a Broad cast engineer. He told me that nearly ALL High power commercial Transmitters run this way. He said it would not be practical to have a Filament choke on the Secondary side as the larger tubes pull 100 amps. The choke would not fit into the cabinet and would be costly. He said they use insulated boards to mount the Fil transformer on and they use small chokes on the Primary side along with bypassing caps.

    I also found this on the Amps list that seems to match what my Friend Said.

  4. W5HRO

    W5HRO Administrator

    That's complete BS because BC transmitters don't use grounded grid amplifiers so there is no need for any secondary fil choke.

    Enough said...
  5. KE7TRP

    KE7TRP Member

    Thats simply not true. There where MANY Big high power rigs that used Triodes in GG that used this method. RCA 50KW Shortwave used it, Gates HF20 also used this. Lots of Big GG amplifiers that sat next to Plate modulated transmitters to boost output used this method. None of those rigs had the Choke on the secondary. It would have been a MASSIVE choke and simply not practical.
  6. W5HRO

    W5HRO Administrator

    Some modulatiors used GG or close to it, but that's AF and not RF.

    And the Gates HF20 final RF amp section had a push-pull-parallel type of configuration.
  7. WD5JKO

    WD5JKO Member

    I cannot say anything myself about BC transmitters and G-G RF stages, but if they exist, then that overrides any opinion to the contrary. If they don't exist, it overrides the opinion stating they do exist. Facts usually trump opinions, but sometimes, emotion based opinions will trump facts. I lose most arguments with the XYL this way. All she has to do is raise her voice and blurt out something and she has already won. With my boss at work, he is often wrong, but in case I want to challenge him, I must remember that the boss is always correct. Same goes for sysops. :-)


    On Sep 16, 2004, at 5:22 AM, lncarman wrote:
    I've heard over the years of some amp builders floating the filament transformer. I was
    told that you isolate the filament transformer from ground and add two chokes
    in the primary side with bypassing. Since the primary side is 115V or 220V the
    amperes demand would be much less on the primary side. Looks like any RF would
    be suppressed greatly by the filament transformer and you wouldn't need a lot of
    uH in the primary chokes to finish the job. Has anyone tried this???

    --- It is common in commercial g-g applications, Larry. All filament xfmr windings are RF-bypassed to the core and the transformer is floated on a insulating sheet of ABS or G-10. A bifilar choke is placed in the primary of the transformer. After installation, the C from the core to gnd is measured and this amount is subtracted from the calculation(s) for C2 in the tuned-input(s). An additional choke is connected to the fil. secondary CT to provide the DC path for electrons going to the the cathode from the –HV. As I see it, filament currents of more than 30A are a candidate for this solution. Thus, a filament choke for a 3cx10,000A7/8160 (7.5v/99A) would typically be Much smaller than a filament choke for a pair of 3-500Zs (5v/30A). In this case, with 240v mains, the filament choke for the 8160 would only need to be wound with #22-ga, wire, while the choke for the 3-500Zs would need to be wound with #8 gauge wire.
    - cheerz

    Larry N5BIP
    Amps mailing list

    Richard L. Measures, AG6K, 805.386.3734. www.somis.org

    Amps mailing list
  8. W5HRO

    W5HRO Administrator

    Already read that Jim before replying yesterday. It means absolutely nothing. Second, I never said nobody had or was doing it. I said it was a cost cutting method of lesser quality.

    The old BC transmitters used class C or class B push-pull RF amplifiers and the bigger ones often used push-pull parallel configurations to get the really higher power, but they were still RF control grid driven. Now, there could have been a unique situation where a GG configuration was used somewhere by some transmitter maker, but it wasn’t the norm and if it was ever done it was rare.

    Today everything has gone solid state so no more filament transformers anyway so the conversation doesn't even apply anymore.

    Having said all of that, I would still float the frame of the filament transformer in a grounded grid amp for the amateur service anyway, but I would never in a million years not use the full size chokes on the secondary.
  9. KE7TRP

    KE7TRP Member

    Have you thought about how big a 100 amp or 200 amp filament choke would be? Think this is the reason they just simply do not run that way.

    I spent another few hours using the amp with the new plate choke. One time near the end of the 2 hours I keyed up and BAM got a big arc in the RF deck. The choke survived. I have no idea what arced and why. I was running 4700 volts on the plates under load. I keyed it hundreds of times that night and only one time it let out an arc. I think its a sequencing issue as I did fast key it back to back when it happened. Frustrating to say the least. It always happens after its nice and hot and I unkey and then key up again fast like if someone askes me a quick question.
  10. W5HRO

    W5HRO Administrator

    There's no reason to ever need that big of a filament choke. The big BC stuff is RF control grid driven and now its solid-state. About the biggest you would ever need is like what you are trying to do in cathode driven GG configuration. I know there are some CB'ers who build humungous amps and even some hams who do it using many tubes in parallel, but that's their problem for simply being stupid :icon_shh: Who would ever need anything bigger than a pair of 4-1000A's or 3-1000Z's, etc? You don’t hardly even see that anymore. A pair of 3-500Z’s is really nothing as far as the filament chokes are concerned.

