Chicago driver xfmr BD-1 pinout?

Discussion in 'Technical' started by K4TQF, Sep 13, 2016.

  1. K4TQF

    K4TQF Member

    Ran across a Chicago Transformer BD-1 driver transformer. Gates used this in their BC-250GY AM broadcast transmitter. Pair of 6L6 driving a pair of 810.
    A google search shows catalogs, etc... but I can't seem to find a pinout anywhere.

    Thanks, Durff
  2. K4TQF

    K4TQF Member

    A quick check with ohmmeter shows Pins 1&3, reading of 238 ohms with pin 2 reading approximately 120 ohms to either 1 or 3. So, from this I assume the primary is 1 & 3, with pin 2 as CT. Pins 4&5 read 40 ohms as do pins 5&6, with no connection between the pairs. From this, I assume 4-5 & 6-7 to be the secondaries. I do know this is supposed to be a 3.5:1 step down transformer.
    Gates loads the secondaries with 2K each side.
    So, loading the assumed secondaries with 2.2K each, results in an impedance reading across the primary to be 4K side to side or 8K P to P as read by my impedance meter. ( @1000HZ )

    See, that wasn't hard was it ?
  3. W5HRO

    W5HRO Administrator

    Easy... below is T9 in the Gates


    That's the same transformer used in the Collins KW-1 and the Desk KW plus in my 4-400 HB transmitter.

    The important thing is to be sure you don't have the dual output windings connected to the 810 grids in the same phase by mistake otherwise they will cancel each other out. Inject a signal into pins 3 & 1 and make sure the phase is inverted at pins 7 & 4. Pins 6 & 5 should be shorted together and fed with the negative bias voltage. The thing is sometimes those transformers were mislabeled and if you have those secondary output windings connected wrong, like if pin 4 is connected to pin 6 by mistake the output will get completely nulled out. For example, sometimes pin 5 could really be pin 4 if they swaped them internally. That did happen from time to time back then. It's always best to test it with a signal first using two scope channels. Even using the 110Vac coming from your wall outlet will work for the test signal.
  4. K4TQF

    K4TQF Member

    This is what I have installed in the "Norton" preamp to use as a driver for the pair of 250TH. I'll be using some sort of common "beam power" output tubes on the primary. The Norton originally used EL-34/6CA7, but I prefer 6L6-5881or 6550.

    Now, to find a decent sized reactor for use with my RCA mod xfmr...
  5. K4TQF

    K4TQF Member

    That was a typo in my earlier post. It is just as your diagram shows. I am reading the impedance with it loaded as shown in the Gates BC-250 schematic I have hanging on the wall.
  6. W5HRO

    W5HRO Administrator

    I have no idea what a "Norton" preamp is, but the transformer should work connected to the 250TH grids. Just short pins 5 & 6 together and feed the negative bias voltage there after you make sure the phasing is all correct like I mentioned.

    As for the input side, just as long as that "Norton" has around a 3K to 6K output. If not then that transformer is not going to work.
  7. W5HRO

    W5HRO Administrator

    Mike, one thing to also mention is that transformer was designed for only around 250Vdc to 300Vdc max on the primary and was setup to produce about 15 watts of drive power. It was designed for 2A3's /6A3's/6B4G's and would work with 6L6's, but using 6AC7's or 6550's may take a higher voltage to get the plate to plate load up to 3K ohms and above. Those tubes can yield less than 3K if you run them under 400Vdc so be careful. You may get a bad match and high current with only 300V and below.

    The main thing is the ratio you need to the 250TH's and it’s possible the transformer primary can handle 400Vdc, but it wasn't designed for it. It might be safer to never exceed about 350Vdc max on it under any circumstances.

    P.S. If you ever come across a BD-2 let me know because I would buy it. I always wanted one for quad 2A3's, etc.
  8. W5HRO

    W5HRO Administrator

    Mike, I had some time this morning and I dug up the original schematic on the Gates using that driver transformer. They were running +330V DC on the primary side and used two 6K swamping resistors from each end to the center-tap to better match the 6L6's and to protect the transformer from excessive power. That transformer was designed for 2A3's that would produce 15W max in push-pull with fixed bias. 2A3's with fixed bias for 15W were 3K plate-to-plate. With cathode bias for 10W they were 5K plate-to-plate. At lot of people would use 6L6's trying to get more power and I had always thought about doing that myself, but in reality 6L6's make shitty driver tubes. There are issues using them and the transformer may burn up if the power is too high.

