BC-1G,T Final Grid Circuit

Discussion in 'Amplitude Modulation' started by AC0OB, May 16, 2021.

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  1. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Sometime around 1965 Gates changed over to 807s in the BC-1T. I believe the reason was that 6BG6Gs were an early sweep tube that was used less and less in TVs, and the price was going up. Gates provided a retrofit circuit board with 807s, with detailed step-by-step instructions for the installation. The characteristics of the tubes are very similar (both 6000 micromhos transconductance, 0.9A filament, etc). I remember, because I was working as CE of a local daytimer that used a 1T, and did the conversion over night so that they were ready to go by next morning's sign-on.


    The 6BG6G is essentially a 6L6G with a plate cap on top. So is the 807, but (except for the very latest production before the tube was discontinued) it has extra shielding to make it more suitable for RF amplifiers along with a 5-pin base, but even so the 807 is still squirrelly.

    The performance of the two tube types in that transmitter was identical as far as I was able to tell.
  2. W3SLK

    W3SLK Ham Member QRZ Page

    K4KYV said:
    Don, I don't have anything that uses 6BG6's, (that I know of). That tube doesn't really click in my memory bank. However, I thought it was prudent for Gates to continue with the 807 since they were made by the thousands during WWII. Hell, if you look closely today you still may find NOS 807's from that era. I stumbled upon a few 'stubby' 807's that I have only as novelty tubes. However, I am tempted to unleash them to the 'audio-phools' where I'm sure they will command top dollar!
  3. K5UJ

    K5UJ Ham Member QRZ Page

    Nice work on the grid input network Don. Never liked the 807s mounted horizontally. I'm guessing the cylinders Gates wrapped around them are to further shield the leads to the electrodes to make them less apt to take off. I know they can be mounted horizontally but the combination of these shields and the pc board makes me think of long term heat stress.
    I'm not so sure about that. I know there are Williamson amp designs from the early '50s using 807s, but I've never encountered any homebrew or commercial audiophool tube amps using them. My guess is that nowadays consumers like for the tubes to be out for display, and they're spooked by exposed h.v., even with porcelain plate grips. Having said that, I have a lot of 807s in case they start getting used anyway. What you really have to watch on the 807 is the screen v. RCA limits it to 300 v. If you get up around 350 you are getting too close to the sun. The plates should not show color. A single 4D32 or pair 6146s would be better as an IPA.
    W8KHK and W3SLK like this.
  4. WZ5Q

    WZ5Q Ham Member QRZ Page

    I never really liked them at all. Saying that, about all my BTA-1M uses other then the 833A's and rectifier tubes is 807's. In the RF Circuit, they use a 807 for the oscillator, then 2ea 807's in parallel to drive the 2ea 833A's which are also in parallel. In order to make up for the lack of drive to the 833A's, RCA decided to Screen Modulate the 2ea 807's from a tap on the secondary winding of the mod transformer.

    LOL, too close to the sun, that's funny.

    I feel that an IPA needs LOTS of headroom and reserve. 813's would work great, Raytheon used them as push pull drivers in their RA1000 for the push pull 833's.

    I'm a Triode freak though, so in my big homebrew, I will use push pull 35TG's as an IPA to drive the push pull 3-1000H RF Finals. I should get about 300 watts of output capability with about 10 - 15 watts of drive. Not only do these triodes look cool, but their tantalum plates color various shades of red under load and they use that beautiful iridescent green depleted uranium glass for the tube seals. It's a Twofer of gorgeousness!
    K5UJ, W3SLK and (deleted member) like this.
  5. W3SLK

    W3SLK Ham Member QRZ Page

    K5UJ said:
    Rob, look up some of those amps that are being built. Someone built a single 833 and is running it in class A. They wanted about $4K for it! I saw one using an 813. Crazy what they will do. And lets not get Don started about someone's 'tesatura' in their audio! ;)
    AD5HR likes this.
  6. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Those black 807 shields were a standard off-the-shelf component made by Millen Company. I still have several n.o.s. ones unused in the box. The little Millen exciter used a single 807 with one of those shields. According to RCA specifications, when used as an RF amplifier the external shield should come right up to the bottom of the plate structure, or else the tube is sub-mounted with the base below the chassis and the plate structure above. RCA says the tubes are mountable in "any position".

    I never had a problem with the horizontal mounting in the BC1-T. It makes the underside of the circuit more accessible. The main problem is that a pair of 807s is marginal at best for driving a pair of 833As class-C plate modulated. The earlier BC1- series transmitters (E and F models) used a single 813 as driver. I didn't like the phenolic circuit board, either; vacuum tubes and printed circuit boards is simply a bad combination. I re-built my RF driver section onto a piece of sheet aluminium to replace the PC board, hoping they would be less squirrelly, but they performed exactly the same, no better, no worse, but at least carbon resistors were no longer burning holes in phenolic. I finally got rid of the squirrelliness when I changed the IPA plate/PA grid tank circuit to what is shown in the diagram above.

    The little 'stubby' version of the 807 is called the 807W. It also has a 4-digit type number, 5933. They are supposed to be electrically identical to a regular 807, but built "to stand up under rough service". The 807W/5933 may not be physically interchangeable with a regular 807 in some cases, since the base has a larger diameter.
    Last edited: May 24, 2021
    K5UJ and (deleted member) like this.
  7. W8KHK

    W8KHK Ham Member QRZ Page

    Rob, I am GUILTY AS CHARGED. But hopefully I will not be considered an audiophool. In March, I spent the better part of a week building an audio amplifier (not quite a Williamson) with four of those 12-volt 807s (1625s) in push-pull parallel.

    My goal was to create a decent speech amplifier for my 810 modulators in the P-P 250-TH rig. I had a bucket of those bottles, and a dozen or so circuit design ideas that I wanted to try, including using cathode followers instead of a driver transformer. The amplifier does 60 watts with excellent frequency response and negligible distortion, even without the two local or single global feedback loops (see the schematic). I was so pleased with the performance that it is now installed as the center channel amplifier for my home theatre system, and I will be building two more stereo versions for the front and rear channels, using 4D32s and 6146s, respectively; I already have the requisite iron. I will continue using the customized UTC MLF as a speech amplifier.

    I really do not understand why 807s and 6146s get such a bad rap for audio, but are considered A-OK for class B linear amplification of sideband signals???

    I did not mean to hijack your thread, but since you made the statement, I thought you might appreciate the "encounter". Now back to the thread topic SweetMusic.jpg ........

    Attached Files:

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  8. AD5HR

    AD5HR Ham Member QRZ Page

    Some of the audiophiles are running 450TLs in class A.
    Check out the Zed page of K5LRX.
    The European audiophiles have discovered the old WWII
    radar pulse tubes also, and drive prices crazy.
    W8KHK likes this.
  9. W8KHK

    W8KHK Ham Member QRZ Page

    AD5HR likes this.
  10. WZ5Q

    WZ5Q Ham Member QRZ Page

    Ohhh my, how beautiful. I'm a Murky Vapor Tube junkie. I use them whenever I can and have boxes of them ranging from 866, 8008, 872A, 575A, etc.
    W8KHK and (deleted member) like this.

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