AL-811H tube swap to 572Bs

Discussion in 'Amateur Radio Amplifiers' started by KW4GT, May 12, 2019.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    Oh, I did not read the four tube AL-811H has adjustable neutralization plate. Best to leave it alone unless the amp shows instability. I believe some older amps such as the Collins 30L-1 did not have neutralization- bad news.
  2. KW4GT

    KW4GT Ham Member QRZ Page

    Apparently the 572Bs are 'close enough'. I'm good with leaving it alone.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Bend to adjust. :rolleyes:
  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    BTW- HRO (Ham Radio Outlet) shows to have Svetlana 572B tubes still in stock at $89 each. If so I would rather own these vs Chinese. New old stock needs to have filaments lighted for 2 hours before HV applied. Filaments and plate have a getter material applied to maintain vacuum and is activated with heat. After HV is applied, try to heat tube plate with constant carrier to heat up the plate to finish the re-evacuation. 10 minutes should be enough time.
  5. KQ0J

    KQ0J XML Subscriber QRZ Page

    I just replaced my 4 newer 811A tubes in my AL-811H with DX Engineering 572B's 2 weeks ago. Main purpose was to get better
    reliability on FT8, FT4 and RTTY ( 200 - 250 W ) levels. Clean replacement , no tuning issues - all stayed the same. I noticed higher power output at the same input levels - so they must have better gain. For example 20W CW in went from 250W out to 325W out on 20M, 20 W in from 150 out to 225W out on 30M. Better max power out on all bands - especially on 15, 12, 10 M.

    Keeping the 811A's as spares, hope I never need them as I rarely push this thing to max ratings - just when working DX. I had melted the plates on a set of 811A's both in my 811 and my 811H running barely 200W in a RTTY contest setting.
  6. N8VIL

    N8VIL Ham Member QRZ Page

    Be careful where you purchase tubes from. The tubes I purchased were all defective. 2 had very low output and 1 had a filament that barely lit. It was a major hassle getting a refund and communication was terrible. The seller had me "return" them to another purchaser but I had to follow PayPal's instructions to get a refund.
  7. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Easy to do with that amp and tube combination. Shedding heat is an issue to keep in mind and the modern 811A's plate structure doesn't have much meat. They can heat up very quickly.

    Tuning for better efficiency in carrier modes like FM and RTTY will help a bit. The Load control in this case will look like it's quite far to the left of normal CW or SSB operation. When doing trials to establish safe operating areas for different modes, I like to run into a dummy and really keep an eye on the plate color which for 811A's shouldn't go over the dullest red (the kind you need a darkened room to see color with)... about 150 watts I reckon from the couple of AL-811's I've had on the bench for repairs (a small sample, I know). You may be able to run 200w with careful tuning, but doing trials and finding out how much heat can be handled is necessary with that amp.

    The AL-811 is inexpensive and a bit fragile, but I think they are fine. Like any other tool, they need to be run within their capabilities or things go wrong quickly.

    Using 572's really helps this because that tube has more meat on the plate and higher average dissipation ability... Yum!
    Last edited: Jul 26, 2019
  8. N8FVJ

    N8FVJ Ham Member QRZ Page

    My AL-811 has three NOS Cetron 572B tubes. It turns the fragile amp into a solid performer. 65 watts drives produces 800 watts PEP on 75 meters. I know an AL-80B is superior, but the AL-811 with 572B tubes is good enough.
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I don't own one of these amps and haven't had to live with one for any length of time, so I don't know... Have you run carrier modes? If so, what kind of average power can that frame safely sustain with 572s loaded?
  10. N8FVJ

    N8FVJ Ham Member QRZ Page

    AM carrier? At 50% duty cycle I would not run more than 200 watts carrier with 572B tubes due to 800 watts PEP maximum output. FM would be about 400 watts carrier IMO with 572B tubes & 50% duty cycle.

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