Dentron MLA2500B on AM

[WB2G]

Has anyone used the MLA2500B on AM,any quirks,precautions,advice,suggestions.I was thinking of driving it with a DX-60 or a Icom 746-pro.Thanks.Joe WB2G.

[KX5JT]

http://www.radioamateur.eu/schemi/acc_pa/Dentron_MLA-2500b_user.pdf

100% duty cycle at full legal limit....

Should be good to go with the DX-60 and the ricebox at 15 watts or so drive.

[KM1H]

Considering the 8875's are only rated at 300W PD each and the known weak PS I wouldnt want to push them beyond around 250-300W carrier if driving with full Class C plate modulated DSB. The DX-60 gives extra breathing room as does the SSBAM Icom but be very careful that the Icom doesnt start with a huge power pulse as they are known to do with many models. That pulse will be hitting the 8875 grids at the full 100W which aint healthy.

OTOH Ive used my 2500 several times with a DX-60B, plate modulated Johnson Adventurer and Elmac AF-54 and 67 with no damage. I use the almost indestructable LK-500ZC with SS rigs.

Also add a .0047 6KV disc to the base of the plate choke if using the 2500 on 80/160. The 500pf that is there is a joke and usually responsible for blown filters and xfmrs, 'lytics dont like RF.

Carl

[AF9J]

Whatever you do, be careful with the amp.  You trash the 8875s, you're going to be doing some searching, and spending a bundle to replace them since they haven't been made in a long time.

[WB2G]

Thanks for the info about 160 & 80.The DX-60 is my first choice and I know about the tubes,I did get a spare set and they were not cheap..It will take a day or two for the 220 line into the shack before I try it out on 40 meters(7.290hz just happen to have this rock)Thanks again.Joe WB2G

[Steve - K4HX]

Bill, W3DUQ used one on AM for many years. He ran it between 200-250 Watts output.

[Jim, W5JO]

Someone has/had a modification to change those tubes to something very common that would give near the same PO.  It has been too long for me to remember but I bet a search would turn up that information.  I would not want to pay the asking price for the originals.

[kb3ouk]

http://dk9ip.ba-karlsruhe.de/pdf/mla2500/mla-2500b_conversion.htm

says you can use 8874s which are really the same tube but require axial-flow cooling, the 8875s have a transverse cooler, 8874s also can dissipate 400 watts, there is another similar tube, the 8873, which is conduction cooled and only rated for 200 watts.

http://scottbecker.net/tube/sheets/140/8/8873.pdf

8874 and 3CX400A7 are the same tube, i know that svetlana makes them

[WD8BIL]

I put these 8874s (broadcast pulls) in this MLA2500 in 1980. They've seen a hard life. I've used the amp for corntesting, RTTY with Army MARS at 600watts outpoot, DXing and now AM for 15 years. The tubes show it but it still does legal limit out.

Tune it in sometime!

MLA Mod

[K5WLF]

Here's another way to deal with the tube thing:

http://www.nd2x.net/WB8WJU.html#MLA2500

ldb

[Opcom]

It'll work OK for AM but it is hard on the tubes and no argument here about the smallish power supply. Might as well use it, then rework to more available tube or tubes  later. Or the other option, just run it at a somewhat lowered power level to preserve it for years of use. It would not hurt to direct a separate fan right on the power transformer if planning long sessions. No one talks about cooling the transformers but it does help.

[KM1H]

Thats a nice 8874 conversion; Ive done several with a single 3CX800A7 by adding a filament xfmr; uses the same socket. With medical full output 3CPX800A7 pulls around $300 it might pay to do the swap and sell your two sets of 8875's and come out way ahead in the wallet.

Ive yet to see a Russian tube swap that isnt a major PITA. With the 3cx800 its no more complicated than a small Hammond or home brew PC board chassis and some holes. Get the chimney from Palstar (ex Command Tech.) or Ten-Tec or score one for a 8930 on Fleabay.

Ive played around with a couple of LK-800's (3x 3CX800's) at 1200-1300W carrier on AM (Into a dummy load naturally  ) and they never came close to excessively hot with the fan on the high speed position.

Carl

[WD5JKO]

I bought a used 2500 off flybay and it was in good shape other than one 8875 had no emission. It was the one that had two diodes hooking the cathode to the tube that received the drive first. Something wrong with that design. The bad tube had a good filament, but zero emission. The other tube with the same date code was fine, and good for > 600w out all by itself.

Being cheap, I was looking for a replacement 8875. Quickly found that these were unobtanium except for at high prices for a well used tube.

I had an 8877 pull in my junk box since the 1980's. I did manage to convert to a single 8877. It wasn't done well, had and has issues, but it works pretty well considering. It will do legal limit on SSB (w/o processing), and ~ 1100w key down (220v AC input) where the limitation is the severely sagging B+ to ~ 1750V @ just under 1 amp. On AM it will do ~ 350W until things heat up. Everything gets too hot on AM unless I throttle the power out to about 250W by reducing the drive.

The MLA-2500 has severe issues with the Pi-Net, but that is another thread.

couple of pictures:
http://pages.prodigy.net/jcandela/MLA_2500_8877/

Jim
WD5JKO

[KM1H]

All Dentrons are like the Model A and V-8 prewar Fords; they all work better with upgrades

Carl