B&W 5100 transmit problem has me stumped

Discussion in '"Boat Anchor" & Classic Equipment' started by WW9L, Jun 30, 2019.

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  1. WW9L

    WW9L Ham Member QRZ Page

    Hello. I have a 5100 (not B) transmitter that I use almost daily for CW and AM phone. Ever since I've owned it I've had a nagging problem and it has me stumped. When you flip the transmit switch to transmit, either phone or CW, the power slowly rises to full output and plate current. Now this takes a few seconds to occur. Starts out about 60 or 70 watts and over about 10 seconds goes up to full 110 watts if loaded heavily. I have swapped the 6146 tubes and no change.

    Just looking for a suggestion on where to start. I am very experienced with working on old tube gear just getting too old to actually pick this thing up and carry it to the lab (90 + lbs). So I'm looking for some good suggestions before I call in a favor to help carry it.
  2. KE0ZU

    KE0ZU Ham Member QRZ Page

    Hi John,

    Is the behavior consistent on all bands and with either VFO or Crystals?
    WW9L likes this.
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The B&W 5100 and 5100B both use a series of 6AQ5 tubes as multipliers including the driver stage. Depending on the band, the number of these 6AQ5 stages increases. The 6AQ5 is marginal at r.f. frequencies and it may be that one, or more, of these multiplier stages is taking longer to reach full operating condition. I would definitely look at these stages.

    Also, dirty / corroded switch contacts could cause problems.

    In addition, any one of the 6BJ6 tubes (3 of them) could be weak. These are in the VFO, buffer, and crystal oscillator / VFO amplifier stages.

    Frankly, there are a number of possibilities.

    Have you replaced the electrolytic capacitors? Those do wear out and can cause similar problems. Also, have you replaced all of the paper capacitors in the unit? If not, those are also possible trouble sources.

    I never really liked the 5100 or 5100B. However, over the years, I have worked on a number of those transmitters and they do work fine when everything is operational. But, unless the troublesome parts have been replaced, any "boat anchor" equipment can start to have any number of troubles.

    Glen, K9STH
  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

  5. WW9L

    WW9L Ham Member QRZ Page

    I have not tried a crystal but yes all bands all modes. Hold the key down and watch plate current and power slowly go to max over about 5 to 10 seconds.

    Just a few minutes ago I also noted that the final needs to be neutralized. Max power out is not at the dip. So before I do anything I have to tend to that.
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Take a good look at the screen voltage on the 6146 tubes from when the transmit switch is thrown until the transmitter reaches full output.

    Are you using 6146B / 8298A or 6146W tubes in the transmitter? Most of the 5100 and 5100B transmitters do not like those tubes and it will be hard to neutralize the final without making some modifications. Those transmitters are MUCH happier with the original 6146, the 6146A / 8298, or 6293 tubes.

    If you haven't read it, my article on the 6146 family of tubes can be found at:

    https://img1.wsimg.com/blobby/go/11... 6146 Family of Tubes 2.pdf?ver=1561837899714

    By the way, Antique Electronic Supply ("tubes and more") has recently added the article to their technical article section:


    Glen, K9STH
    WW9L and W1TRY like this.
  7. WW9L

    WW9L Ham Member QRZ Page

    Using the original 6146 tubes. I know the 5100 doesn't like the newer B or W variants. I have a couple of spare 6146A that I will swap in and see what happens. Also have the modulator 6146's to try as well. Thanks for the suggestions on the articles. Good read! Further note, the 5100 does NOT have a neutralizing cap like the 5100B so I may have to improvise.
  8. W2WDX

    W2WDX Subscriber QRZ Page

    I would also take a look at the primary of the plate transformer. Sometimes people place varistors (like a CL-60 type) on them to eliminate the inrush current that causes a "kathunk" when you go to TX. This will cause a slower rise time in plate voltage. If that device is on there, (sometimes on the center tap of the secondary as well) it's not a bad thing, in fact it's a positive addition.
    WW9L likes this.
  9. K5UJ

    K5UJ Ham Member QRZ Page

    We have an AM forum right here on QRZ.com Several fellows with experience who would have some good suggestions.

    Agree with W2WDX on looking at the p.s.; I'd start out seeing what the h.v. supply to the final PA is doing under load. Put a VOM on the unloaded B+ and measure it. Transmit and see if the B+ sags down and comes back up or stays pretty stiff at specified voltage. Make sure it's the rig and not something else downstream changing value. put the rig into a different load, a DL and see if the same thing happens; always do as much simple stuff first as possible. Do the neutralizing. What's the AC mains to the rig status? Are you running it on a dinky variac? That can cause your v. to the rig to sag. As was already stated, lots of possibilities.
    WW9L likes this.
  10. WW9L

    WW9L Ham Member QRZ Page

    You might be onto something. I seem to remember the 5100B I had years ago would "thunk" on transmit. This one doesn't. It's scary silent when you throw the transmit switch.

    FYI I think you mean Thermistor which is usually an NTC device used as current inrush limiter.
    Last edited: Jul 2, 2019

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