Mla-2500 output tank

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, Apr 14, 2019 at 10:55 PM.

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    I finally got around to building a new filter cap/bleeder board and installing it with new caps in this Mla-2500 I picked up. I guess I got lucky, because the tubes seem very strong and works well on every band......haven’t checked 160 yet. The only issue I see concerns the loading on all bands. Does anyone know if the minimum capacitance of the antenna load cap is too high? The reason I ask is because on all bands I’ve tested 15,20,40, and 80, when I have this thing tuned for peak output and low grid current, the load cap is all the way to the left (if looking at the front of the amp). I assume this is close to fully unmeshed but I could be wrong. I can pull the lid again and check but I figured I’d ask here first. Wouldn’t it be odd to have too much loading capacitance on 15-80? Thanks in advance!
     
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    Disregard.....I looked and the load cap is darn near fully meshed, not unmeshed, on these bands. These amps obviously could’ve benefitted from caps with more range
     
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Try loading it more (with less load capacitance) and see what happens.

    Usually, that will drive the grid current down even lower without losing output power, or at least "not much" output power, and that's actually the sweet spot for almost everything including minimum IMD.
     
    KN4CTD likes this.
  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    I’ve done this, and yes, it brings the grid current down without much loss in output. It just seems odd that max out power is always close to fully meshed. From 15-80, the load cap barely needs to move and is pretty close to “1” on the scale on each of those bands. It makes it easy to load, but acts like the load cap is low on capacitance from the get go. It seems to work fine as is so who knows. I have no previous experience with these amps so I didn’t know what to expect. I’d feel more better about it if the load control acted more like the plate tune cap when changing bands. I bought this to fix and sell so I don’t wanna go crazy and replace the load cap. The grid current shunt on these definitely need changed. The grid current is very hard to monitor as is!
     
  5. K6BSU

    K6BSU Ham Member QRZ Page

    Many amps have fixed loading capacitors you can switch in for the lower bands. Having the load capacitor fully meshed at optimum setting on lower bands just means you could use another fixed cap parallel with the variable.
     
  6. KN4CTD

    KN4CTD Ham Member QRZ Page

    Yes, on the low bands, but makes little sense on say, 15 and 20 meters.
     
  7. K6BSU

    K6BSU Ham Member QRZ Page

    Maybe I wasn't too clear. The fixed capacitors are "switched" in. Meaning you switch them out for 20, 15 Meters. I can't vision an amp not being able to do that, and still cover all HF bands.
     
  8. KN4CTD

    KN4CTD Ham Member QRZ Page

    You were very clear lol. Sure, it’s possible, if using tune/load caps that have low enough minimum capacitance and high enough max capacitance to match the output impedance of the tube/tubes back to a 50 ohm load.....taking into consideration inter-electrode capacitance and stray inductance. My point was that no C-L-C output tank designed for multiband hf coverage, should need extra padding capacitance on the higher bands.....unless it was never designed correctly in the first place
     
  9. KN4CTD

    KN4CTD Ham Member QRZ Page

    I finally did check 10 meters and it loads half mesh on the load cap......so that’s good I guess. I’ll have to take a closer look at everything in the bandswitch/tank circuit. It doesn’t appear that this amp has ever been modified though, except for what I’ve done......replace bleeders and caps and change the value of the grid current shunt
     

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