TL-922A - no reported plate current!

Discussion in 'Amateur Radio Amplifiers' started by KK6VQK, Aug 19, 2018.

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    Adrian, are you running the amp on 240v? You should be!! These older amps like the 922, SB-220, L4B, Swan Mark II those in particular with the CW/SSB switches, were originally designed as full power amps "at the time" which was 1KW DC input. Therefore those amps did not need plate transformers that are as heavy duty as those amps produced these days that would be considered "full power amps" 1500 watts output. Also with the newer power rule there is NO LONGER a need to have a CW/SSB switch on an amplifier. Those switches are only there to conform to legal issues at the time those amps were made. These days those switches are unnecessary and on the ones that do have them are no longer needed. These days these same amps can be pushed beyond the old power rule of 1KWDC input and approach the new power rule of 1500 pep output. The aforementioned amps will approach the newer power output level with a full 100 watts of drive. Of course this is pushing them a bit harder than intended but it is not a big issue when used for SSB mode with no heavy audio processing. Running an SB-220 between 1200-1400 pep output and likewise for the others mentioned is perfectly acceptable. They can handle it on a casual SSB basis. CW would be a different story of course. Put to obtain these numbers the amp would most definitely have to be operated on 240v. Once these amps are pushed to these levels the first thing that goes to hell in the hand basket is the HV stability. If the amp is set to 120v these 1200-1400 watt numbers would be nearly impossible to obtain, but with a decent 240v line with #12 wire this is no problem. I generally advise not to even bother with the CW/SSB switch unless you are just learning how to load up an amp. Use the CW mode to get the amp at resonance and then switch to SSB and tweak the settings. Once you know your preliminary settings you can go right to SSB mode and load the amp up with no problem and the need for the CW setting is moot. Another thing, too many hams are anal about grid current especially with the 3-500 tubes. The 3-500 does not have a wimpy grid, they are pretty robust and they will have no problem whatsoever handling 300ma on occasional SSB voice peaks. Don't forget, the 3-500 can operate at 125-150ma of grid current, key down 24/7 so a total of 300-350ma for a pair on SSB is no sweat. No need to baby these tubes in these amps. Henry ran these tubes to produce 2000w PEP in their amps so you can see these smaller amps will never approach that number. Don't be afraid of loading that amp up quickly with a 100 watt carrier for max output and over couple the load a bit and run SSB with it. No problem.
    KK6VQK and WE6C like this.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Certainly as Lou mentioned, you should be running the amp on 240V. I'll assume you are, but maybe not. Let us know.

    The very shallow plate current dip when you're "fully loaded up" is normal with low-Q plate tanks such as we have in multiband amplifiers. I find an easier parameter to observe is grid current: When plate current dips, grid current peaks at the same tuning point and the peak is often easier to observe than the dip.

    Yes, LOADing is critical for linear operation and should be the last adjustment made, to tune the LOADing by reducing its capacitance slightly after achieving an output power peak, to bring down the Ig.

    Also as Lou mentioned, and I have also, three times now, there is zero reason to use the amp switched to "CW."

    The HV drop you see is very normal when the amp is run on 120V, might be slightly excessive if you're running it on 240V. I wouldn't replace the capacitors unless they need to be replaced. One way I "easily" check for excessive power supply ripple is to run the amp at full power (CW steady carrier) into a load and listen with headphones on a receiver to see if there's any detectable 120 Hz hum. A scope is better for this, but if you have hi-fi headphones that reproduce 120 Hz just fine (I use Koss PRO-4AAs, which let 120 Hz through without any rolloff at all), failing filtering in the power supply becomes audible as a slight hum on the signal.
  3. KK6VQK

    KK6VQK Ham Member QRZ Page


    I promise I'll try with the SSB switch today. I figured that at least since these tubes haven't been pushed hard in some unknown amount of time, I should really spend some time running the heaters first (which I did) and then push them a little less hard before I pushed them hard. I don't know how long they sat there unused before I acquired the amp.

    On a side note - why was the earlier limit "1KV DC input" ? Why wasn't it based on output power?

    Yes, I got 220v run upstairs. Yes, it's done safely. Holy crap there's some stupidly bad wiring in these old houses. Fixing it is expensive - I should've just bought a new tube amp. :)

    Ok, hm. So listening for ripple - is it just radiated 120Hz? I mean, I can hear a physical hum that's loud when the amp isn't tuned right and not-quite-so-loud when it is. Again, I have no baseline for what a correctly behaving linear amplifier is, so it's all new to me.

    Thanks for being patient all y'all.
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Because back in the 40s and 50s (and even later) very few people had an accurate way of measuring their RF output power.

    Measuring your DC Input power was easy - you just multiplied the PA voltage by the dipped PA current. (you normally knew what the Voltage was on any given rig, and you nearly always had a PA Current meter, so you could see what your input power was)

    Another reason was that - years ago - many transmitters were designed to feed 600 ohm or even higher antenna impedances (not 50 ohms) directly . . . so you couldn't actually measure your output power as you didn't know exactly what your antenna impedance was !

    So in nearly all countries, the power limit on different bands was quoted as maximum Input Power - not Output Power.

    Roger G3YRO
  5. KK6VQK

    KK6VQK Ham Member QRZ Page

    ah, that makes more sense. ok! thanks!
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    First, running the filaments only on 3-500Zs generally does "nothing." They don't create enough heat to cause a grid short, and that doesn't re-getter the tubes in any way since the Zirconium gettering material is applied to the anodes, not the filaments. AFAIK running the filaments without actually using the amp just helps shorten the tube life and make the filaments burn out a little bit faster.:p

    The 1kW DC input to the final stage rule was a long-standing Part 97 regulation, but it went away more than 30 years ago. After it did, most amateur amps no longer have any kind of "CW/SSB" switch.

    The 120 Hz ripple cannot be heard audibly from the amplifier -- if there's a filtering problem, it can be heard on the signal that's amplified by the amplifier because power supply ripple can modulate the carrier. You can only hear it on a receiver, and usually only with headphones.
    WA7PRC likes this.
  7. KK6VQK

    KK6VQK Ham Member QRZ Page

    ok, so it makes around 1000-1100w on 20m, and a bit less on 40/80/160. It just gets a crappy SWR match for 15/10, and it looks like some of the mica caps in that area are a bit cracked.

    So far so good. I'll go poke at it a bit tonight/tomorrow when i get some more time and see if it's input SWR related on those higher bands.

    The HV line sags from around 3500->2800v when TX'ing in SSB mode though!
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    Verify what the AC source is doing. If the lights in the room dim, that's a clue. If the AC voltage is fairly "stiff" under load, look downstream. :)
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ^Good idea.

    Remember the B+ is ~13x higher than the AC line voltage, so any drop in line voltage under load produces a much larger drop in HV.
    WA7PRC likes this.
  10. KK6VQK

    KK6VQK Ham Member QRZ Page

    house lights aren't sagging. :) I wonder about getting a power meter for this 240v circuit though to monitor it!

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