Dentron 2500B grid current?

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

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  1. K7GLD

    K7GLD Ham Member QRZ Page

    I have a Dentron MLA 2500B amp - tuneup and meter reading info seems a bit thin in the owner manual, but seem to indicate there SHOULD be about 100 ma peaks in SSB operation, and grid current should not exceed 200 ma when tuning up or operating.

    Mine has full output, and works great - but the absolute MOST grid current I can produce, is about 100 ma in tuneup - and the meter doesn't even move in full power SSB operation.

    There's not much in the grid circuit - just a bypass cap and a .0462 ohm meter shunt resistor. I don't have any test equipment accurate at that low a resistance, so before I begin any further testing or parts substitution, am hoping other 2500B owners might offer what their typical grid current readings are. I suspect the possibility the grid shunt might be defective, or the bypass capacitor shorted - all other meter readings seem right on, so doubt any issues with the meters themselves.

    I'm getting slightly over 1.5 KW out key down, and over 1.8 KW out in SSB peaks into a reasonably accurate outboard watt meter - so pretty sure the tubes are OK, and that is on 115 vac...

    Any help?
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    80mA peak grid current will run these tubes at full power, usually.

    1800W output from a pair of 8875s? That would indicate an overdrive condition which is hazardous to the health of these fairly rare tubes; I'd never even approach that number.

    The MLA-2500B has an untuned input and if used with a solid state exciter, will create lots of IMD. It was designed for use with a tube type exciter having an adjustable tuned output stage. It was also designed for "2000W PEP input" service, which would yield maybe 1100-1150W PEP output. I'd never push it beyond that.
  3. W9GB

    W9GB Ham Member QRZ Page

    Listen to the advice given, and follow.

    As you likley already know, the Eimac 8875 tubes in this amplifier are no longer made -- for at least 2 decades now.

    WATCH the grid current on these 8875 tubes, Dentron did not install any grid protection (that I am aware of).
    BABY this amplifier ... and you should have long-lived operations.

    BTW, Alpha used two Eimac 8874 tubes in the Alpha 76A model ... and Alpha did not install grid protection either --
    but Dick Byrd provides that modificaiton.

    A majority of these Alpha 76A series amplifiers (properly operated and maintained) are still using the original Eimac tubes -- after 30 years!

    Last edited: Dec 13, 2010
  4. W8JI

    W8JI Ham Member QRZ Page

    That's idiotic advice. 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.

    You should be able to run 300mA or more of grid current, but it would also be pretty rough on the tubes. 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.

    Those were the darkest days of Dentron. That amp should have had a good grid trip circuit set at about 125 mA, better cooling, the right filament voltage, tuned input, proper tank values, and fault protection on those expensive fragile tubes. Early amps never even had a grid meter!!!

    At that power I would expect you are hammering the bandwidth pretty good and within a few hundred hours of transmitting time, you will probably have bad tubes.

    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.

    73 Tom
  5. K7GLD

    K7GLD Ham Member QRZ Page

    WELL guys, thanks for the input, but my primary goal here was actual use reports from other 2500B users I could compare my meter readings with - not a scolding or advice on linear use - I've been licensed since 1959, got the Extra Class in 1970 - and have built and operated several legal limit amps myself, I'm not a dummy in regatds to overdriving, spurious emissions and splatter...;)

    And yeah, I fully realize the hazards of abusing these scarce 8875's - or ANY power tube - and only used my original figures as test info to illustrate that the tube ARE in good condition, not as any indication of what I do in normal operation - a reasonable bit of data that CAN slew plate and grid current capabilities.

