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Dentron 2500B grid current?

Discussion in 'Amateur Radio Amplifiers' started by K7GLD, Dec 12, 2010.

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  1. W8JI

    W8JI Ham Member QRZ Page

    1.) I answered your question specifically, and quoted a link to the Eimac data sheet.

    2.) I was a consulting engineer who looked at Dentron stuff for a friend at a bank (Toledo Trust) when they were going into default because of warranty expenses and other problems, and I've seen dozens of 2500's.

    3.) I worked with Heath on the 8873 amp, which was a similar tube. That amp had chronic tube failures because of excessive grid current by users, the number 1 warranty failure was from running too much grid current. The grid in that tube is identical to the 8875.

    Sometimes when people don't like the message and want a different answer, they shoot the messenger. The fact is the tank values are screwed up, the amp really desperately needs a grid trip circuit, the amp needs a HV fault resistor, and the amp needs a tuned input. I would never run an 8875 family tube at more than 50 mA Ig per tube on CW, or more than about 60-70 mA per tube under any tuning condition.

    It cost Heath tens of thousands of dollars in warranty expense because they didn't include a $5 grid trip circuit in the SB230.

    Gold plated grids erode over time from elevated grid currents, and the gold redeposits on the ceramic insulation and on the cathode. The damage is cumulative over time, a little here...a little there.

    That's just a fact of life.

    73 Tom
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Well, that's certainly true. The meters can't respond to voice peaks and don't move much on SSB.;)

    My 3K could output 1500W PEP on a real PEP meter with the meters barely moving. If I ran compression, they'd move a bit more, but still not very much.

    The 3CX1200D7 is a great tube, and it will survive without damage if the B+ fails and you keep driving it: It has a lot of grid dissipation capability.:p

    Unlike an 8877, which is a great tube but won't survive such abuse.

    I'm not wild about all the protective circuitry in some of the Alphas, although they do make the amps rather idiot-proof. I try not to be an idiot if I can help it, and they drove me nuts. SWR >2, the amp trips to standby. That's a bit conservative. I don't have any Alphas anymore, although they do make good products I don't prefer so much "protection." I've loaded the 3K into SWR >4:1 and with proper LOAD control adjustment, grid current remains well within ratings and the amp survives just fine without arcing anything.:)

    The only damage my 3K ever had was from the earthquake, when it literally fell over on its side. That must have made a loud noise, but we were too busy getting the hell out of the house to notice.

    The power supply sustained no damage at all. In fact, through dozens of contests, including ones that were multiop where the operators didn't know anything about amplifiers, it never failed in any way. The 5500V oil filled filter caps were very conservatively rated for a 4200V supply and they never failed in any way, nor did the 10kV PIV 5A avalanche HV rectifier assemblies.

    I miss Henry making amps. I'm sure they never will again, but they had a good run.
  3. K7GLD

    K7GLD Ham Member QRZ Page

    You are correct, your info related to the recommended 2500B grid current was there and helpful - just got sorta muddied with the other only peripheral info - the tube specs (for instance, and I'd already seen and was fimalier with them) are only generally helpful, since what Eimac specifies for the 8875 is NOT particularly typical in the way Dentron had them running in the 2500B - and THAT was the specific and precise info I DID ask for, want and need - yes, you did provide that, and thanks.

    All in all, it seems my own grid current readings fall into the same general area as other 2500B users are seeing, so I'll rest easy, and let it go at that.


  4. AI3V

    AI3V Ham Member QRZ Page

    You cannot use a analog meter to give you anything but a GUESS as to what your voice may or may not be doing.

    Depending on the average to peak caricteristics of YOUR voice, the analog meter may not even move when the tube is being overdriven severly on peaks.

    If you want to do a single or multi tone test, then you can compare the oscilloscope readings with your meter readings.