    Anyway, is the cathode bias designed to change or shift as you key the amp? If everything is on 24/7 including the plate voltage then I don’t know what else it could be other than something it just too close to something else and it just arcs over at random. If in some way the cathode bias opens up via a relay or something due to switching when the plate voltage is at the anodes full bore then the load wouldn't be there so it would just arc to the shortest path to ground it could find.
  11. KE7TRP

    KE7TRP Member

    The Plate voltage is on anytime I turn it on. It is sitting there on if I am in the shack actively using the amp. Just like any other ham amplifier. There is a large contactor that switching is cut off bias, 20K resistors and handles the RF switching. When I key, the bias goes to my Bias supply and the tubes idle up at the same time the input and output coax is switched. I am thinking of sequencing this so the coax is switched and then the Bias is changed moments later. But yes, Its a very random arc. You can get a night of use out of the amp and its fine but then, POW it will arc.
  12. W5HRO

    W5HRO Administrator

    But what I'm wondering is if it is switched by a relay could the cathodes be open for a second or two between that switching? If the plate voltage is on the tubes and the cathodes (filaments) to ground connection floats for an instance bewtween the switching that could be it. There would be no load for the plate supply if the cathodes are floating while the relay contacts are open and switching. It would just arc over to anything it could find. Relays are not always that reliable and there could be slight delays in the switching every now and then unless it's not relay switched.

    You have never posted any drawing of the input switching or the bias circuit so it's just a guessing game. I dont know how the bias and everthing is actually connected, but maybe if there is a relay then using a make-before-break relay can work if it doesnt short anything out. It may require using a DC blocking cap in the right place, etc.
  13. KE7TRP

    KE7TRP Member

    It is the same circuit used for 60 years or more. Its a 20K Cut off resistor per tube. Normally the 20K is in circuit to cut off tubes. when you key, the resistor is out of circuit and the bias voltage is applied. -14 volts is what I have it on now. This gets me 150MA per tube at idle. The only time it is not in one of those two positions is the mechanical time the contactor in in motion. I certainly do not see any jump in plate current, however, It might be to fast to register.

    Your theory might be correct here.. It might be unloaded for that split second and as a result, it arcs. Its a single pop and it typically happens when I double key or short key the amplifier.
  14. W5HRO

    W5HRO Administrator

    I think it's worth looking at. If I understand your circuit correctly and if you were to find and use a make-before-break relay then all it would do is throw those cutoff resistors across the bias supply for a second or two which wouldn't hurt anything. Then at the same time the bias supply would still be there for a second or two until it fully switched back to the resistors again. You would just need to find a relay that would do that when it closes and opens both. Make-before-break and never unbreak before-remake. I think most all make-before-break relays do that anyway.

    If by some chance that is the problem then it’s all because the cathode is losing its path to ground just long enough to make the plate supply arc over to wherever it can.
  15. Opcom

    Opcom Member

    It sat in a garage for many years before I got it. It didn't seem to have been completed, but several people have definitely worked on it.

    I can see how it could have worked 'OK' but the original builder didn't have the right parts to make the best of it.

    I wonder how the project is coming along.

    The transformer in that one could be run with a C input and make enough voltage but with a small load like 50-100mA would be >4KV

    I like to use a choke input for good regulation. Something like ths would be a decent power supply for two 4-1000 at 3300V or a bit more..
  16. KE7TRP

    KE7TRP Member

    Just an update. The amp has been working great all this time. Mainly used on 80 meters. I have run it hard and up to 6000 volts on the plate with no issues. The plate choke from Floyd was what I needed. It shows no color change or hot spots of any kind. It runs cool and the amp just works. I have not had any Arcs. They stopped. I am not sure what it was but I did have a ton of RF on my audio. After exhausting all efforts to try to remove that RF, I swapped out my MD100 Yaesu microphone for a Second MD100 I had in the closet. RF on my audio was gone and everything was back to normal. The original MD100 is clearly at fault, I can swap back and forth and hear the RF on the mic. I suspect, This RF was inducing some kind of high frequency in the amp and causing those arcs. Possible? Either way, Its working great without issue for now.
  17. KX4DV