    Is there any way you have a pair of 2A3's or 6B4G's laying around? Low Mu triodes like those always rule for audio drivers anyway and will produce the best overall sound. I have a few pairs of 6A3's for mine and they were a short-term 6V filament version of the 2A3 until they came out with the 6B4G in the octal socket. They didn't make the 6A3's for very long. They only issue today is that the audiophools have jacked up the price up on those tubes to astronomical prices so they are no longer cheap anymore. The main problem you are going to have though is that with either 1500V or 2000V on the 250TH's you need around 50W of drive power. Well, that's only a 15W transformer. At 1500V you can run "0" grid bias by grounding pins 5 & 6 of the transformer, but you still need almost 50W of drive.

    I think you are going to need to find a different driver transformer if you are set on using the 250TH's unless you are only going to modulate a single RF amp tube. There is no way you are going to get full power out of the 250TH's to fully 100% modulate 1KW RF finals. If you are only going to use one RF amp tube though at half the power, then you could probably get away with under driving the 250TH's. I know those old driver transformers are really hard to find, but if you were planning on modulating the two 4-400A's in the other amp you will be lucky to get 50% modulation using that driver transformer. If you try to modulate the single 4-400A amp instead, then it may be ok. The Chicago BD-2 transformer was for quad 2A3’s to produce 30W and is closer to what you really need.

  9. K4TQF

    K4TQF Member

    Your Gates schematic has a date of 9/5/63. Correct ? My schematic for the Gates BC-250GY has a date of 6/27/50 and differs slightly, but the facts remain. Mine does not have the swamping resistors and draws B+ from the main HV supply thru a 8K 160W resistor. The bias setup is the same, as far as I can tell.

    I think I sold all of my 2A3, I may have some 6B4s left. I'll need to dig thru my boxes & tube caddies. I would like to use the 250th and I am only using a single 4-400. I pulled the other one out to make room for the T-3 tank coil and also to allow for the xtra plate current consumed by the modulators. I have some new 211 & may have a pair of 810s. I have a couple dozen NOS, NIB, GE VT-4 / 211s back in Arkansas.

    When Tim came by he mentioned that he wouldn't run over 1500V thru that RCA mod transformer I have. I didn't have a chance to ask him why he stated that. It was originally rated for 8000-11000V. He probably assumed that it was old, used and beat to hell. I didn't mention that it was NOS, NIB. I plan to put a load on it and let it sit here to run warm and dry out really good before I put any voltage on it. Maybe I can get a hi-pot test run on it before that transformer guy near me goes out of business.

    The Norton originally had a pair of EL-34s driving eight 845s in class A PPP. it used a Kenyon T-301 driver xfmr and a Kenyon T-496 300W output xfmr. Both of which, I still have. He also dropped off a UTC PVM-5.
  10. W5HRO

    W5HRO Administrator

    Yep, they probably had to fix the design. The 6L6's were probably closer to 7K to 8K plate-to-plate so they swamped it down a bit with the 6K resistors, etc.

    Here's the very first one from 1948 and they started out with swamping resistors on the output side which was stupid. That was a mistake, so they removed them. Does your 1950 drawing have the ones on the output side of T9? Looks like they were trying to fix the problem then finally realized they needed the swamping on the input side instead. This is a good example of why 6L6's made shitty audio drivers.

    That 8000-11000V may have been AC ratings and not DC ratings which was probably the case. At the same time even though it's NOS the fact remains it is very old. The varnish between all of the layers is probably dried up just due to it's age alone. 2000VDC would probably be ok though.

    It’s just like when someone finds an old antique piece of equipment that is still new in the box and was never used. They think just because it was in the box and never used that all of the caps are still new too until they start to use it. After a while the electrolytics start crapping out due to the fact they dried out just due to age alone.
  11. K4TQF

    K4TQF Member

    Yup, 2K each side. Without the swamping resistors on the input, 14K impedance P-P, or 7K each side, according to my un-calibrated impedance meter @ 1000HZ.
  12. W5HRO

    W5HRO Administrator

    When you take those measurements you need to ground pins 2, 5 and 6. to the ground of your meter if possible. Then you will see the real P-P for each side from pins 1 to 3 and then from pins 4 to 7.