    To simply speak and expect the meter to give you any accurate info...:rolleyes:

    My experience is maintaining 40KW transmitters used by the NAVY, Those baby tubes you have we used as the pre-pre-driver tubes.:D

  5. K7GLD

    K7GLD Ham Member QRZ Page

    Well, yes - and believe it or not, I HAVE managed to pick up that tidbit of info in my 50+ years as an active Amateur Radio operator, which was sorta why I specifically asked for grid curreent readings typical in both TUNE, and SSB operation...:D:p
    Last edited: Dec 15, 2010
  6. KM1H

    KM1H Ham Member QRZ Page

    I have a pair of MLA 2500's here. One was used as the 2nd station amp, until a Alpha 76PA 3 holer came my way as payment for other amp work (it was part of a Caribbean Multi-Multu contest amp in rather sorry internal condition), and is now the HF station back up.The other I converted to 6M and is the backup amp for that band. There is even a DTR-2000L riding a shelf. I sort of liked the challenge of turning Dentrons into rather decent and reliable amps; it certainly didnt require much money or time.

    At no time do I remember requiring more than 30-40ma total grid current to achieve the sane and safe 1200W out running 70-80W drive. The one on HF has the original tubes and both have all necessary protection circuitry added.

    A major problem with the 2500 were users who clipped out the 50 Ohm input resistor thinking that going from 1200 to 1500W really made a difference except to quickly fry the grids of a 300W Pd tube.

  7. KD7MSC

    KD7MSC Ham Member QRZ Page

    More power from the MLA 2500 :D

  8. AI3V

    AI3V Ham Member QRZ Page

    Then you understand why the meter is a poor thing to use to measure (over)drive on voice.

    It will ONLY give a accurate reading with a steady signal.

    You allready have a power output metering problem, Don't compound it with trying to use a average reading grid current meter to measure peak grid current.

    Unless of course you don't like the guys talking up and down the band while you are on.

    Read the care and feeding of power grid tubes.:)

  9. K7GLD

    K7GLD Ham Member QRZ Page

    Why do you steadily insist on making a simple question so complex? :confused::rolleyes:

    My question FROM THE START has been related ENTIRELY to obtain the MAXIMUM observed Dentron 2500B tuneup and SSB grid current readings from other 2500B USERS, so I can compare them with my own in order to determine whether MY grid circuit readings are reasonably typical, or not!

    That is ALL I was asking, not about what regular operating techniques I should use, or whether a meter is a better measuring device than something else. NOT what a manufacturers spec sheet sez, NOT what a "similar-but-different" tube in an entirely DIFFERENT amp displays, not what is expected to display on a scope, not about bandwidth, distortion products or splatter.

    NOR am I particularly interested in "recommended", or "suggested" peak readings from well-meaning guys who are NOT 2500B owners - and have never actually operated or seen one...

    SO, WHY are we having this tedious, largely off-topic and pointless "debate" on issues I was NOT asking or concerned about? You trying to somehow prove you're smarter than me? HEY I'll make it easy for you - you're smarter than me - even if you ARE apparently unable to focus upon and comprehend the quite simple QUESTION I have steadily asked:

    "WHAT are the peak Dentron 2500B grid current readings other users are seeing on tuneup, and in normal SSB operation?"

    Now, Rege, I've carefully re-read that question to be sure it's in English, and not some foreign language - so what's so hard to comprehend? :rolleyes:;)

    Now, since a few here HAVE comprehended and accurately responded to that question, I'm done here - and will leave you free to to pick some other target to impress with your broad knowledge and understanding...:p
  10. W0BTU

    W0BTU Ham Member QRZ Page

    My friend, I'd like to say something to you, if it's OK. Nothing against you. There's something very profound happening here that I don't think you appreciate.

    You have received the best information here, THE solution, by some of the most highly qualified people on planet earth, to your inquiry. You will never read anything even remotely as good as this thread in ANY book or ANY source available anywhere. Period.

    Can't you see that since that amp is designed wrong, that what other users are seeing is irrelevant, and if you somehow applied that info, that your amp is going to fail?

    That's all I'm going to say. I hope you appreciate that we're simply trying to help you. :)
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