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Single 8417, "unknown linear".

Discussion in 'Amplifiers' started by LeapFrog, Mar 12, 2017.

  1. LeapFrog

    LeapFrog
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    Rare Vocoder Pepe

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    Hello everyone, I have a friend who came across an amplifier.
    I have not found any model numbers or labels on the outside of the unit, the meter says "Sonar Radio Corp" on the insert.

    It uses a single 8417 tube, and it has a "Grand" transformer.
    I noticed a makes-shift nut on the ground terminal.
    The unit hums to life when the ?main? button is pressed, the other button *seemingly* does nothing.
    I have no information on this amplifier, does it look familiar to you?

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    Sorry for the poor quality photos.
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    .

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    Any information on what this is, and how it was ran back in the day, would be greatly appreciated!
    Thank You
    &
    73
    -"LeapFrog" aka Sam
     

  2. nomadradio

    nomadradio
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    It's a Black Cat JB-150.

    The oddball knob on the rear is the carrier-drive adjustment. Should be set to keep the carrier below about 10 Watts. Will typically swing over 100 Watt PEP. A hot 8417 tube might get you 150 Watts PEP.

    They were popular as drivers for ham-type linears. Especially when using a radio that had no way to adjust the carrier power, like a tube-type base radio.

    It's a genuine hot rod. Crank the carrier too high and the tube will melt.

    Literally. Too much carrier, or a radio that's too large will melt holes, or at least cracks in the tube's anode.

    Thought I had a pic of that, but I'll have to track it down.

    Yours is a "late" version that has a 12-Volt relay and a transistor keying circuit. The older ones used a relay with a 110-Volt DC relay coil and a tiny 6AQ5 tube to key it.

    The outer button is the power switch. The other one should be for standby.

    Bad filter caps are normal for an amplifier that old. The 1000uf 25-Volt cap that filters the power for the relay will frequently cause the keying circuit to behave in quirky ways.

    If you hear a "hum" in the room when it's turned on, there could also be a bad rectifier diode in the high-voltage.

    73
     
    543_Dallas and LeapFrog like this.
  3. LeapFrog

    LeapFrog
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    This is a wealth of knowledge in one post, thank you nomadradio!
    I cannot get a clean waveform with the carrier set below 30-40 watts, that's with a clean radio going into it.!?! It wants to swing.. a lot? ..And it is pinching the negative peaks I am going to lower the modulation and try again.

    Got it semi-clean, it really needs close to no audio to modulate the carrier!
    Thank you :)
    -LeapFrog
     
    #3 LeapFrog, Mar 12, 2017
    Last edited: Mar 13, 2017
  4. Jimbo165

    Jimbo165
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    Black Cat--Seeker or Wawasee all same amps basically. I have about 5 or 6 of these. They will increase modulation so do not use them with a power mike or mic gain turned up too much because it will cause a squeal, fuzziness or over modulation they increase modulation themselves. Also do not use a radio that dead keys much over 4 watts or swings over 25 watt pep into amp because it just will not work as it should or last. These amps I have found are fantastic for a low output radio. The swing is caused by the dead key setting of radio or amp. I use a radio that dead keys 2 1/2 watts and then set Black Cat at 3 watts and that seems to work the best for me. Depending which one I am using Seeker or Black Cat I can get 50 watt pep or 100 pep output. I also have never been able to get these amps to work on Sideband .
     
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  5. LeapFrog

    LeapFrog
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    Running a tame Cobra 89 XLR into it, I set the box for 8.5-9 watt key and let it swing to 40+ on the peaks, it really does modulate the signal to an extreme, peaks are off the scale (lol not literally) on the scope but the owner is happy and wants to run it like that, i'm glad I had a chance to have one on the bench! Neat little unit for sure! I don't think it is setup for SSB, something tells me this box is a class-c with "modulator" in the pet-name.
    Thank you gentlemen, 73.
    -LeapFrog
     
    #5 LeapFrog, Mar 14, 2017
    Last edited: Mar 14, 2017
  6. nomadradio

    nomadradio
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    This what the old-timers called a "Modulator", or "Modulation Box", or "Swing Box". I remember hearing this design called a "Mule Box" because of how your wattmeter would "kick like a mule" when you speak.

    Never have found out how the term "Jewel Box" got associated with them. Maybe because the cabinet is not much bigger than the better half's box of jewelry? Maybe? I'll probably never really know how that name caught on.

    I am pretty sure that's what the initials "JB" found on Black Cat amplifiers is supposed to mean.

    The tube is biased way below cutoff. This permits you to use a much higher power supply voltage than the tube would normally tolerate without immediate damage. A PA amplifier meant for a high-school gym in 1955 might use two 8417 tubes running at 450 or 500 Volts. The JB150 puts 800 Volts on the tube's plate circuit. This vastly improves the tube's peak-power output.

    Your carrier is only partly amplified, since only the upper part of the carrier's waveform can turn the tube "on". The high negative bias cuts off the lower part of the carrier waveform. But that's not so bad by itself, since the tuned circuit in the tube's output side restores the 27 MHz carrier back to a RF sine wave, more or less.

    The real action occurs when you modulate the carrier. The audio waveform is "riding" on the reduced carrier, and gets amplified at full blast, as though the carrier really were turned up to 20 or 30 Watts.

    Gives it that "asymmetrical" audio 'swing'. The tube's power gain applied to audio peaks is several times the gain that is applied to the carrier alone. This is where the asymmetry happens, in the tube's grid circuit. Adjusted correctly, the receiver at the other end hears a higher level of modulation than he did from the barefoot radio.

    Sure, it's a primitive way to achieve this, but it was cheap and sounded good enough for the old tube operators. And it permitted you to adjust the carrier level feeding into your "big" box down to a safe level that wouldn't melt your 3-500 tubes. Variable "key" on a tube radio can be done, but it's tricky if you want to retain full power on modulation. Turning down the carrier in a tube CB generally turns down the peak power in direct step with the carrier.

    Or you could use it to "nitrous" your D&A Phantom 500. Wouldn't wear out the tubes the first day, maybe not the first week if you kept the carrier turned low enough. But the impressive wattmeter readings still took their toll on tube life. But for a wattmeter-worshipper, this was pure gold.

    Looks a bit odd on a 'scope, but the base-station operators 40 years ago didn't use one.

    Bear in mind that a large radio 40 years ago would bust a gut to show 18 Watt peaks on AM. A typical low-priced tube-type base radio would show a 3 Watt or 3.5 Watt carrier, and maybe 14 Watts PEP. If the final tube was a little tired, an average-reading wattmeter would show backwards swing on modulation. The JB would fix that. This is the drive level that the JB150 was optimized for. The smaller the radio, the bigger this thing makes it become, more or less. A modern radio with more than 20 Watts PEP on AM will make it work too hard, unless it's modified to tolerate more drive power.

    But what you're seeing on the 'scope is probably what it's supposed to do.

    73
     
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  7. StrangeBrew

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    A bit off topic but do the transistor type "modulators" also work in this manner?
     
  8. LeapFrog

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    Thank you nomadradio, I was concerned but your description explains it well.


    73
     
    #8 LeapFrog, Mar 15, 2017
    Last edited: Mar 15, 2017
  9. nomadradio

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    Never have seen a method to make transistors do this same trick. It's a tube thing, far as I know.

    73
     
  10. StrangeBrew

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    OK thanks, I have a transistor modulator here and was just curious about the way it worked.
     
  11. LeapFrog

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    I think if a high gain transistor is used like the 2SC2290, maybe the box produces more PEP & "adds modulation" like the older modulators do, but I don't know for sure... Yet
     

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