    A pair of 6L6's in push-pull at only 250V with fixed or cathode bias is 5K P-P. They are just like 2A3's with cathode bias. The problem is when you run the voltage up on 6L6's that P-P goes up to like 7K to 8K P-P. Certain brands can even be higher like 8.5K P-P. What probably happened is Gates ran the 6L6's at +330V to get a bit more voltage swing and drive power to the 810's. They may not have used 2A3's because they wanted more than 15W so they went with the 6L6's instead, but it was probably a mistake. Since it was only a 250W transmitter they just should have used 2A3's at 300V instead. That BD-1 transformer would have worked perfect with 2A3's to the 810's without the need for any swamping resistors or problems.
  13. K4TQF

    K4TQF Member

    Yeah... quite a difference. 3K P to P and 2.5K across the full secondary with two 6.8K swamping resistors, one across each primary winding to CT.
  14. K4TQF

    K4TQF Member

    That "Norton " audio output deck has 8 of the jumbo 4 pin sockets on it. I could stick a pair of 211s in there as a another stage between the preamp chassis and the 250TH outputs. I have the original Norton power supply and the original Kenyon T-496 output transformer.
    The 300 watt T-496 is one of those multi-tap transformers with about a gillion different impedance configurations listed in the spec sheet. the 211s take very little input drive and would drive the pair of 250TH. 60amp filament xfmr is already on the chassis as is the T-496.

    Norton front:


    Under the preamp chassis:

    Norton output chassis: partial of the original supply below...

    Top of preamp chassis:
  15. W5HRO

    W5HRO Administrator

    By the way, the reason for the dual windings on the secondary of that BD-1 transformer was so you could adjust each of the final modulator tube inputs (bias) if everything was out of balance, but in most cases it’s not necessary unless you have two very mismatched tubes. You can just short pins 5 & 6 together without adding the adjustment resistors and feed the negative grid bias voltage there. It was done for the same reason 2A3's needed separate adjustable filament supplies. Now, with 250TH's you might possibly need the adjustment resistors, but I would try it first without them. They were really only needed with the early directly heated cathode tubes and its possible 250TH's might fall into that category. 810's also have directly heated cathodes, but they are not like the 250TH's.

    Also, do you have any 803’s ? I’m asking because that would be a much better choice for the final modulator tubes. Unlike 810’s they don’t require very much drive so that would eliminate that issue and then the BD-1 and the RCA mod transformer would work perfect together. 803’s would be a great choice because you can run those up to 2000V in a modulator and just short the screens to the plates and I think they are still available and cheap if you ask around. I bet one or two of the AM guys out there where you are probably have a pair laying around they would sell you or trade.
  16. K4TQF

    K4TQF Member

    I'll look around at NEARFEST...
  17. W5HRO

    W5HRO Administrator

    I know your old RCA transformer is good up to 10K plate to plate on the primary side so you would run the 803's at 2000V instead of 2500V and that would work fine.

    I also take back what I said about tying the screens to the plates. You tie the screens to the control grids and then tie the suppressors back to pin 2 of the BD-1 transformer which is the driver plate supply line center-tap connection.

    The configuration below will actually work great and would be the workaround that would do the trick :icon_thumbup: Then you could go back and use the 250TH's for the RF amp in push-pull at 2000V as well unless you are dead set on using the 4-400A. If so then you would need to use a different modulation transformer other than the 1:1 RCA anyway.

  18. K4TQF

    K4TQF Member

    The 4-400 is in the old plasma rig which also contains the 3KV power supply. The 250TH were to be the modulators driven by the Norton preamp... output thru the RCA xfmr .

    Maybe this thread is better moved to the thread "another fine mess..." as I have two projects going.

    The "4-400 ex plasma RF unit with the "Norton" audio preamp / modulator & T-3 VFO:
    FullSizeRender (1).jpg

    and the PL-175/4-400 amplifier:
  19. W5HRO

    W5HRO Administrator

    Then the issue you will have is the BD-1 will not handel the power you need to drive the 250TH's.