    KX4DV Member


    I'm late to this party. But I've been intrigued by your videos KE7TRP. I actually have a power pole transformer and two 4-1000a's I'd like to build into a setup like this. (Selectable Left/Right/Left+Right) I've been trying to read through this thread during the day and pick apart out it works. I know you don't have schematics for the amp. But I picked up on a lot of things you've corrected. I'm thinking about getting a variac to drive the power pole transformer to allow more adjustability for the HV side of things. Maybe I'm mistaken but you've got 3 variacs in your amp right? One for the HV transformer and two for the filament transformers? I don't think I'll follow the route of variacs for the transformers and just pick up some of the 7.5v 22a transformers from Mouser. (I think there was mention that you were using the variacs to dtive up the filament transformers you had to 7.5v) I'm thinking a little more modernization in what I'm going to build. I see that the input tune has been a problem along with the choke. I was thinking about using a pair of these input tune boards for mine: http://home.earthlink.net/~wd7s/TU-6B.htm (One being setup for ~100ohm and the other being setup for ~50ohm input) Then use vacuum relays to switch things over in and out. I'm still trying to piece together how you have the tubes selectable in the system. I'm guessing they have HV to them all the time and you're turning on/off filaments? I've got a lot more parts to collect and a lot more reading to work all this out. But just wanted to say I've enjoyed this so far.

  18. W1VTP

    W1VTP Member

    Just talked to Timmy HLR yesterday about using a 4 by 1 in linear service and he is asserting that such a hookup will result in higher than normal IMD because the sg needs some positive bias and the the cg needs some RF along with the cathode excitation. Have you built yet and if so, have you done any testing?
  19. WA5VRL

    WA5VRL Member

    New to this site. Read through this whole discussion and saw a lot of very good information. Hope all the ongoing projects turn out well.
    The last 4-1000A I built was back in 1987 and it has been rock solid ever since. Used materials I accumulated over the years and made a
    few upgrades as I had time and funds available. Attached the schematic just for viewing pleasure. Take care and 73... WA5VRL

    Attached Files:

  20. W5HRO

    W5HRO Administrator

    I still have the 4-1000A tubes and the plate transformer, but whether I will ever actually build the linear is unknown. If I did I would only use one tube. I think somewhere back in this thread I had the idea of actually using RF chokes from the screen and control grids to ground instead of tying them directly to ground which could possible help. Using only one tube though there is no way in hell I would ever add positive bias to the screen in GG configuration the normal way. Wouldn't want to lose any power and it would doing so. If it had a bit of IMD then too bad.

    Welcome, I think I saw you schematic on the web somewhere back when. Do you have a webpage?

    I think the last idea I had was this which did add just a hair bit of controllable screen bias via a 4-65 to obtain exactly "0" control grid bias. Doing it like this might not reduce the output power.

  21. WA5VRL

    WA5VRL Member

    Thanks for the reply... No webpage. I have also built the other usual amps over the years including the 811s, 572b, 813s, etc.
    I usually built table top stuff and sold them at the local swap meets but the 4-1000 power supply is too heavy to transport.
    Will sell it out of the home QTH when I get ready to part with it... There is a recent image of it on my QRZ listing.
    The screen bias is something I have been thinking about. That is partly why I never bothered metering the screen yet.
    Thanks for the schematic. I may go that way next time I want to do another "upgrade". 73 - WA5VRL
  22. W5HRO

    W5HRO Administrator

    That circuit was just an idea and I'm trying to remember exactly what I was thinking at the time. It was almost two years ago before this thread was hijacked and went in the other old 2-tube direction.

    I think the idea of using the RF chokes from the grids to ground was to try and reduce their capacitance for the higher bands like to try and make it work on 10-meters. Then the 4-65A screen voltage control circuit was so you could adjust the control grid to "0" so it would help with that along with the power out.

    The problem with grounding both the control and screen grids is that they never really want to be at "0" at the same time so you get excessive current draw and usually from the control grid. There is a point to where you can adjust the screen along with the cathode bias to obtain "0" on the control grid which is what you really want. You would just look at the control grid’s current meter and adjust the screen for the null after you figure out the correct fixed cathode bias setting first. It should work and you would only need to adjust the screen control from that point on as you change bands. The only other thing is both the screen and control grids may also need a bypass cap to ground on the tube side of the RF chokes because those chokes could lead to oscillation. It would just take a bit of experimentation. It they did need caps there they would at least be in series with the grids which would reduce their capacitance. Tying both those grids directly to ground just throws a bunch of extra parallel "C" from the input/output to ground.

    Anyway, my idea had nothing to do with IMD, it was about trying to reduce the control and screen grid capacitance for the higher bands and to get the power out.