Tube Radio Forum - The friendliest radio forum on the net!
Would you like to react to this message? Create an account in a few clicks or log in to continue.

Recreation of a Knight kit broadcaster amplifier

Go down

Recreation of a Knight kit broadcaster amplifier Empty Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Thu Sep 03, 2020 12:47 am

I have always liked the novelty of a small AM transmitter. After seeing some videos of the Knight Kit Broadcaster Amplifier I wanted to try and make one, since I had most of the parts sitting around and a schematic in front of me. once I finished assembling it and worked out a few bugs the amplifier part of it works great! However I don't think is actually transmitting properly. the issue is that there is possibly a couple of variables. First the tuning capacitor is just stolen from another am  radio, but it does have the 470ish Pf maximum that the original one did. Second issue is that I had to make my own IF coil, which is where I think the issue may be. According to a website I found where someone had recreated the same circuit they used 80 turns for the CD coil and 45 turns for the AB coil (schematic is attached). In videos of real knight kits however it looks like there are far more than 80 and 45 turns on the coil, and I have no idea where to find specs for a coil. If anyone has any ideas on this or has ever worked on a knight broadcaster that would be cool.


Recreation of a Knight kit broadcaster amplifier Knight10

Recreation of a Knight kit broadcaster amplifier Am_tx_10

Recreation of a Knight kit broadcaster amplifier Am_tx_10

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Chas on Sat Sep 12, 2020 11:39 am

Very nice for a first post to this group, Welcome!

Any "replacement" vacuum tube oscillator coil would do the job.

To test, unless you are familiar to listening to an unmodulated carrier, using another radio the broadcasters output should be across the entire BC band and be tuned with the 470pf variable...
Be aware because this is a "warm" chassis circuit, there may be an unavoidable hum in the modulation.

If 400hz to 1khz audio can be used to modulate, using a 'scope it is possible to determine what level of input will yield 90-95% modulation as well as any distortion.

Because the oscillator is directly modulated there will be some "FMing", frequency modulation to the signal. For music, especially solo instrumental this may sound like distortion on the signal.

Should check the plate current for the oscillator, may find the output much greater than allowed under FCC Part 15 regs. FWIR is 100mw...

If the location for use is an isolated property. Though still not O.K.

Looks good!

Chas


Last edited by Chas on Sat Sep 12, 2020 12:35 pm; edited 1 time in total
Chas
Chas
Member
Member

Number of posts : 149
Age : 73
Registration date : 2008-07-24

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Recreation of a Knight kit broadcast amplifier

Post by Ed in W. PA on Sat Sep 12, 2020 12:18 pm

Chas, on the percent modulation measurement how would one determine the amount? Are you referring to using a trapazoid  display?

Ed in W. PA
Member
Member

Number of posts : 118
Registration date : 2014-02-17

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Chas on Sat Sep 12, 2020 12:36 pm

Yes...
Chas
Chas
Member
Member

Number of posts : 149
Age : 73
Registration date : 2008-07-24

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by AJ2X on Sat Sep 12, 2020 1:28 pm

Yes, ElectroMaker, that looks like a fun little project. Building it as the later "Knight Wireless Broadcaster -Amplifier" has a few advantages: the output transformer (to the speaker) allows you to test the audio amp separately from the AM oscillator; the output transformer replaces the harder-to-find 5.5H choke; and the dropping resistor in the filament circuit reduces the strain on the tubes from the now-normal 120VAC line voltage.

An oscillator coil (not an IF coil) can be found at Antique Electronic Supply https://www.tubesandmore.com/products/coil-miniature-adjustable-coil-70-oscillator, though it's $15 plus shipping. Winding one's own coils is educational and money-saving, though it's a bit of an art in itself. Salvaging an oscillator coil from an old AM radio would work too, but not all of them would work in this circuit.

Ask more questions, there are lots of answers here (and elesewhere).

AJ2X
New Member
New Member

Number of posts : 10
Age : 72
Registration date : 2018-09-11

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Recreation of a Knight kit broadcast amplifier

Post by Ed in W. PA on Sat Sep 12, 2020 2:25 pm

Chas, I am not shure how to connect the scope to get the correct pattern on the display. I think I saw it once in a AARL handbook but can't find it now.

Ed in W. PA
Member
Member

Number of posts : 118
Registration date : 2014-02-17

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by AJ2X on Sat Sep 12, 2020 5:02 pm

Trapezoid display on a 'scope is done with the modulating audio applied to the Y deflection plates of the CRT, and the RF to the X deflection plates.  No timebase needed.  https://www.testandmeasurementtips.com/analog-and-digital-modulation-and-modulation-measurements/  and https://www.radioworld.com/misc-1/measuring-modulation
Maybe not easy to do with many scopes nowadays.

AJ2X
New Member
New Member

Number of posts : 10
Age : 72
Registration date : 2018-09-11

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Sat Sep 12, 2020 7:20 pm

AJ2X, I have a couple of coils from an AM radio, though not very old, which should I use if either( picture attached, let me know if it doesn't work). I wouldn't mind winding my own coil except that I have no idea what the specs should be. I also should have mentioned that I attached two different schematics of a similar thing, my device is the top one.

Recreation of a Knight kit broadcaster amplifier Am_coi10

(for some reason I cant get this pic to be right side up)

Another question I have is about one of the 50C5 tubes I have. On a mighty mite tube tester the tube reads good emission, but when the tube starts to get hot, the tester says that there is alot of grid leakage. I am not sure weather the tester is accurate and if it is what it would do in the circuit.

