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Yaesu FT-101ZD PA Mod for 6883B/8552 Tubes

Discussion in 'Radio Circuits, Repair & Performance' started by KM4ALK, Dec 2, 2018.

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  1. KM4ALK

    KM4ALK Ham Member QRZ Page

    Hello all. This is my first post here on QRZ. I've been a licensed ham for a few years now, but not had a chance to do much with it. I have a background in RF and radios, but am very green in the Amateur Radio world. I have an FT-101ZD MkII that I have been bringing back to life/cleaning up. When I got it, it had 6146A tubes in it instead of the correct 6146B tubes. They looked like they had been quite hot even though they would tune up just fine, and would not achieve full output so I decided to replace them. 6146B's can be had for anywhere from $45-65 each from what I can find. I am on a finite budget so I did some research and found that the 6883B/8552 tube is identical except for the heater grid and the price. 6883Bs and Motorola 8552's are readily available and much cheaper. The 6146B has a 6 volt heater and the 6883B is 12 volt. I dug out the schematic and discovered that the FT-101 heater circuit is 12 volts, and the 6146B's heaters are in series. I bought a pair of RCA 6883B's from a seller on fleabay for the tidy sum of $18 shipped and they appear to be brand new. Today I pulled the PA board out and removed the blue jumper from between pin 2 on V1701 and pin 7 on V1702. I added a new jumper from pin 2 on V1701 to pin 2 on V1702, and another from pin 7 on V1701 to pin 7 on V1702. Now the tube heaters are in parallel. I re-installed the re-connected PA board and seated the new 6883B's in their sockets. Each sees 12 volts and appears to be happy. With the set put back together I tuned it up on 29MHz into a dummy load and checked the neutralization. All seems well so far. I switched bands to 80M and tuned it up again. Everything seems happy and it was putting 100W into the dummy (could have gone higher but I felt the point was proven). Just waiting to try it out on the air. I don't currently have an HF antenna up but hope to change that within the next few weeks.

    Just wanted to post this up in case it might help someone else. I'll post an update as soon as I can get it hooked to a real antenna. This may be common knowledge to many but I couldn't find any examples of this being done online.

    73's

    Eric KM4ALK
     
    W1TRY and AI3V like this.
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Well done !

    By the way, if ever you replace the Driver or PA valves in these rigs, it's normally worth going through and adjusting all the RF stage trimmers . . .

    Roger G3YRO
     
    W1TRY likes this.
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The 6883B / 8032A / 8552 are all the same tube and many are cross-branded with 2, or even all 3, part numbers.

    The "Motorola" branded tubes were made by RCA and just marked as "Motorola". Those tubes were installed in the new, primarily MOTRAC, units when shipped from the factory and for sale, as replacement parts, by the Motorola C&E parts department. During the almost 10-years that I owned the Motorola reconditioned equipment center for the south-central United States, I literally saw tens of thousands of those tubes!

    The 6146A / 8298 tube works fine in the equipment designed for the Japanese 2001A, 6146B / 8298A equivalent, tubes. However, that tube is rated at 85-watts input for SSB and 90-watts input for CW versus the 120-watts input for the 2001A for both modes. As such, for SSB the power needs to be kept at, or below, 70% of the output power for SSB and at, or below, 75% for CW. Otherwise, the tube ratings are going to be exceeded.

    The rewiring of the heaters, of the final amplifier stage, is also possible with the Heath SB-100, SB-101, SB-102, SB-110, SB-110A, HW-100, and HW-101. I do this fairly often because of the differences in prices among the tubes. However, I definitely recommend putting a label, on the final amplifier cage, stating that the heater connections have been modified to use the 12.6-volt tubes. Otherwise, it is possible that, at a later date, you could forget that change and install the original 6.3-volt type tubes. Or, the transceiver can end up in the hands of another operator who would then replace the 12.6-volt tubes with 6.3-volt tubes. I put such a label on all units in which I make this modification.

    Glen, K9STH
     
  4. KM4ALK

    KM4ALK Ham Member QRZ Page

    Roger,

    I have been slowly going through this radio with the service manual in hand and checking things out. Appreciate the recommendation and will make that next on my "to do" list.

    Break-- Glen,

    I had figured that may be the case with the 6146A's, but wasn't sure even after reading spec sheets. Glad to hear good gouge from someone with firsthand knowledge. This is my first foray into the world of valve driven/controlled radios. I was planning on putting a label on the outer shell, but agree labeling the amp cage would be wise. Stickers can get peeled from the outside.

    Now that you mention it, I should probably also place a label mentioning the frequency counter replacement I installed (sourced from Mr. Teruhiko Hayashi JA2SVZ).

    Thank you both for the feedback and recommendations!

    Eric KM4ALK
     
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    KM4ALK likes this.
  6. KM4ALK

    KM4ALK Ham Member QRZ Page

    Very nice! Saving a copy for future reference. Thanks!
     

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