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Tube Amp adjustment.

Discussion in 'Amplifiers' started by 190, Mar 12, 2009.

  1. 190

    190 Active Member

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    Ok, i got 2 old vintage sweep tube amps. onr is a mini scout, the other is a scout 20. there is a adjustment screw on the back of both of these amps (as well as on other old sweep tube cb amps ive seen) with a lock nut on it. what are these for.? how do you adjust them, how can i tell if its right. i know what the tune and load knobs are. i took the bottom cover off of the mini scout and this set screw thing goes into a ceramic peice with a copper coil hooked to it. just dont want to screw these 2 amps up because they are both as new. thanx.


     
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  2. 295

    295 Member

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    I am not familar with the amp you specified, I have seen this set screw before on a different model and it was for tuning the input for low SWR. Perhaps a few photos showing the outside and inside of the area around on your amp and I or someone can provide more assistance.
     
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  3. 190

    190 Active Member

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    say 295, i dont know how to post pictures on here. sorry. im not sure what the little screw in the back is for. just trying to find out. ive seen these on other old cb tube amps. i thought i read somewhere about it might be the plate voltage adj.not sure. these are sos electronics scouts. the mini scout has one tube. the scout 20 is a 4 drivin 4.
     
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  4. Moleculo

    Moleculo Administrator Staff Member

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  5. 190

    190 Active Member

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    ok ill try it. the other scout amp the screw is a little brass slotted screw. the mini scouts looks like a little bolt. i was told these are input tuning screws.
     

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  6. W5LZ

    W5LZ Crotchety Old Bastard

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    I think my first guess (<-guess) is that it might be an input adjustment. But since I know nothing about the thing, except what you've told us here, I don't think I'd put much faith in that guess. Unless, or until I can find a schematic for it, or someone who does know about it, I'd just leave it alone. Not much help, huh? Sorry 'bout that.
    - 'Doc
     
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  7. linearone

    linearone King of NY

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    tough for me to tell but it might be a bias adjustment or a input tune, or a ground screw. pop the bottom cover off and take a pic of what its connected too. please be careful as there could be stored voltage in the caps present and I wouldnt want you to get hurt.
     
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  8. linearone

    linearone King of NY

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    whoops I must have blacked out you said it has a coil etc, seems like an input tune to me.
     
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  9. ethomas

    ethomas MudDuck

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    I have a 20 and a super scout. the way i set mine are on high power i key and turn till i find the middle of key. Like if it goes from 2oo to 400 i set at 300. Works fine for me I think they are a ant. match
     
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  10. Suicide Ride

    Suicide Ride Member

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    Tuning input cap

    The tiny little "slot" in the back of most tube amps (& old tube radios) is your input tune capaciter.

    Use it for tuning (highest output - swing) everytime you change something in the "circiut" ie: radio, mini-Scout, papa Scout, wattmeter, low pass filter, antenna.

    Change, add, remove any of these & you will need to re-tune the back of each piece of equipment that has one of those capaciters. That includes coax jumpers.

    Tune your driver w/ amp in stby, & then tune the amp. Use a watch/type screwdriver for the cap, & be really carefull. The thing you are turning is just a slot cut into the end of a machine screw, & it WILL break if you crank on it!!

    As you "un-screw" (counter-clockwise) the post, it compresses the mica wafers that make up the cap, so start the whole process w/ the cap backed all the way out (turn counter-clockwise till tight - won't turn), & usually, as you turn "in", it will have a "sweet spot". Don't mess w/the nut as that is how you mount the cap to the equipment.

    Dead-key your mic, & turn the cap clockwise until highest output (w/o modulation-I turn my mic gain down). Turn mic gain back up, re-key & tune the front like you normally would do, & then adjust the cap in the back for highest swing. It should be within a 1/4 to 1/2 turn of where you started, if not the same. (sweet spot)

    Don't know if you have a dummy load, as that really helps in the tuning process. Mine has a wattmeter & is oil-filled made by B&W. Normal price on feebay is around $125 - $200 max, & every bit as good as a Bird meter. Best $$$ I've EVER SPENT!!!

    Hope this helps.... Let me know if you need more info, or if I didn't make sense.
    PM me if you get into a jam (or jelly :D )

    Just my 2 cents.....
     
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  11. Bionic Chicken

    Bionic Chicken Active Member

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    The small trimmer on the back of the Mini Scout is to set the dead key. Usually from 2 to 10 watts. That is all the tuning for it....just set the dead key. It will modulate forward from there.
    On the Scout 20 set the Hi/Lo switch to Hi and then max the two tuning knobs for a peak dead key without any audio. This will pretune the amp. Once you have this done key the amp and say AHHHHHHH while moving the tuning controls slightly to acheive the highest peak reading. Only one of the controls will make a difference in the peak power, the other one will change the key slightly. Once you have the peak reading continue to move the control (that makes the peak higher) clockwise until you see a drop in wattage of about 30 watts or so. You then can tune the input tuner on the back of the amp for max peak from there. The amp then is tuned and will require no further adjustment. If it is in good shape it should run cool and have great audio.
    Also you do not want to drive the Scout 20 with the Mini. The driver tubes in the Scout can't take that much wattage for a long period of time. With the price of tubes now it is not something to do. Both are good amps and are reliable. I have owned both and still have several Mini Scouts. They are keepers.

    BC
     
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  12. 190

    190 Active Member

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    Thank you bionic chicken and suicide ride. your info has been quite helpfull. i really like the scouts and the ones i got are keepers.
     

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  13. Paulwe4

    Paulwe4 Member

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    what transistor is in the keying circut
     
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