Dentron 2500B grid current?

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

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  1. KM1H

    KM1H Ham Member QRZ Page

    Or put another way that he may understand if I use small words.

    Read any 2X 8874 Alpha manual. The 8874 is identical internally to the 8875 but with 100W more plate dissipation (Pd) per tube. It is comfortable at 1200W out key down (A0) and many owners still have the original tubes.

    Try it with the MLA-2500 and dont push it beyond 1200-1300W on SSB using a peak reading wattmeter of known accuracy. Then record the grid and plate current readings for posterity as meters can have different responses. Dentron used Prime, and Alpha used Modutec for their meters.

    If you still want to see 1800W PEP than I suggest another forum or a frequency between the 10 and 12M ham bands to impress folks. You are not doing a good job of it here Im afraid.

  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    I strongly suggest that you calm down a bit. Then, go back and read your comments about putting out more power than the legal limit allowed by the Federal Communications Commission as expressed in 47 CFR Part 97 Section 97.313. Then take note of the policy of NOT to promote anything having to do with illegal operating.

    Therefore, I strongly suggest that you do cease to keep commenting on illegal operating procedures. If you wish to continue with illegal operating procedures then please take the discussion to another website that condones illegal operating.

    Glen, K9STH
    Head Moderator
  3. W8JI

    W8JI Ham Member QRZ Page

    Carl makes a good point. The 8874 is like an 8875, but the 8874 because the tube has a forced air axial flow cooler, has 100 watts more dissipation per tube.

    The point I am making is this:

    We better be lucky or very attentive and very good operators when using an amplifier using gold plated grids when that amplifier has no fast reliable grid trip circuit.

    Gold is displaced by the kinetic energy of electrons striking the grid, and the more grid current the more energy is at work depleting the gold. It's like the process of water evaporating, except gold being a metal is almost like ice. At a certain point gold will start to very slowly move, and this is far below the melting or boiling point of gold.

    I have ruined a brand new pair of 8874's prototyping by having too much grid current for just 20-30 seconds.

    It is an incredibly poor idea to have any grounded grid amplifier using gold plated grids without some form of fast electronic disconnect.

    This is also why it is bad news to hammer the tubes and elevate the temperatures. A 3-500Z can be run and will generally fully recover from having a grid so hot it glows red or orange. A gold plated grid will totally poison the tube long before the grid shows color.

    Worse yet, with a gold plated grid damage is cumulative over time...even if the tube has cooled since the last overload.

    Keep the grid current low. I'd also install a $10 grid trip circuit to protect an $800 tube. It is good economics.
  4. W1BR

    W1BR Premium Subscriber QRZ Page

    Don't shoot the messenger, but the average power in one person's voice is different than another. Also, many rigs run RF processors, ALC or or other forms of compression that will also make dramatic differences in the apparent grid current reading in SSB operation.


    Pete k1zjh
  5. K7GLD

    K7GLD Ham Member QRZ Page


    Another well-intended, but long-winded NON-answer that fails to answer the SIMPLE question I have REPEATEDLY asked:

    "WHAT grid meter readings are OTHER 2500B USERS actually seeing in tuneup and typical SSB operation?"

    Sure seems to be a killer/confusing question, since so many have ALL SORTS of long-winded commentary on all sorts of peripheral subjects - but are totally unable to answer in a single sentence...:D:p

    For the terminally dense, here's a sample, suggested type of reply:

    "I usually see 100 ma in tune, and voice peaks of 10 ma in SSB, at 1200 watts out..."

    THERE, see how simple, short, and EASY that is...:p

    BUT, seems lots of guys have this ego-driven, "I'll show this guy and all other readers how SMART *I* am, while at the same time demeaning HIS intelligence and operating ability..."

    I can't speak for other readers - but in view of the REPEATED posts I have made in this thread FULLY and CAREFULLY outlining what I was looking for, and WHY, these well-off-topic posts unrelated to what I was ASKING, go lots farther to make the ego-stokers look like asses than they do ME! ;):rolleyes:

    Then, there's THIS:

    Since you ARE a moderator, I would normally expect that you would "go back and read", and be fully familiar with the other posts I have made in this thread, and be as "efficient" at calling and culling out those posts that do NOT answer the supplied question, as you are here in applying accusation and criticism of what you ASSUME is my typical operating practice that is TOTALLY inaccurate ASSUMPTIONS on your part - or do you ENCOURAGE off-topic post here?

    Most specifically, that the power output readings I supplied earlier were under TEST conditions only, (when did I EVER say I was, or INTENDED to actually operate under those conditions on the air?) done purely to determine the condition of my tubes - since WEAK tubes will not display the same plate and grid currents as GOOD tubes! Perhaps you MISSED this info I offered back in post #5?

    Those of you doing your damnedest to make ME look bad with off-topic and silly. frivolous posts, sure aren't doing anything to help YOUR appearance in the process! :rolleyes:

    MEANWHILE, there WERE enough in this thread who DID have sufficient reading comprehension and understanding to actually properly ANSWER my question, that I have the info that I was looking for, and have accordingly replaced the .0462 with a .462 shunt that now makes full scale grid readings 100ma, instead of the 1 amp as it originally did, and grid current far easier to properly monitor - and for other readers possibly looking for the same info I was - here's my reply:

    I see about 90 ma grid current in tuneup, and about 10ma on voice peaks in SSB at about 1200 watts output, that is with about 50 watts drive, and the 2500B 50 ohm input swamping resistor in place.

    Yeah - a bit long winded - but all info actually PERTINENT to what I wanted and asked for...

    And any moderators rushing frantically to chastize me for this post, do us ALL a favor and carefully READ all the posts in this thread first for better understanding as to what REALLY is going on here - and leave your baseless accusations and assumptions and off-topic advice on YOUR side of that keyboard! ;)
    Last edited: Jan 7, 2011
  6. W0BTU

    W0BTU Ham Member QRZ Page

    Let's all chip in and buy Glen and the other moderators each a beer. They deserve it. :)
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Is the last week in March too early to begin spring planting? I'm thinking of using wire grids to support my Currant bush canes which are typically heavily laden. I'm thinking a 0.046" wire will work well. Others say this strategy is golden.

  8. W8JI

    W8JI Ham Member QRZ Page

    It might be a good idea to relax a little bit and stop being so demeaning or insulting to people trying to help.

    My very first post gave you values that you apparently just ignored. I said:

    An 8875 has a grid dissipation of 5 watts. It has a fragile gold plated grid that can be ruined in a matter of seconds with more than ~150 mA of grid current, sometimes less.

    and I also said:

    In normal operation in the Dentron circuit you should see maybe 50 mA or so of grid current on CW carrier at full power. You should see nothing, or just a slight wiggle, on SSB voice. If you had a two-tone generator, you would see about 25 mA grid current in that amp when properly driven and loaded.


    I'd keep that amp below 50 mA of CW grid current and 1000 watts pep or carrier out. It was barely adequate as a 1000 watt output amp. It was really designed as a 1 kW INPUT amp, not 1 kW ourt....but you can safely push it a little. Just remember the tubes are unobtainable, and they are VERY fragile.

    IMO the lowest form of humanity is someone who asks for help, gets it from several people, and then takes the extra time to come back later and run down all the people who tried to help him.

    I certainly regret trying to help you.
  9. KM1H

    KM1H Ham Member QRZ Page

    Amen Tom. His over the top arrogance is akin to throwing a grenade into your bosses office and then walk in looking for a raise.

  10. W0BTU

    W0BTU Ham Member QRZ Page

    A perfect description of what took place.

    Hey Carl, are those 304TL's in your avatar? Is that your amp?
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