N9GGG
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« on: February 15, 2004, 02:58:17 PM » |
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even if i found a mod for this amp will i be able to use it with the neweer tubeless radios....
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N5RMS
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« Reply #1 on: December 10, 2012, 06:57:28 PM » |
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??Mod, what mod? Mine is bone stock A model with original tubes, drive it with K3/100 at 40 to 50 watts!
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W1QJ
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« Reply #2 on: December 11, 2012, 03:48:01 AM » |
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The major draw back to the MLA-2500 is the fact that it does not have a tuned input circuit. SOme solid state radios are very sensitive to small amounts of swr and will fold back starting at about 1.3 to 1. You could use your internal tuner to help match. But the amp wa designed with use with older radios that have adjustable output networks. SOme solid state radios are more tolerable to the swr and still work ok with the amp. The tubes have sensitive grids with no protection so be careful.
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K1ZJH
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« Reply #3 on: December 11, 2012, 09:53:35 AM » |
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The L network also provides a flywheel to present a proper load over the full RF cycle. You may be able to drive it, but when driven by a solid state exciter the IMD may suffer.
I'm pretty sure Dentron at one time offered an external box to correct the problem with newer rigs, but they sell for big bucks when they show on eBay.
Pete
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K8AXW
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« Reply #4 on: December 11, 2012, 10:37:04 AM » |
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If your solid state exciter is SWR sensitive and won't drive the amp at full output, consider building an external "input circuit." (I have no idea if your exciter has an auto tuner - Just considering the possibility it doesn't)
An external input circuit (ususally low Q) is no big deal to build and use.
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WX7G
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« Reply #5 on: December 11, 2012, 12:56:12 PM » |
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An external antenna tuner can be placed between the SS transceiver and the Dentron.
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K8AXW
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« Reply #6 on: December 11, 2012, 08:24:03 PM » |
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X7G: I seems that I read somewhere that using an antenna tuner between an exciter and amp increased the IMD. Have you ever heard of this?
The only difference I can think of between a tuner and an amplifier input circuit is the difference in Q.
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K1ZJH
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« Reply #8 on: December 12, 2012, 06:53:04 AM » |
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X7G: I seems that I read somewhere that using an antenna tuner between an exciter and amp increased the IMD. Have you ever heard of this?
The only difference I can think of between a tuner and an amplifier input circuit is the difference in Q.
The SWR on the feedline between the matching network and the amplifier tube cathodes can have an adverse affect. I recall reading that was a problem on 10 meters when the sB-220 was being designed. Ideally, the L match belongs right at the tube socket. Pete |
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WX7G
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« Reply #9 on: December 12, 2012, 11:33:00 AM » |
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It does.
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K7MH
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« Reply #10 on: December 12, 2012, 11:47:37 PM » |
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The L network also provides a flywheel to present a proper load over the full RF cycle. You may be able to drive it, but when driven by a solid state exciter the IMD may suffer.
I'm pretty sure Dentron at one time offered an external box to correct the problem with newer rigs, but they sell for big bucks when they show on eBay.
Pete
They did, the CM-1. It didn't have band switching for the WARC bands though although it may work ok on them. Amp Supply had a tuned input kit for Dentron amps that was internal. I put it in my Clipperton L. It was about $100 back then. |
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KE2TR
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« Reply #11 on: December 13, 2012, 07:46:02 AM » |
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IF I recall back when I owned a MLA2500B the amp had a padding resister in line to the input of the tubes which gave a decent match, even with the touchy solid state rigs back then. Many hams would take that resister out of line to get over 1500w output but that did two things, it would shorten tube life and give a greater miss match at the input. That amp does real well around 1Kw to 1.2Kw output on ssb and those tubes will last a long time at those power levels but if ya push it hard then pop goes those hard to find 8875's. I am not sure but I do believe the 1st series 2500 had that resister also. The real issue with that amp was hams would bypass that resister and dive that amp to 1.8 to 2Kw output and if you ever would look at that output on a spectrum analyzer there would be VHF parasitics all over the place but under 1.3Kw is was nice and clean. Most modern rigs today have internal tuners that should take care of and missmatch.
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K1ZJH
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« Reply #12 on: December 14, 2012, 01:23:17 PM » |
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VHF parasitics and IMD performance are two different issues.
Pete
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NO9E
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« Reply #13 on: December 18, 2012, 06:43:02 AM » |
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KE2TR wrote parasitics AND IMD. IMD caused by input being less sine and perhaps by excessive HV variations at higher load. And parasitics because of feedback less attenuated. It is interesting how one action (removing the reistor) can lead to many different problems: short tube life, parasitics and IMD, and perhaps more. I am one of the many who knew someone with fried tubes in MLA-2500. Ignacy, NO9E VHF parasitics and IMD performance are two different issues.
Pete
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KE2TR
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« Reply #14 on: December 18, 2012, 09:14:45 AM » |
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The problem was that in the original QST review of the 2500 they stated that Dennis of Dentron had taken a prototype mla with him when he visited the ARRL and he pinned a 2500w bird slug with that amp, mind you that was not a production 2500. So many who though that this amp could do 2kw were thinking that it would be cheaper to buy a 2500 over an Alpha, yes cheaper but the amp was not made to do that type of output and those tubes were very overdriven.
Its funny that the PS didn't pop but that amp converted to other types of tubes seems to work well round 1500w and even the 8875's in that amp work real well around 1200-1300w. At the time I owned my MLA I had brought home a IFR service monitor from work and did some testing into a 2.5KW dummy load, it was not a hi end way of looking at the spectrum but it told me were the amp had its best/cleanest output so for those out here that think I was talking out my ass well if you could see the IMD at and around your xmt at 14.2 and when you looked at the VHF range you could detect these two problems when the amp was pushed over 1500W. I ended up getting rid of the amp cause at the time I HB an 8877 and that amp didn't even have a burp or fart at 1500 or when tested into a dummy load 2500 so running 1500w was a walk in the park. The MLA in today's used market is a good buy at around $500 but that with good tubes, given that you can no longer buy the tubes for the amp it seems funny to me to spend any more for the amp cause then at $650 to $900 your into a used AL80 series amp which may not give you as much output but has excellent serviceability over the MLA. I have seen the MLA's sell for between$400 to $700 used and some try to get more, some fool will sometimes pay that amount but once those tubes go its gonna be painfull. I liked the amp for its size and it was easy to drive but there is no way I would pay any more then $500 for one use and that goes for these nut bags that try to sell there SB220's for $600-$700, these amps are 30 plus years old but therefore allot of buyer's who don't really look into what they are buying.
Jim
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