W8JI FL2100B bias and feedback mod

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

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  1. NJ9P

    NJ9P Ham Member QRZ Page

    Looking for opinions from those who have made these mods to their FL2100B.

    While the mods make perfect sense, from an engineering standpoint, I know how reality sometimes works.

    So, any opinions either way?
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Well I used to work for Yaesu when these Amps were a current model, and also had one of these amps for 40 years (sold it earlier this year).

    Firstly, most people DON'T have a problem with the original Bias circuit, as long as they don't use the Russian Svetlana 572B valves. These don't even look the same as the original Cetron valves, have smaller anodes, and much higher gain. I would never use them, and have advised others the same.

    However, the Chinese 572Bs now made are perfect copies of the originals.

    The original Bias circuit used in the FL2000B, FL2100 and FL2100B, although very simple, is extremely reliable (unlike the later circuit used in the FL2100Z, which often fails)

    There is also no problem with it cutting off the 572Bs, if you use the correct type.

    As W8JI points out, there are several Amps using this design (based on the Collins amp), but most of them DON'T have any neutralisation capacitor.

    I personally never had any problems with instability with these Amps . . . but I suppose it depends on how you use them. I rarely used them on 10m, which is where you MIGHT get a problem.

    A lot of people criticise these Amps (I think because they are Japanese) . . . but in my opinion, comparing them with many other Amps I have worked on, I think they are extremely well-made, and very easy to work on. The mains transformer in particular is very rugged, and the filter capacitors Yaesu used in those days are also extremely high quality - I never had to replace mine.

    I notice W8JI also talks about the possibility of destroying the Valves if you had full continuous power for 60 seconds . . . well of course ! I would never key down at full power for more than 2 seconds at a time. That's the way we were taught to tune up SSB valve PAs or Linear Amps. Do that, and your Valves will last for 20 years.

    Do you have any problems at present? Do you use 10m a lot?

    Roger G3YRO
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Can you post a picture of your tubes?
    Last year, a JA amateur was having trouble with his FL2100B after he hurt his original tubes and put a brand in that was problematic.
    I smuggled some genuine T-160Ls in my carry on over to him and that solved his problems.

    As Roger posted, the new Taylors seem fine.

    Here's the thread:


    IMO it’s easier to fit the correct tubes and use the amp.
    Last edited: Jan 10, 2018
  4. G3YRO

    G3YRO Ham Member QRZ Page

    I bought my 572Bs direct from China (for a fraction of the price), and they work fine too.

    Roger G3YRO
  5. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    I was getting some oscillation on 10m when in standby. I did the transformer center tap mod (with relay) and took care of the problem.

    I replaced the clunky original relay with a pair of Omrons wired in parallel (16A total current capacity). One set of N/C contacts opens the center tap on the transformer when in standby.

    Never had a problem in well over a year since the mod was done.

    Max VE3TMT
  6. NJ9P

    NJ9P Ham Member QRZ Page

    It still has the original Cetrons in it, I'll know if I need new tubes once I hunt down an affordable dummy load (or break out a pile of 50 ohm sand resistors and a bucket of mineral oil...)
    KD2ACO likes this.
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    If an amp oscillates in standby, with no load, it has a stability issue.

    Biasing to cutoff does really address what caused the amplifier to take off.

    Parasitic? Neutralization?
  8. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    From the W8JI site:

    In the FL2100, rumors are some tubes oscillate because they have higher mu or gain. This is actually the opposite of the truth. The FL2100 will oscillate, on standby, with any 572B tube if the bias is reduced enough to allow conduction with the antenna and exciter disconnected. The reason some tubes oscillate in the FL2100 while others do not, while on standby, is some tubes have slightly lower mu and draw current at idle while in standby. If bias is reduced on any brand tube, the FL2100 series will break into oscillation. The HF instability has little to nothing to do with gain, the instability is rooted in the lack of neutralization in early FL2100's, and a terrible neutralization system in the later FL2100Z.

    The center tap mod completely eliminates this problem.
  9. W1BR

    W1BR Premium Subscriber QRZ Page

    They hosed the neutralization.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    Yes but the point is that most people don't get any instability with these Amps !

    As I mentioned before, there are LOTS of different Amps using this design, without any neutralisation, as it wasn't found to be necessary.

    Yaesu produced this design for 12 years and sold thousands of them (FL2000B, FL2100 and FL2100B, all the same inside) . . . do you not think they would have changed the circuitry if there was an issue? It's only when people started using the Russian Valves (which are nothing like 572Bs) that people had problems.

    So in my opinion there seems little point in doing any mods unless you actually have a problem.

    Roger G3YRO
    Last edited: Jan 11, 2018
    KD2ACO likes this.

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