Hammarlund HXL-One Low output (2x 572b)

Discussion in 'Amateur Radio Amplifiers' started by SA5VOX, Sep 10, 2018.

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  1. SA5VOX

    SA5VOX Ham Member QRZ Page

    I recently got my hands on a real nice amp, a Hammarlund HXL-One from a SK sellout.
    (Its layout is somewhat a basically simplified version of a Heathkit SB200.)
    The problem was that it was in a terrible state when I got it. The big fat bleeder was broken, the HV diodes gone in smoke, a cable to the neutralization varicap had almost welded it self to a nearby edge and the psu caps was long gone dry and some of the resistors over the caps was way of. Since the former owner never mentioned what had happened and is not around anymore I had to suspect a lightning strike thru the mains where there also was a black welding spot on the casing near the incoming chokes.

    All in all I had some fixing to do.
    Ok, a lot of hours later and all faults fixed (I hope) I fired it up and lo and behold it worked without fires! :)
    But the output was not very good, at all.
    Here is where my question for help comes in.
    With 1900 volts it topped 400w with 100w input at 80m and then dropped to ~175w out at best on 10m. This with a Heathkit 1kW Cantenna attached to it. (No Bird meter to measure with unfortunately, just two regular cross needle meters, one for max 1kW and another for max 3kW.
    Booth shows reliable at 200w as far I know.)

    I have measured every component, cleaned all contacts and switches and checked the gear that goes in or out to this one but no luck.
    Made a neutralization that pushed about 10w more out of it but that was it. Soft tubes? Check!
    Bought a used but tested and good pair of Cetron and did the whole soft start / burn in procedure.
    Now I get 400w 80m thru 10m even though 10m really makes the tubes go red in a jiffy at 10 and 15m.

    Why can't I get the 572b's to max out on 6-700w with 100w in? They has been tested in another amp before and gave 700w out.
    As of what I have read for the Hammarlunds cousins a pair of 572b's should at least do 500w out, that with about 65~85w in, I did 100w.

    Noticed: The plate voltage drops from 1900 volts to 1600volts at load. I guess this might be one thing? (Though I have read about those that get's out 600w at that.)
    At 15 and 10m it is easy to run over the 550mA max grid current specified but not on 80 to 20m.

    My future plan is to run this amp on 4-450w out when needed, otherwise I like go "barefoot" but I want this amp to run well and not have to brute force it cherry red to get those watts out of it. :)

    Any ideas or usage tips are welcome!
    Last edited: Sep 10, 2018
  2. W1QJ

    W1QJ Ham Member QRZ Page

    I had one of those amps years ago. I don't recall if it has a tune input network but my gut says no. That said, how is your input swr? If you don't have a good input match you will be loosing a lot of drive. The HV is also very low for those tubes. That voltage is more for 811 tubes. The SB-200 runs 2400v and the Dentrons run almost 2700 volts so it is hard to compare the output to those amps. It has been so many years I ago I had mine. One day the filter caps blew up on me one at a time BOOM BOOM BOOM
    KI4AX, KD2ACO and SA5VOX like this.
  3. SA5VOX

    SA5VOX Ham Member QRZ Page

    OUPS! Yepp, one of the filter caps did blow, I almost counted on it but scared the wits out of me anyway! :D
    Your absolutely right there, I run a manual tuner on the input to match the impedance so always below 1.5:1, mostly 1:1.
    The manual state that the voltage should be 1900 so that seems to match up, but I wish I could get a little bit higher, perhaps that's why you have to kick it with 100w?
    Manual also says that it should give you 1kW Input at 100W which I take as it would give about at least 500~600w out? (I do have some newbie learning to do with those figures of "Input vs Output". :p But I know that ~400w gives you a S1 and next step is 1200w to get a S2, no need with that much, just want to raise above the noise those really bad days. ;) )
  4. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    That high voltage sounds a bit low, especially at full load. I read the posts again but could not tell if you replaced all of the filter caps. If not, I'd try that.
  5. SA5VOX

