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W8JI FL2100B bias and feedback mod

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

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  1. W1BR

    W1BR Ham Member QRZ Page

    The tubes are now produced in China. What came with the amp 20 years ago is not what is going to be used for replacement tubes today.

    Didn't folks go through this with Collins when RCA came out with the 6146B?

    Regardless, I'd rather be proactive than be surprised by an unexpected event down the road. It was a design fault, regardless if they got away with or not.
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Well all I can say is that I have found the Chinese 572Bs perfect copies of the Cetron valves that came with the amps when new.

    Roger G3YRO
  3. N8VIL

    N8VIL Premium Subscriber QRZ Page

    I performed a few modifications to my FL-2100B along with a rebuild. Was abused by someone in the past. I had to replace the RF choke and doorknob caps in the tube compartment along with the suppressor resistors when I got it (for free) to get it working. I also had to move the 10 meter tap on coil to original position as it had been moved for 11 meter. I had to rebuild the input tuning circuit as 20 and 40 meter band input tuning could not be adjusted. (replaced large rectangular caps with 1000 volt silver micas - 2 of them were bad) I did do the bias modification (voltage doubler) for good measure. I added a soft key board from Harbach but had to modify to work with the -36 volt bias and relay voltage (keying voltage now +1 volt). I added variable ALC and grid current metering. All this was probably un-necessary but I enjoyed doing them. I also replaced the tubes with NOS Cetron (gifted to me). I like the amplifier and works great.

    16 Rebuilt input circuit.JPG

    Input tuning rebuild along with grid resistors

    11  Adjustable ALC pot location.JPG

    Adjustable ALC pot location

    15 Grid current meter and switch under meter.JPG

    Grid current labeling. Switch under meter switches between HV and Ig

    10  Botttom Chassis mods.jpg

    Bottom chassis view of circuit changes with legend
    KD2ACO likes this.
  4. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    Roger, correct me if I am wrong, but Yaesu came out with the technical bulletin to modify the bias supply because of the tube sources. It wasn't a defect of design, but a lack of bias adjustment range that was being addressed.

    If the defect of design was addressed, then they would have eliminated the RF grounding of the grids and used true grounded grid design.

    The W8JI mod is a design change if you go to true grounded grid and eliminate the bias scheme Yaesu was using.
    Which is what I and some others do with the amps when we get them.

    Actually, it isn't really W8JIs mod, but the original design of the grounded grid amps using triodes.
    W8JI just pointed out the obvious solution to the poor design the amp manufacturers were using because Collins originated the adjustable grid bias design...

    If you want to change the bias of the tubes with grounded grids, you could go with cathode (filament) bias if it is necessary.

    By the way, I am using the bias scheme W8JI mentions because I have removed the bias off the grids and wired the grids directly to ground. By doing that, I have seen a reduction of input SWR fluctuation during tune up as well.

    You still need the bias power supply to run the lamps and relay. In my case, I added the voltage doubler circuit as I am using a 24 volt relay with 3 poles for the bias switching addition. The relay has a fast pick circuit as well, so the original bias resistor is also used to limit current thru the relay once it is pulled. You also need 24 volt lamps, or LEDs with the proper limit resistor for 24 volts.
  5. G3YRO

    G3YRO Ham Member QRZ Page

    I never found any need to ever adjust the bias resistor on any FL2000B, FL2100 or FL2100B . . . as long as you used PROPER 572Bs.

    I'm sure it would have been necessary if you tried using Russian 572Bs . . . but I don't even consider that they WERE 572Bs, as they had much smaller anodes, and very different gain etc !

    Using Cetron 572Bs, and then the excellent Chinese copies, I always found they were fine, and never needed any adjustment. (sure, the standing current would drop slightly once the Valves started losing emission . . . but then so would the maximum input current, so there was really no point in adjusting it)

    The original circuit may be simple . . . but I have always found it 100% reliable. (unlike the circuit used in the FL2100Z, which goes wrong all the time!)

    Roger G3YRO
    N8VIL likes this.
  6. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    The russian tubes were more like the low power cousins of the 572s, but have they updated the design recently?

    I believe you about not having to adjust the bias of the 572s, as they are zero bias tubes to begin with and the bias is built in.
    The external supply is unnecessary, but it gave the idea you were making a difference...
  7. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Last I heard, the Russian tube factories stopped making glass RF power tubes. That’s been left up to the Chinese now.

    When the hv is up where 572s sing, the idle current may be a bit high without a couple volts of negative bias.
    Last edited: Jan 5, 2019
    N8VIL likes this.
  8. N6UH

    N6UH Ham Member QRZ Page

    Do they do requests or just the regular set of "Great balls of fire" and "Burn baby burn" !:p:p
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    My favorite version with pyrotechnics!
    N6UH likes this.
  10. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    What is the HV that the 572s begin to sing at for you, and is that for original tubes or Chinese tubes?
    And is the idle current in spec with the original design, or is it too high for the original design as well?

    My biggest complaint about the 572 tubes is there are so many different ways of using them, and they all have different characteristics with the different configurations that it becomes difficult to talk about them in general terms. I have original Cetron tubes, but if they go bad, I will get a new pair and see what I have once it happens. If that means putting in a small bias supply for the cathode, then that is what I will do, but before I do that I will see what they do without it to be sure it is needed. As for the grid bias supply my amp originally had, it now runs the low voltage for lighting and relay use. The grids are tied directly to ground at the sockets and all the excess parts removed.

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