MLA-2500 question

[WA3RSL]

Hello all,

I have an MLA-2500 that just put on the air last night. All seems fine but the wattmeter on the MLA-2500 was only reading 600 watts out (CW) when the plate voltage is 2600 volts and plate current is 700 ma. Is it possible that only one tube is working or is the problem an inaccurate wattmeter? At this time I don't have an outboard wattmeter.

Thansk

73

WA3RSL

Frank

[KA5N]

  That's 1800 watts input, my guess is the wattmeter is off.  Since you have just fired it up
you are really in the check out phase.  You really need an external wattmeter (that is fairly
accurate) and a dummy load that can handle the power.

Allen

[WA3RSL]

Hi Allen,

So there is no way I would have 2600 volts with 700ma if only one tube was working?

Last night I was on the air for 3 hours with the amp on 40 meters and it worked cool and pretty much excellent. The fan is a little loud.

One last thing I did do. I drive it with a brand new Kenwood TS-590S. I noticed when I put the am in standby the wattmeter in the MLA-2500 would read my output from the radio. The radio (SWR 1:1) wit 100 watss reading on the radio, the MLA-2500 read 50 watts. So maybe I am 50% off on the reading on the amp. LOL

73

[ZS5WC]

 
The wattmeter reading is off --there is a calibration procedure in the MLA-2500 manual requiring an external wattmeter.
Other point to note is to fine tune the amp with full HT applied, ie the 'tune' button out--to properly match the tubes to the load under dynamic conditions.
Also keep in mind that you must engage the TS-590s atu when driving the amp. , (unless your MLA has tuned input circuits), as most (including mine) are far from 1:1 swr across the HF spectrum at the input.

73 de William
ZS4L

[WA3RSL]

Thanks William,

I feel better. Was concerned I was only getting output from one tube.

I haven't tuned with the tune button out. Will do that. Also I have been using the TS-590S tuner to match to Amp. I just ordered a peak reading SWR/Wattmeter to tune up with. And I did see that cal procedure in the manual for the MLA-2500. I'm on mostly 40 meters where I just put up a 6 element wire beam aiming 300 degrees from southern VA!

73

Frank

[W8JX]

Well the real test is comparing bare-foot and amp signals down range as that will tell you more than watt meter.

[W8JI]

Hi Allen,

So there is no way I would have 2600 volts with 700ma if only one tube was working?

Well, if you have a good single tube you can actually hammer it that hard. It won't last long, but the 8873-5 series has a lot of reserve emission.

One last thing I did do. I drive it with a brand new Kenwood TS-590S. I noticed when I put the am in standby the wattmeter in the MLA-2500 would read my output from the radio. The radio (SWR 1:1) wit 100 watss reading on the radio, the MLA-2500 read 50 watts. So maybe I am 50% off on the reading on the amp. LOL

That sounds like the power meter is 50% off, assuming SWR is low on your antenna.

Be really careful with that amplifier. The 8875 tubes are unobtainium now, and they are VERY easily damaged from excessive grid current. The grids can be popped in as little as 30 seconds!

If your 2500 does not have the mod to read grid current, you should add it. Also it is a good idea to add a grid trip circuit to disconnect if the grid current gets too high, and to lower the filament voltage so it is in the MIDDLE of the range at your median line voltage. Dentron ran the filaments a bit too hot.

73 Tom   

[WA3RSL]

Hi Tom,

Thanks for the ideas. I do plan to do the grid meter mod and a couple of other mods I found on the net.

My SWR is flat.

So this Friday night I will connect up my new wattmeter and also cal the MLA-2500 wattermeter on 40 meters where I am mostly.


73

Frank

[K9MHZ]

That input circuitry is a real mess, and certainly not good for a nice, modern rig like the 590S.  For some reason, the MLA-2500 has had a following over the years.  I owned one for a short time back in the 80's....and that was enough.  It was marginal in design and low quality back then,  and I can't see that it's somehow improved since.  That wire-wound swamping resistor can't be good for any rig.

Sorry to all of the owners.....just another opinion here.

Best,

Brad, K9MHZ