Tube suggestions for 811H

Discussion in 'Amateur Radio Amplifiers' started by AC2MM, Dec 30, 2018.

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    I don't think the Ameritron Al-811(h) amps have the same cut off bias system as the Yaesu amps which were the amps that seemed to have a problem with low cut off bias voltage. What is the cut off bias voltage on the Al-811(h) amps anyhow? There was a mod to increase the bias above self cut off if necessary.
    Last edited: Jan 29, 2019
  2. N8FVJ

    N8FVJ Ham Member QRZ Page

    BTW- The Svetlana 572B are available at RF Parts for $89 each. I would buy Chinese 572B vs the Svetlana 572B. I seen Chinese 572B for sale at $55 each in a lot of four tubes. And, NOS Cetron 572B if found are superior tubes vs the Svetlana at about the same price.
    Last edited: Jan 29, 2019
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    It's been a while! You're right, Lou.

    The 811H has the schematic drawn incorrectly at relay 1B. If the schematic was drawn correctly it would show the cathode goes open when it's not keyed, so the cathodes will just go to where they need to be. That voltage will be pretty high with a low mu tube.

    I haven't tried low mu tubes in an AL-811 but I suspect the idling current would be way hi with a low mu tube.
  4. W1QJ

    W1QJ Ham Member QRZ Page

    W8JI claimed that the AL-811 did have some problems with some tubes and he made a mod using the voltage across one of the filter caps to form a voltage divider to produce 40vdc to force cut off on some pesky tubes. As far as low vs high mu tubes, I think you may have it backwards. A low mu tube will draw lower idle current than a high mu tube. High mu tubes may require some operating bias to lower the idle current whereas a low mu tube might run at zero bias and draw less plate voltage at idle. This is why if you use 4-400's in an SB-220 instead of 3-500's, you would short out the zener diode.
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Hi Lou,

    I'm sorry I wasn't clear that I was thinking about WD4IGX's post. These guys would just blow the fuse in an RF amp (hopefully!) :eek:
    Click for big pic.
  6. WD4IGX

    WD4IGX Premium Subscriber QRZ Page

    Cetrons are clearly superior, yes, but similar price NO. They were at one time but no longer. The only place I can find NOS Cetrons right now is RF Parts and they are $199!

    Chinese it is. I confess I'm a little hesitant, though, to order glass tubes direct from China. Shipping and return issues mainly. Return might not be such a concern as what I'm reading above seems to indicate they've upgraded their QC and they work pretty well, but that's a long way to ship items made of glass. Probably worth the risk though.
  7. WD4IGX

    WD4IGX Premium Subscriber QRZ Page

    I just did some more testing (all tests into dummy load.) And BTW I checked the circuit breaker and it's a 20 amp, not 15.

    With all other major loads connected to that circuit off, I still saw some variation from key down to key down, but less than last night. Results did not vary significantly:

    Ig - 132ma
    Vp - 1400v
    Ip - 575ma

    DC Input - 805 watts
    Output - 500W (Per "Mighty Fine Junk" wattmeter built into tuner - tuner in bypass to feed dummy load of course.)
    (62% efficiency)

    Ig - 128-138ma
    Vp - 1380v
    Ip - 610-620ma
    DC Input: 842 - 856W
    Output - 500W (virtually no change in output at best tuning, regardless of variation on tube readings from amp's meters.)
    (59% efficiency)

    Both of these were with exciter drive set to 80W. Grid currents were still obviously well below the Ameritron speced 200ma so out of curiosity a very quickly and briefly tuned the amp for max output after setting exciter drive to 100W. I only made a very quick glance at Ig to make sure I didn't exceed the 200ma (it didn't) and output power. I didn't take time or key up again to look at the plate readings, but I was able to get only the tiniest amount more power anyway, perhaps 510W.

    So it looks like my tubes will do the level at which I should be running the amp at (or the level I should not actually exceed in normal repeated use, anyway) with maximum specified drive, whereas most people seem to say they REDUCE drive to run it at 500W.

    Looks like my 811As are plenty serviceable but perhaps past their prime, run hard and put away hot in the past as it were. :D I can live with that. I'll get four Chinese 572Bs and keep them aside, or swap them out when/if I get some time and take a notion, but continue to run these in the meantime.
  8. W1QJ

    W1QJ Ham Member QRZ Page

    I've serviced many AL-811H amps and I can easily obtain 700+ watts of key down power. Of course this is with good tubes. You should be able to get the same if your tubes are good but it could be the 120v you are using. I always use 240v on the test bench. I have done testing using a dedicated 120v line and a dedicated 240v line and the 240v line always beats the 120v line every time with every amplifier no matter how small especially noticeable on SSB voice peaks. ONLY a peak digital hold watt meter like my LP-100a will catch this. You would never see this with an analog meter.
  9. KR2C

    KR2C Platinum Subscriber Platinum Subscriber QRZ Page

    I have Svetlana 572B in my 811H. They work great. As I understand it, these tubes are great for the audiophile crowd. Rumor has it that these will fail quickly in an amp that is designed for 572B. The 811H being undersized for a set of 572B tends to make a good match for the Svetlana 572B. I've been running them hard for about 5 years.
  10. N8FVJ

    N8FVJ Ham Member QRZ Page

    Yes, lower voltage on Svetlana 572B works well. The tube did not work for some amps that operated the 572B at 2500 volts due to arcing. The plate spacing and size is a little less than Chinese or Cetron 572B. But, the Svetlana is rated at 160 watt plate dissipation. Much more robust vs an 811A. The Russians always made a great long life direct filament too just like vintage USA tubes. It is a thoriated carbonized filament.
    Last edited: Jan 30, 2019

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