    Rather, as stated above, I'm simply LOOKING FOR actual grid current meter readings from other 2500B USERS to determine if mine is typical, or whether I should start replacing components. :p

    Any other Dentron MLA 2500B users out there?
  6. KD7MSC

    KD7MSC Ham Member QRZ Page

    I have had more than one MLA2500 and can tell you in SSB you want to see as little current as possible. All of the above advise is great. I always kept the output below 1KW and barely a wiggle from the grid current meter in SSB. I am sure that Tom and Steve have both owned or worked on this amp before as well. If I ever had any questions about any amplifier I would ask them for sure. If the power supply has not been rebuilt I would keep an eye on the caps. You may also want to look into adding a step start and glitch resister.
  7. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    My Tempo 2001 uses a pair of Eimac 8874 (3CX400A) in the final. They max out at about 1400 watts output on 80-meters and drop down to about 1200 watts on 10-meters. I normally run them a little lower, between 1200 and 1300 watts on 80 and around 1000 watts on 10. The tubes have been going strong for over 35 years.

    One thing about the Tempo 2001 is that the plate tuning capacitors are fixed and you tune the output using a roller inductor. Of course the antenna loading capacitor is variable. Because of the roller inductor the amplifier works very well on both 17-meters and 12-meters.

    Glen, K9STH
  8. K7GLD

    K7GLD Ham Member QRZ Page

    NO offense or disrespect intended here guys, but....;)

    A guy gets on an automotive forum, and asks "what should my speedometer register in my '05 Ford Escape with OEM tires, at 2000 RPM at freeway speeds in OD?"

    Responders then comment that the guy should keep his tires properly inflated, and use a good brand of oil - and keep his eyes on the road, not on his gauges...

    All good info perhaps - but NONE answer the actual QUESTION! :rolleyes:

    2 most common issues on Internet forums in regards to posts seeking help in specific issues, is the "asker" not including proper, or sufficient, info to obtain the help he's looking for. The other, is the number of responders NOT reading or fully comprehending exactly WHAT the seeker is looking for - and then offering "helpful" info that is well off target, and provides precious little of the assistance that was really wanted - I see both examples wayyyyy too often in forums just like this one! ;)

    As I've already said, I KNOW running an amplifier at or above it's actual rating as a regular practice is hard on the tube(s), and can create a dirty signal. I KNOW that the lower the grid current (as well as the plate current!), the easier it is on the tubes - and the longer they will live.

    And yeah, I know that the guys so far offering advice are probably smarter than I am - but as much as I appreciate their input, they STILL have failed to actually answer the simple, basic QUESTION I'm asking - and "advice" is only "good", when it actually ANSWERS the question! :p

    As much as I appreciate "scatter-gun" replies, where info hits all around the target - but NONE in the center ring, there has YET to be that single, to-the-point reply that actually gives me what I am looking for - so, tho' it's looks to be a lost cause, here again is what I'm looking for:

    WHAT is the maximum grid current meter reading you (an actual MLA 2500B OWNER/USER) typically see when tuning up, and then see meter peaks to in SSB operation?

    We will ALL agree that keeping the grid current LOW is a worthy goal, but guys, FIRST, I need to know if MY grid current metering circuit is reading properly - THEN I can best operate well within proper settings - but at this point, I'm uncertain MY readings are correct - and THAT is why I'm asking other 2500B owners what THEIR amps display - CAPEESH? :D
    Last edited: Dec 13, 2010
  9. AI3V

    AI3V Ham Member QRZ Page

    Maybe the question you should be asking is what is the grid current meter trying to tell you?

    Here is the classic text from the folks that designed and manufactured your tube:

    The care and feeding of power grid tubes

    Or, you can ask a bunch of guys on some internet forum.

    Every Dentron "amplifier" I have ever used or repaired has been a poor excuse of a linear, anything even remotly approaching "good engineering practice" takes a distant back seat to the output meter. Asking how badly everybody else overdrives their amplifier will do you no good.

    One other thing, there is no way you are getting 1.8KW output on a 115vac line, not even close.

    Last edited: Dec 13, 2010
  10. KD7MSC

    KD7MSC Ham Member QRZ Page

    50ma - 60ma
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