Amplifier bypass options?

Discussion in 'Amateur Radio Amplifiers' started by K3HMG, May 1, 2021.

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  1. K3HMG

    K3HMG Ham Member QRZ Page

    Good day all,

    I have another rookie question or a couple maybe ;). As you’re reading this some of you most certainly already know that I’m fairly new to the hobby and I am trying to learn as much as I possibly can. I have recently acquired a Heathkit SB 200 amp. That I want to put online in my station. @WB2WIK and many other have been absolutely wonderful offering their time and years of experience in helping me get off and running. I am forever grateful to all who have helped me so far and a future thx to all who will lend their advice in the future. On to my current question(s).

    when it comes to the SB 200 I am going to be using a footpedal that will not only key my mic and radio but will also key the amp. I’m using a Heil FS-2 foot switch. My automatic tuner of course is after the amplifier. So if I depress the foot pedal it will key up the amp and radio.

    1. If I want to bypass the amp let’s say for allowing the tuner to do its job with out the amp in line or simply if I don’t want or need the power from the amp what are my options?

    2. Would placing a simple switch in line on the amp keyer line from the foot pedal work? Or will the amp only pass a TX rf signal when the amp is keyed?

    3. Or is something like the MFJ-1705H RF bypass switch needed to accomplish this? If so and the switch is in bypass and I depress the foot switch the amp will still key up. Does that hurt the amplifier to be keyed with no TX rf going into it?

    Again thx all for helping!

  2. N0NC

    N0NC Premium Subscriber QRZ Page

    Since the SB-200 as designed does not have a standby/operate switch, the normal way would be turn the amp off if you're not going to use it. However you could put a switch in the key line between the foot switch and amp to open the key line when the amp is on, but you don't want to use it(tuning for instance).

    73, Chuck
    WA4SIX and K3HMG like this.
  3. K3HMG

    K3HMG Ham Member QRZ Page

    If I installed the switch you mean not to have it in the on position while tuning? Keep it in the off position until I want the power from amp?

    the easiest solution may be to turn off the amp while tuning for the band like you said. Then once tuned tuned turn the amp on and tune the amp.

    In either case, if I understand correctly the amp will pass a TX rf signal wether it is on/off or keyed/not keyed is that correct? If so either option seems to be fairly simple. May lean toward the switch on the foot pedal line that way I can keep the amplifier on and warmed up.
  4. N0NC

    N0NC Premium Subscriber QRZ Page

    Depends on how often you want to use the amp. I use my AL-80B mostly on 75 meters for checking into the state traffic net. Otherwise they can't hear me (I use a ground mounted vertical which doesn't do well on short distance skip). Mostly I work CW, and 100 watts seems to be all I need most of the time.

    73, Chuck
    K3HMG likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's what Heathkit intended. There is no "warm up time" for an SB-200; it's ready to use about one second after you switch its power on.

    Adding a "standby" switch in line with the keying cable to the amp works fine, too. But for many years thousands of hams just used the on/off switch to use or not use the amp. Of course it passes signals through it just fine when switched off. The only thing likely to "wear out" by using the power on/off switch is the switch itself, which can wear out but can also be replaced.
    K3HMG likes this.
  6. K3HMG

    K3HMG Ham Member QRZ Page

    sounds good understood. If the original engineers designed it that way then no need to changed it. Thank you
  7. N8CBX

    N8CBX Ham Member QRZ Page need the bypass switch installed. I installed one in each SB200 of mine. Always enjoyed it.
    Jan N8CBX
  8. K3HMG

    K3HMG Ham Member QRZ Page

    where did you source it from so I can take a look at it?
  9. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    I would add the standby switch, since the fewer times the filaments are switched--think inrush current--the longer the tubes will last.

    Jim . . . . . . . . .EARTH: LOVE IT OR LEAVE IT

    K3HMG likes this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That is intuitive for me, too.

    But the SB-200 and SB-220 were sold by the thousands to people who never added a standby switch and the tubes lasted decades with a lot of on/off switching.

    I recall Rich Measures AG6K actually measured the inrush current (since he did it and published the data a long time ago I never bothered to try) to find it was so limited by transformer saturation it wouldn't be deemed destructive. I had the opportunity to visit with Rich a few times (he lived about 30 miles from me) and although he had quite a test lab at home, he had access to a way bigger one at the Navy base where he worked as an instrumentation engineer or something and had lab notebooks full of so many notes and so much measured data on these amps I could hardly believe it.:p

    I think since adding the switch costs about nothing and is really easy to do, it's a worthwhile mod and might make the power on/off switch last longer.:) But probably not really necessary.
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