    So what else do you have in the way of a a driver transformer that will handel 50W?
  20. K4TQF

    K4TQF Member

    Thus, my idea to employ the original output transformer ( Kenyon T-496 ) in between a pair of 211 & the pair of 250TH.

    In that config, the modulator would look something like this:
    ( insert beam power tube of choice) > BD-1, > pair of 211 > Kenyon T-496 > 250TH> RCA mod iron.

    The Kenyon T-496 has a gillion taps, configurable for almost anything needed and is rated for 300 Watts.

    The Jumbo 4 pin sockets are in place, and will accommodate the pair of 211, & the pair 250TH. Also, the Kenyon T-496 & the 60A filament xfmr are on that same chassis. The low voltage and HV supplies for the Norton amp are below all this in the same rack.

    Attached Files:

  21. W5HRO

    W5HRO Administrator

    Ok, I understand what you are wanting to do, but 211's will produce 200W and overdrive the 250TH's. The 250TH's only want 50W of drive and that's it.

    It sound like you are just wanting to use what you already have (what's left of those old built-up racks) and try to connect all of them together and make it work, but I seems a bit cumbersome. You would have drivers driving drivers and you would need to attenuate the power way donw coming from the 211's unless you reduced their plate voltage. They make not work like that.

    From the pics any everything it looks like you have enough tubes, transformers and racks to get started, but you may need to just build up one audio rack using the parts from all of them to simplify everything. Keep thinking about it I guess, but don’t rule out finding a pair of 803’s and using the 250TH’s in push-pull in the RF deck instead like I mentioned. You could do that or maybe even something else that would be much easier.
  22. K4TQF

    K4TQF Member

    So, it really comes down to the impedance match between the ( insert beam power tubes of choice here ) to the 250TH. I could use the Kenyon T-496 due to the wide range of impedance configurations ( 620 )... even if 300W power rating is overkill, .

    I don't think I would need to run everything balls out... full tilt boogie. I was thinking more along the line of utilizing what I have in a conservative manner.
  23. K4TQF

    K4TQF Member

  24. W5HRO

    W5HRO Administrator

    Maybe 6550's like you wanted to get the 50W of drive to the 250TH's using that Kenyon transformer.

    The only concern is will the 50W and the low 6550 voltage be too low for that transformer and will the power transfer across it to the 250TH's as a result?
  25. W5HRO

    W5HRO Administrator

    By the way, I've have actually been tempted to install the 803's back in my HB transmitter in place of the 810's. I originally started with the 803's because I had my heart set on using them, but my driver transformer (BD-1 equivalent, but for 5K P-P) was mislabeled. That's how I know about those transformers. I had it all buttoned down on the rack with the wires cable-tied and everything and had connected the transformer per the numbers per the spec drawing. Well, when I fired everything up I only had about 5% modulation and after a few days assumed the 803’s I had (and still have) were just bad. Since everything was buttoned down I only took my volt meter on AC and clipped the black probe to ground and then touched the red probe to each 803’s grid and with audio applied I saw the drive voltage on each side and went “Ok I have drive”. What I didn’t catch at first was that both of the grids were in the same phase because the transformer pins had been mislabeled so it was not swinging through and was being canceled out. I should have known better.

    Well, to make a long story short I bought a pair of 810’s thinking the 803’s were bad, changed the sockets, and I still only had 5% modulation and went WTF? Needless to say that’s when I pulled the scope out and realized what was wrong. From that point on I have always scoped out those audio transformers and all audio transformers to make sure the phasing is correct. I’m running 6A3 drivers and with cathode bias so the drive requirement needed is really more using the 810’s. The 803’s would have been perfect with only 10-watts of drive coming from the 6A3's. At the same time though I’m only using one 4-400 in the RF deck so if I switched back to the 803’s I’d probably have even more modulation like 200% for sure which would really be too much. Right now it’s about 180% with the 810’s limited down to only 150% and with the negative peak limiter. When I started the project it was back when a lot of us were trying to figure out ways of obtaining 200% modulation and were experimenting

    If I could go back though and do it all over again I would still use the 803’s. Maybe one of these days I will switch and put them back in.