    SA5VOX Ham Member QRZ Page

    Oh, yes, right, my bad, should perhaps list the main components that I swapped out for new ones.
    I got new filter caps of known brand to bee better safe than sorry.
    All resistors over each filter cap was replaced as well.
    All the diodes in the HV section was replaced, even the still working one (was only one that was still intact).
    The high voltage cap from the plates to the output was replaced though it matched it's original value but it had a black hole in it from some kind of arcing.
    Both caps on the isolation choke to the filaments as well even though value and optical said they where ok.
    The big 75k 100W bleeder resistor was shattered on the outside and was a open circuit. Replaced. Hard to find! Was lucky to find one on eBay. Was about to make my own with multiple small 9w 5k6.
    Got a new cap and diode to the antenna relay.
    Cleaned all the rotating switches with a 800 fine granule paper at those worst parts and simple white paper on those which was nearly ok.
    Resoldered al joints that looked suspicious.
    Measured all other components that could in one or another way lead to short circuit, loss or any kind of interference to the amp side.

    I use a Kenwood TS440s, a manual tuner with built in Watt/SWR crossmeter, a Heathkit 1kW Cantenna and yet another Watt/SWR crossmeter 1kW for the output. (Even have a 3kW to change with if necessary.) I use RG58 between all contacts, considering the short distances and short time I key the amp there should not make so much difference, or?

    I tune up the amp very polite since the input SWR on the amp tends to change a little before the input power reaches 10W and it all gets tuned in, after that it is usually rock solid.
    Idle current for the 572b's is about 190mA. At full 100w input and tuned out max onto the load ~400w the grid current is 500~550mA for 20 thru 80m. But as mentioned 10 thru 15m it tends to go of up to 700mA if your not careful.

    I can't seem to find anything weird about the amp otherwise. Yes, the plate voltage is a bit low but still I've read others that has been using the HXL-One to be able to get more out of it.

    I will try to adjust the neutralization to the tubes again to see if I missed something. I didn't turn the cap very much but perhaps that is the wrong approach? (Using a long plexiglas rod for this!)
    Last edited: Sep 10, 2018
  6. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    That sounds like a lot of work, and all in the right direction! :p

    I'm thinking the 500ma+ must be plate current.

    I don't have experience with this amp, so I'll leave the smart comments up to others, but at 1600 volts key down under load, I'm thinking that 400 watts is about what you could expect out of it.

    Hopefully, someone with this amp can tell you what HT voltage to expect.

    all the best
  7. W4WTS

    W4WTS Premium Subscriber QRZ Page

    I have one that I use every day. I think my high voltage is 2300.....

    Are you checking your power out using a dummy load?
  8. KI4AX

    KI4AX Ham Member QRZ Page

    You may find some of the answers at this link:

    I do not have a drawing or manual to look at. Assuming it is similar to the SB-200, as you have suggested...

    I agree with others that the HV voltage is low. SB-200 HV runs about 2400 VDC.

    In order to get 500 - 600 watts out of a pair of 572B, in a Grounded Grid application, you will need about 2400 VDC on the Plates and about 500 - 550 mA of Plate Current.

    Just out of curiosity does the HXL-One have Parasitic Suppressors on the Anodes of the 572B's? And, are they made with 3 or 4 turns in the coils?

    Dan KI4AX
  9. SA5VOX

    SA5VOX Ham Member QRZ Page

    Thank you Dan,

    Yes, the HXL-One have Parasitic Suppressors on the anodes, but just one for both 572b's, there is however a MK2 version that has separate for each tube.

    Well yes, the HXL-One is very much alike the SB-200 but lacks the input tuned filters (it seems to be added at the MK2 version though that I have found some schematic to, maybe a future mod?).
    A link to the original MK1 version schematic:

    It seems that the dipping HV is the bad guy in this so question is... There is 2 x 3 4700uf/450V with a 470k resistor over each capacitor, are the capacitors ESR level to be in any specific range?

    Is it possible to add another two diodes, capacitors and resistors to raise the HV a notch or is that going to wreck the transformer in any way?
  10. SA5VOX

    SA5VOX Ham Member QRZ Page

    2300?! Oh, strange, the manual says 1900v but perhaps that is under load?
    Or do your HXL-One been modified with more diodes, caps and resistors?

    As many has pointed this out I'm now convinced. ;)

    Yes, I'm using a Heath Cantenna 1kW dummy load, wouldn't dare running it straight on a antenna. :confused:
    Last edited: Sep 17, 2018

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