W8JI FL2100B bias and feedback mod

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

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  1. K1ZJH

    K1ZJH 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 Ham Member 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.

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