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Ebay AL-811H, What Have I done?

Discussion in 'Amplifiers' started by 338_MtRushmore, Dec 17, 2018.

  1. 543_Dallas

    543_Dallas Sr. Member

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    Some transceivers do have problems with overshoot. They don't want to deal with a lot of tube failures so they give instructions so that maximum safety to the equipment is insured as BlowinSmoke said.

    Hams are no different than the rest of general population. Some are very smart and some aren't. You have to write manuals for the ones that need it the most.



    Tuning at max exciter power is not always the best way because you're beating the tubes to death for no good reason. Some HF rigs produce 200 watts, what do those guys do? You have to know your equipment and use good judgement.
     
    NightThumper likes this.

  2. 338_MtRushmore

    338_MtRushmore Sr. Member

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    What method do you use to verify your radios have no overshoot? I would like to check my TS-940S, because I suspect it has significant overshoot. My meter usually shoots to the moon and I have to wait for it to decay before I get the readings I expect. Tom claims the ameritron meter I have is very sensitive to short overshoots, but maybe he designed it and is a little biased?

    What would you tell someone that wanted to drive a 4 pill with 150 watt radio? You would say it really isn't a good idea due to the power output of the radio and the drive requirements of the amplifier. This issue is also addressed by Tom:

    NOTE: This text assumes your exciter does not have greatly excessive drive power level compared to drive power requirements of your amplifier. If your exciter has significantly more power output than your amplifier requires, you really should add an attenuator between the exciter and the amplifier input. Using power controls in most radios to reduce drive more than 50-70% for amplifiers is generally a bad idea. This is because many exciters (radios) have ALC-overshoot issues. The ALC or power overshoot problem worsens as output power is reduced below maximum.
     
  3. 543_Dallas

    543_Dallas Sr. Member

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    I would use a peak hold watt meter or an oscilloscope. The 2 awm-35 watt meters I had were very sensitive. The first one was ok but got broken when I rearranged the shack. The one I bought to replace it was a later revision with the led back light and I was not happy with it.

    Driving a 4 pill with a 150 watt radio? It can work but if you have to ask for advice you probably shouldn't do it.

    Sometimes you need an attenuator but it is a rare when you're using a commercially built ham radio and amplifier. W8JI and Ameritron are covering their butts so a customer can't hold them responsible for flat tubes or amp damage.

    If your ts-940s has issues with overshoots you should eliminate that problem or load the amplifier heavily enough that it won't damage the tubes. If your overshoots are hitting 80 watts you would need to tune up at 80 watts. If 80 watts is beyond the safe drive level for tune up we have to go back to good judgment and advance the load control just enough.
     
  4. BlowinSmoke

    BlowinSmoke Member

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    Unfortunately I'd rather not discuss "pill" amplifiers. But to answer your question about transceiver overshoot, there is really only one way to see it and that is with a device that can respond to the instantaneous "spike" of power above the carrier level selected. You would need a scope or a digital watt meter that holds the highest peak in an envelope. You can't rely on a analog meter due to the damping or lack of the meter has. A digital watt meter with a hold feature will show it very easily. One of the better ones that comes to mind is the Post LP-100a or any in that line. Alpha makes one and so does Array Solutions. They are digital read out meters that will display the highest peak power that it sees. Use one of them into a dummy load, set your radio RF power level to various levels BELOW full power, use a carrier mode like CW or FM, set the RF power control and key up the radio several times in a row. Look at the meter, it will display the power of any spike it sees. You won't be able to get any accurate information from an analog meter. Many amplifiers especially those running tetrodes, and sensitive triodes like 3CX800 and 8877's which require very low drive to obtain full power, will most likely have overdrive or grid overload protection. The amplifier will go into stand by with any overload beyond the threshold of the circuit. Other tubes like 3-500, 3CX1200's and the like usually don't have overload protection because their grids are very hardy and can withstand momentary overloads with no problem.
     
  5. 338_MtRushmore

    338_MtRushmore Sr. Member

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    I am actually glad I pushed this so hard. Now that i know why it can be a problem I can make knowledgeable operating decisions.

    I looked at the specs of a few of the digital meters mentioned and I couldn't find any sort of response time. I will have to do some more research to see if they can actually detect short peaks in the 200-300 ┬ÁS range. Tom believes oscilloscopes are not fast enough for an accurate reading. Could be bs, I don't know yet.

    I am just trying to understand the possible dangers before I ignore the best operating practice. Once I understand the dangers, and how to avoid them, I can safely make my own decisions as you guys do.
     
  6. BlowinSmoke

    BlowinSmoke Member

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    The spikes are so fast that they really can't do much damage to a thoriated tungsten filament tube.s It is basically a non-issue. And if you are talking about spikes that are 2 to 3 times the full power of the transmitter you certainly will not compensate for that with any loading changes. Don't get to anal about this because there is only so much you can do and then it is out of your hands. Believe me, you don't have to re-invent the wheel, many of us have been there done that and can save you a lot of agonizing. In fact that is what we have been doing here already. Once again, these spikes only become problematic in a case where an amplifier overload circuit constantly trips and you just about can't use it because it constantly trips. And like I said any tube with a TT filament will not care. 811's, 572b's, 3-500's, 4-1000's etc. Don't agonize!
     
    338_MtRushmore likes this.
  7. 338_MtRushmore

    338_MtRushmore Sr. Member

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    I was under the impression that it was the high voltage in the tank circuit the was hard on the components and the amplifier in general.

    I also assumed that the risk of being undercoupled during undetected peaks would be greater than the slight efficiency gain by tuning at a reduced exciter output.

    Further research shows that alc overshoot is actually very common. Aparently the IC-7300 is so bad it trips the protection circuit in some modern amps. AD5X published a very interesting article "Amplifier Overshoot-Drive protection". Definitely with the read if one is interested in the subject.

    Will an amplifier last many years taking undercoupled overshoots? Probably. Would an operator be worse off for understanding the dangers? I highly doubt it.

    I feel better knowing I have a rudimentary understanding of the subject now.
     
    Slowmover likes this.
  8. drgrant

    drgrant Member

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    When Adam AB4OJ and this guy here
    https://blog.febo.com/?p=321
    did overshoot tests on the 7300 they didn't find any, but I suspect at some point when the radio was initially released that there were some issues.... either that or there are issues with their methodology for testing. It would not shock me if there have been several hardware and firmware revs on it. I just looked on Icom's site, and the 7300 has been updated like 6 times, and the radio has been out for several years now, so there's a good possibility of some secret squirrel hardware revs / ECOs in the mix,
    too.....
     
  9. 338_MtRushmore

    338_MtRushmore Sr. Member

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    The mention of the 7300 was an example of it being a fairly common issue, even with modern rigs. I searched for alc overshoot and was surprised how many rigs do have this problem. My TS-940S appears to overshoot 20 watts or so.

    I wasn't my intent to trash the 7300.
     
  10. drgrant

    drgrant Member

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    Oh I definitely realize that I just think it's worth noting that a lot of times they end up fixing these things....
     

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