Sommerkamp FL 2100

Discussion in 'Amateur Radio Amplifiers' started by PD0BN, Feb 12, 2018.

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  1. PD0BN

    PD0BN Premium Subscriber QRZ Page

    Hi Hams,

    I've got a little problem with this amp. Let me explain:

    I've bought this unit from an older Ham. He received it from a friend and replaced the old 572B tubes with new ones. He noticed that R201 and R202 were heavily burned, so replaced them.

    Now in my shack. I’ve replaced almost all the C’s (except for the KV unit, power voltage is ok). Also rewound the choke, was burned at the 20m segment, tested the choke for resonant issues, all looks fine.

    Now the actual issue:

    The output with 30Watt input is around 300Watt. Sounds good he, but when inputting 100Watt the output increases a little bit, max 400Watt. Furthermore, the earlier mentioned R201 and R202 are smoking a little bit (getting black). The Bias voltage increases sometimes to 1KV. This is much, much, MUCH too high. Especially at 10m with 100W input, the Bias voltage jumps to the roof.

    Another thing when just applying one tube (the one attached to R202), the strange change happens that the resistor in the “parasitic suppression circuit” blows out with a big bang. I’ve swapped the tubes, same problem.

    Who can be of help with this problem. Hope to here from one of you fellow Hams around the globe.

    ’73 – Bor – PD0BN

    Attached Files:

  2. G3YRO

    G3YRO Ham Member QRZ Page

    Firstly, what make are these 572Bs you are fitting?

    The Russian ones are not at all like the original Cetron American 572Bs, so are not suitable in these Linear Amps - I suspect this is the cause of your issues.

    You need to get the Chinese-made 572Bs as these are exact copies of the PROPER 572B valves.

    Roger G3YRO
  3. W1BR

    W1BR Ham Member QRZ Page

    I've never heard of anyone having an issue with the original Svetlana 572B tubes that were mad in Russia, although they haven't been made for sometime.

    If those are grid resistors, I'd suspect there is an issue with one of the tubes.
  4. PD0BN

    PD0BN Premium Subscriber QRZ Page

    Hi Roger,

    They are from China,so I don't think this is the issue. Thx for your response.
    '73 - Bor - PD0BN
  5. PD0BN

    PD0BN Premium Subscriber QRZ Page

    Hi Peter,

    Thx for your reply and yes, it's all about both grid resistors. You mention an issue with one of the tubes, but as you could see in the attached picture both of the resistors are showing burn marks, and if used on 10m they really are smoking and the Bias is groing to the roof (+/- 1KVolt)

    73, Bor –PD0BN
  6. G3YRO

    G3YRO Ham Member QRZ Page

    If those resistors are burning up, then either you have a flashover inside the valve(s), or it is going unstable.

    Either way, it's most likely due to defective valves.

    Does this happen on all bands? And when fed into a Dummy Load?

    Also, if only one valve is actually working, the most you will get out is 400 watts, even with 100 watts of drive.

    You can simply unplug one of the 572B valves in these Amps and run it with one, to discover if one of them is faulty.

    In addition, there is no way that the bias voltage can go high (unless you have a short inside the Valve from Anode to Grid) . . . I suspect your meter is reading RF and giving you a false reading.

    Roger G3YRO
    Last edited: Feb 12, 2018
  7. PD0BN

    PD0BN Premium Subscriber QRZ Page

    Hi Again,

    It's all tested on a dummy (1KW load) and it's happening on all bands. I think you are right when saying they are going unstable. It's happening when the input power is above 30-50Watt. As mentioned in the first thread, when the input is around 30Watt, the amp gives around 300Watt output. I think that’s pretty good.

    Any suggestions?

    ’73, Bor – PD0BN
  8. W1BR

    W1BR Ham Member QRZ Page


    Both grids are tied together. Over voltage on one grid will backfeed to the second tube.

  9. K4LMP

    K4LMP Ham Member QRZ Page

    What type of resistors are you using in the pictures? Most times, I see carbon resistors, and I think you can't use just any old resistor, they have to be of a specific type. Just a suggestion, as I don't know much, but thought I would throw it in.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    Yes I'm presuming you are using Carbon resistors, both for the Grid resistors and for the Anode stoppers ?

    (if you used wirewould, that would explain the instability !)

    Roger G3YRO

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