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Home Brew Amplifier First Start

Discussion in 'Amateur Radio Amplifiers' started by KC0VVB, Nov 5, 2019.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The parasitic suppressor resistors aren't a very critical value; I've used anything from 47 Ohms to 150 Ohms and they all seemed to work okay. But since original carbon comps have gone the way of buggy whips, and they drift with temperature and age anyway, the new Ohmite OYs seem not only a good replacement, but a better one.

    The original T-bolt wasn't difficult to tune at all. The LOADing of course had two separate controls, coarse and fine, but once you have defined what is needed for your particular antenna situation, you know how to pre-set those to save time. For me it was easy to tune by just watching grid current: Dip the plate tank by watching for a grid current peak, then if that was more than about 10mA, just crank in more LOADing to bring it back down. 650W output was easy while running grid current at nearly zero, and screen current would be somewhere around zero also (it could swing slightly positive or negative with variations in drive).

    I drove mine with 100W rigs for years, just using a 10 dB 100W attenuator in line. Although I built my own attenuator, commercial ones that are good from DC through UHF are all over on the surplus market cheaply today and with nice finned aluminum heatsinks they can be pretty small compared to what I did.

    But you've now got a GG amp and hopefully will get it going properly.

    The description of how your amp came to you originally sounds like it fell off a truck! (Maybe it did.):p
     
    KD2ACO likes this.
  2. KC0VVB

    KC0VVB QRZ Member

    Steve after your suggested changes both tubes lit up 120 ma on the grid
    UP to 500ma on the plate 400w cw carrier output at 3885
    2800 volts at idle drops to 2400 volts in transmit
    Of course I will have to lower the drive on the ranger to about 20 watts to give 200w carrier out and allow for some headroom on AM.

    In your opinion Steve what are some safe numbers I should be looking for as far as tuning.
    Not much response from plate coupling or loading capacitor into the 50 ohm dummy load. IMG_0793.JPG IMG_0794.JPG IMG_0795.JPG
    Erik KC0VVB
     
  3. KC0VVB

    KC0VVB QRZ Member

    Before I gave this old amp a rest for the night i got some tuning numbers off the meters.
    50 cw watts carrier rf input from the ranger. 480 ma on the plate, 180ma on the meter that
    shows g1 and g2 combined with the switch in the grid position so i will just call it grid.
    500 watts out on the mfj-267 peak reading watt meter.
    I am getting a response from the plate tuning and load coupling capacitors.
    They each want to be meshed about 20 deg.
    When i turn down the drive on the ranger to 20 watts or so by turning down the actual drive knob I do show 200w carrier through the amp
    The grid curent and plate curent meters are modulating. The peak reading watt meter is not modulating to
    800w peak on AM. It probably will when it cools off right now its hot enough to fry eggs on.
    Do these seem like good numbers? Is there something i need to look out for and adjust?

    Erik KCOVVB
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Your numbers seem in the ballpark.

    To test for stability, I'd try it on all bands, 80 through 10 meters, adjusting the tuning both with drive (to peak output and assure efficiency is about the same on all bands as it should be) and also without drive, but with the amp "keyed" to produce idling current, tuning the tank adjustments through their range to assure the amp doesn't try to oscillate anywhere.

    On AM, the grid and plate meters should not change with modulation -- that would be weird. A typical wattmeter shouldn't, either, but a real PEP wattmeter design should. If the MFJ is a powered, real peak reading meter, it should indicate output power that tracks modulation; but normally neither plate nor grid current would change (on AM, with a steady but modulated carrier).

    Not sure why it's getting so hot. Remember for AM work, you need to tune up at full PEP power level, so if you expect to achieve 800W PEP output (not sure if it can really do that, but maybe), you'd tune up at 800W steady carrier output, then reduce drive to 200W output and should have reasonably full modulation. You NEVER tune up a linear amplifier at "operating carrier power" for AM work; that could be very damaging to not just the tubes but the whole plate tank, especially the tuning capacitor.
     
