Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Jan 11, 2019.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    No, it was about my 6th or 7th 2m amplifier, and the little article describes mostly putting a commercial VHF RF deck on 2m, and building the power supply/metering/relay/control system for it, as back then 2m 1/2 kW RF decks were all over the place, often taken out of VHF "paging" service. The paging amps were very rugged as they normally operated 24/7/365 completely unattended, on tall towers or mountaintops and nobody ever looked at them until they failed. "Desense" and "noise" were never a problem, since they had no receivers! So they could run out of tune for months, getting hotter and hotter until something finally bit the dust and then they'd go fix it.

    That sounds like a terrible thing, but it was good for hams, as VHF pagers eventually became obsolete a lot of those decks hit the market very inexpensively, and they're built like tanks.
    N2EY likes this.
  2. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    One thing the articles don't talk about is how to adjust the little butterfly cap across the grid coil. Maybe it balances the signal to the tubes. Hope separate screen and Grid meters aren't used for each tube. The W1HDQ Killerwatt from the 1972 VHF Manual is the closest I have seen to what I actually have. I noticed in Steve's 73 Mag article he used a separate Transformer and PS for the Grids and the Bias. Really like that idea, but this PS is Heavy enough!!! Still worried about dropping the +2500V to the screen regulator tubes. Thats gonna be a 2000 V drop atleast across that resistor!!!
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I thought you wrote that what you have tapped down the HV filter equalizer string so only +750V or something goes to the screen regulators.
  4. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Sue doesn't like that idea, cuz if there ever was a screen short, the voltage across the remaining caps would be way overvoltage and the caps woud explode!!!
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I didn't really like that idea either, frankly.

    My reasoning was: You'll be drawing 30mA for the regulators from the middle of an equalizer string forming a voltage divider. By drawing current partway down that string, the "equalizing" is pretty defeated.:p

    Probably (if you don't want to use a separate screen transformer) better to drop from the 2500V B+ with ~70K, 100W resistance. Because one end of that resistor will be 2500V above ground, that end must be very well insulated, so the "mounting clips" that come with the hollow WW power resistors won't cut it -- you'd want to mount those clips onto good insulating ceramic pillars or something to keep at least that end way off the chassis. And a 100W resistor will get very hot dissipating about 65W all the time, so I'd either use two ~35K 100W resistors in series to spread the heat out more or at least use a fan blowing across or through a single 100W resistor to prolong operating life.

    For me, easier to just add a screen supply with a small transformer (it only needs to handle about 15W -- if you can find one with the right secondary winding, the transformer can be smaller than your fist).
  6. N2EY

    N2EY XML Subscriber QRZ Page

    Well, that all depends on the design, doesn't it? It's a pretty simple Ohm's Law problem to determine the effect of drawing that much current from a particular point in the capacitor string.

    Another approach is to use a string of smaller resistors mounted on suitable ceramic standoffs. I did that in the Southgate Type 7 transmitter power supply, because I had lots of suitable small (10 watt) power resistors, but no really big ones of the right value.

    At Southgate Radio, it has been found that power resistors run at half rating or less will last pretty much forever.

    Yes, BUT - it needs an interlock system so that you cannot apply screen voltage without plate voltage. Otherwise it's easy to toast the screen-grid final tubes.

    The power supply and metering needs of the W1HDQ amplifiers are well documented in the original 1964 article. One just has to read and understand it.

    73 de Jim, N2EY
    N2SR likes this.
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    I have a Gonset 903 2 meter amp good for 375 watts output. It does not need a 60 amp power supply for the brick type 350 watt SS amps.
    N2EY likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    This is true, and one advantage of deriving screen voltage from the B+ supply; if B+ fails, it all goes down and if just the screen resistor opens, no damage is done.

    Another precaution which could be worthwhile with external anode tubes is an air flow switch, so if the blower stops or slows down, the air pressure sensor shuts down the amplifier. These tubes will last just seconds (if dissipating any power) without the air cooling. (They can last a long time with only heater current, as heater power in these is very small.) Many of the forced-air cooled external anode "repeater" or "pager" amps did have such an air flow safety control system. My homebrew amps never did.:p
  9. N2EY

    N2EY XML Subscriber QRZ Page

    Exactly. Although, if the screen resistor is properly designed and sized, the chances of it failing are tiny.

    All it takes is a practical application of Ohm's Law. No big deal; the math is super-simple. Fourth-grade stuff at most.

    It's also a good idea to make use of the VR tube jumpers in the screen supply, so that if a VR tube is not in its socket, no screen voltage is present, rather than abnormally high screen voltage. (This is why the jumpers exist). Note too that octal VR tubes of the 0A3/0B3/0C3/0D3 family have an operating range of 5 to 40 mA, while the older VR-75/90/105/150 and the miniatures are 5 to 30 mA.

    It is also a good idea to put a resistor across the entire VR string of sufficient size to hold down the voltage if something opens up. Otherwise the screens could see very high voltage if a VR fails to fire. Just another simple Ohm's Law problem. Easy-peasy.

    The pager amps were unattended, so of course such a switch makes sense. Not really needed for typical hamshack use.

    btw, All Electronics has nice analog panel meters for $12 each, 10 for $10 each.

    73 de Jim, N2EY
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  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, they have some amazing deals. That's a "walk in" place for me, as it's about 10 miles from my home, and just blocks from the county courthouse in Van Nuys, a place I usually visit a few times a year, anyway. Fun place to visit! -- All Electronics, not the courthouse.
    N2EY likes this.

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