W8JI FL2100B bias and feedback mod

Discussion in 'Amateur Radio Amplifiers' started by NJ9P, Jan 10, 2018.

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  1. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    The amps I've seen with 572s run a pair of them at around 2250v and ~45ma idling current per tube. This seems to work well in my modded SB-200.

    It's a directly grounded design with 3 diodes on the filament transformer center tap to put the cathodes at about 2 volts idling. The gain was a bit over 13dB and I lowered that to ~11dB with a 15 ohm resistor feeding the cathodes to better match my exciter.

    This put the idle plate dissipation at about 100W per tube. It was 130W per tube without bias when I was using a jumper to figure out where I wanted it to idle. 65 watts drive lets it deliver over 700W PEP on SSB and it can sing like that without breaking a sweat.

    The original SB-200 gets idling bias from pulling relay (and a resistor across it) current through a 33 ohm resistor to drop a couple volts.

    A couple years ago, I gave my still working Cetrons to a JA who couldn't find good tubes for his FL2100b. A couple Taylor 572bs from RF parts went into mine and the idling current, gain and power output stayed the same. The new tubes were going strong when I put that amp on a shelf last year. I'll use it in the AM stack that is going together next summer.

    SB-200 back.jpg SB-200 bottom.jpg

    I haven't dealt with an FL2100 but the numbers in the manual look approximately the same as the Heathkit.

    I like Sean N8VIL's pictures. It looks really nice in there.
    N8VIL likes this.
  2. N8CMQ

    N8CMQ Ham Member QRZ Page

    You like pictures?
    FL2100 bottom.JPG FL2100 RFdeck.JPG FL2100 input.JPG

    This is the FL-2100 I am working on. I just need two caps for the RF deck, and I may have to replace the plate cap as it was burnt with arcing. Someone painted the plates with glyptal.
    Did you notice the new relay? It switches the in and out as well as the bias. It is 24 volts, so I need to mod the bias supply to a doubler configuration and change the LED resistors for the higher voltage too. Time will tell if I did it right!
    KD2ACO and N8VIL like this.
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Very nice!

    I've looked over the scheme a while ago but reading the manual again now, it's a bit odd how the amp is described in the theory of operation as a "zero bias, class B" system, when R4 drops a couple volts to set the idle current and it's a class AB output.

    24 volts 'ain't enough Captain' to cut the tube off all the way in standby reliably either... at least that's easy to take care of.

    Anyway, it looks industrial strength in there! A couple nips and tucks with a soldering iron and all will be well. :p
  4. N8CMQ

    N8CMQ Ham Member QRZ Page

    Thanks. The entire amp is a surprise for me, but the technology was what, the late 1970s or early 1980s when the design was being worked on? We have unlearned some design flaws since then.
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Another look at the T160L curves shows that I was wrong about the 24 volts not being enough to cut off the tubes.

    The red dot is approximately where the cutoff voltage sits as built.

    Doubling that (blue dot) does indeed put you into cutoff territory. It's a bit close, but that counts as probably OK!
    Cetron T160L_Page_2.jpg

    A quick look at the Svetlana "572" (which was made in several transconductance groups) shows that even the 160u version of this tube doesn't belong in an RF amplifier (and they didn't claim that it should). At RF amp high voltage, even the high mu one won't cut off in standby unless you put the grid waaaay down, and getting >2kv onto the plates would make me turn it on with a very long stick, if at all... I wouldn't try it. :eek:



    ouch... 30u
    Last edited: Jan 9, 2019
  6. N8CMQ

    N8CMQ Ham Member QRZ Page

    However, I only measured -18 volts on the grid when it was wired that way in the amp I have.
    But the amp I have also had many problems before I started working on it.
  7. N6UH

    N6UH Ham Member QRZ Page

    Ok. I'll bite...whats the green dot for?
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Idling current set (bias voltage ~-2v).

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