  5. KC0VVB

    KC0VVB QRZ Member

    Got it Steve thank you. An 80 watt rig may put out 800w carrier. If the little peanut whiste ranger puts out 50 watts max carrier for 500 watts out. I am ok with that I should then drop the drive to 125 carrier out. To be comparable with a Viking valiant Or a Apache . That should be enough to get me half way across the country with band conditions the way they are 73 thanks again
    Erik KOVVB
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yep, I agree.

    The amp (the "original" T-bolt, not modified in any way) could run 150-175W carrier power on AM, producing 600W-700W PEP output on AM if powered by a stiff 230V (now 240V!) line. And it could do that with less than 10W drive.

    I actually did have an Apache at the same time, and the difference on the air between the T-Bolt running full blast on AM and the Apache just operating normally wasn't much, maybe a dB or two.

    On a band like 75m, the antenna does all the work! (That's really the case on "any" band, but on 75/80m the antenna needs to be a lot bigger and higher than it would for higher frequencies, so if I wanted to really get out on 75m I had to put more work into the antenna system than on other bands.)
     
  7. K5HU

    K5HU Ham Member QRZ Page

    Hi Erik, KC0VVB
    I could almost feel your excitement, as I recalled building a 4-811A amp in 1975. The following year, I converted a T-Bolt for Billy Byrd, WA4HGN. Recently, I made some changes to the schematic diagram and revised the accompanying article. Please visit my blog to see what’s involved. …glad that you felt encouraged to try!

    I wrote this procedure without having a T-Bolt available for inspection, so please answer some questions. Did you find any errors on the schematic? Are you operating with the swinging choke L101 installed? Did the tubes idle properly at zero bias or did you need to include one or more bias diodes? Did you get all issues resolved and did it finally meet your expectations? 73, Bill K5HU

    Johnson Thunderbolt Linear Amplifier - Grounded Grid Conversion - 4-400A or PL175A Tetrode Tubes
    https://hamradioskywave.blogspot.com/2018/12/johnson-thunderbolt-linear-amplifier.html
     
  8. W2VW

    W2VW XML Subscriber QRZ Page

    Hi Erik,

    If the amplifier is now OK for use then the next task is to come up with a way to reduce Ranger drive power on AM.

    First it's good to be able to tune up the Thunderstein with 75-125 watts drive.

    The amplifier tuning should be left alone and AM carrier drive reduced to the ballpark you mentioned. 20 watts or so.

    The Ranger will not make proper am when the drive control is reduced enough to make output drop.

    The Ranger final needs grid current in order to behave as a square law mixer.

    Check the Ranger manual again to refresh memory.

    There are several ways to reduce drive power and still allow the Ranger to make proper AM. Proper meaning minimum possible distortion for the circuit.

    Some people have used a Tee in the drive cable and a dummy load. The Ranger has a wide range output network and will work fine into a wide variety of RF loads. The key is to load it up at rated plate current.

    Another method is to reduce plate voltage on the Ranger final by using the low voltage power supply for just the 6146 and leaving the modulator at the factory high voltage. Mic gain will need to be reduced. This method increases the dynamic headroom of the modulator and should reduce transmitter distortion. It also has the advantage of maintaining the factory modulating impedance which also means modulation transformer ratio.

    Another two methods which seem popular on the internet but not heard a lot on the air is to simply reduce the screen voltage on the Ranger's 6146 or just underload the final. These reduce output power but also increases the modulation transformer's load impedance by more than twice in most cases. The two solutions are poor at best but ok for the very short term or a dummy load test.

    Hope to hear you on the air soon.

    Dave W2VW
     
  9. KC0VVB

    KC0VVB QRZ Member

    It is an honor and a priveledge to hear back from all the big guns, Especially Bill K5HU the designer and the man him self. The rig worked perfectly i went as far as i could go with it in December I was missing pieces of the RF deck the band switch that controls the tuned input.
    The load switch for the capacitors above the tuning coil. The gear drive and pulley system for turn counter on the plate tuning capacitor.
    It wasn't real practical to use on a daily basis. I found a donor rig on ebay tha of course has all the transformers and everything else that goes wrong missing but otherwise seems to be original, The rf deck looks really good. I plan to finish up the modifications when i install
    The remaining parts.
     
  10. K5HU

    K5HU Ham Member QRZ Page

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