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by AJ2X on Sat Sep 12, 2020 9:47 pm

The device in the can is most likely an IF transformer (usually 455 kHz or thereabouts). The smaller, open, one looks like an oscillator coil at least. It may not have two separate windings like your schematic requires -- some oscillator circuits use a tapped coil. You can check with an ohmmeter to see if there are two separate windings.

There are articles about coil winding all over the 'net if you are keen to learn how. Almost any Radio Amateur Handbook (published for over 90 years by ARRL) will have plenty of practical info about it. Ones from the 1950s and earlier are probably best for tube stuff.

As to the 50C5s, I'd try your "good one" in the audio section (V2) and see how it does with driving a speaker. Maybe keep an eye on the cathode voltage (across the 330 ohm resistor R10) -- it should stay pretty close (+/-20%) to the listed 10 VDC once the tube warms up. Then swap the tubes and see how the "leaky" one does. If it's truly leaky, then you'll need a replacement.

AJ2X
New Member
New Member

Number of posts : 10
Age : 72
Registration date : 2018-09-11

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Chas on Sun Sep 13, 2020 1:41 pm

Ed in W. PA wrote:Chas, I am not sure how to connect the scope to get the correct pattern on the display. I think I saw it once in a AARL handbook but can't find it now.
I can't remember the 'scope hookup either! I found it in a 'scope handbook (which one ?) I know I took a sample of the RF with a 1" link and a sample of the modulation using a ceramic capacitor voltage divider. I have a resurrected Globe Scout that long losts its cabinet and has been re-housed for rack. So, putting bulkhead BNC's and the "mods" was a non-issue.

I wanted to be sure I was looking at the audio at the last point of modulation practical thus the voltage divider was in the modulator plate circuit across the Heising choke. FWIR that is all there is. The scope CRT does the work, no amps as one amp would have to pass 50mhz and that was not going to happen with the old Millen scope. Horizontal and vertical plates. If the connections are reversed the pattern will look like a tree or a pyramid Rolling Eyes I had no usable RF at the scope terminals so a parallel tuned circuit was made with some 4 turns of #12 house wire and a 25pc variable cap mounted on a Pomona connector. So, simply plug into the Millen's front jack. Done... I think that was it without finding the book...

I was using 2.5khz amplitude modulated signal for the FAX transmission. I did not want to over modulate as distortions would show up on the FAX print as "dirt" in the images. I often sent sketches or photos along with hand written test...

Though my signal was perfect, the local 6M rag chew boys belly ached about the tweedle, tweedle the FAX made in their sets... Yes, I one made a transmission via meteor skip that stayed in long enough for a SWL'er in Minnesota to copy. I got his copy as a report. I decided that he may want the copy for posterity so I sent it back. I had the original scan Smile GL Chas WA1JFD

There is no reason why a similar monitoring of the modulation cannot be performed on a home built BC broadcaster. Chas
Chas
Chas
Member
Member

Number of posts : 149
Age : 73
Registration date : 2008-07-24

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Sun Sep 13, 2020 4:09 pm

AJ2X, I measured the voltage on the cathode of the two tubes and they are both within spec, the "good" tube measured around 9V and the "leaky" tube was around 8.7V.

The oscillator coil I have has two separate windings, one of the with a tap. Unfortunately though I think I damaged it whenever I took it out of the AM  because one if the 5 terminals does not measure resistance to any of the other terminals despite there being a wire attached to it, oops. one winding measures about 8 ohms and the other measures about 1.2 ohms, would it still work? I know the resistances are quite a bit different than the ones listed on the spec sheet for the coil you recommended.

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Sun Sep 13, 2020 4:34 pm

I found a listing for an AM oscillator coil on eBay for about 10 bucks. The listing says that one winding is 1ohm 3.15-3.97uH, and the other 5 ohms 126-226uH, this one could would work if mine doesn't.

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Chas on Sun Sep 13, 2020 4:53 pm

Electr0Maker wrote:...The oscillator coil I have has two separate windings, one of the with a tap. Unfortunately though I think I damaged it whenever I took it out of the AM  because one if the 5 terminals does not measure resistance to any of the other terminals despite there being a wire attached to it, oops. one winding measures about 8 ohms and the other measures about 1.2 ohms, would it still work? I know the resistances are quite a bit different than the ones listed on the spec sheet for the coil you recommended.
Some oscillators use a "gimmick" winding that couples the signal. It can be ignored if not required for the Knight circuit.

Keep in mind if the oscillator does not work, reverse one or the other winding connections, not both, to establish positive feedback..

A common failure for 50C5/B5/L6 is heater cathode leakage. None of these are "pricey" tubes choose any reliable vendor.

Try: http://www.findatube.com/

GL Chas WA1JFD
Chas
Chas
Member
Member

Number of posts : 149
Age : 73
Registration date : 2008-07-24

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by AJ2X on Sun Sep 13, 2020 7:49 pm

Chas beat me to the comment about the "gimmick" capacitive coupling some coils have. The resistance of the coil windings is, at best, a secondary indication of its construction, and of almost no use in determining this one's suitability for your oscillator. The principle characteristic of interest in a coil is its inductance. That, and the capacitance across it (or in series, depending on the circuit), determines its resonant frequency.

The equation you need (memorize this - it will be useful in radio work forever after) is F=1/(2*pi*SQRT(L*C)). F is frequency, L is inductance, C is capacitance. pi=3.14159etc. * means multiply, / means divide, and SQRT means take the square root. In words, it's "Frequency equals one divided by the result of two times pi times the square root of the product of Inductance times Capacitance." The formula can be rearranged so that if you know any two of the three variables you can find the third. I say all this because I don't know what you know about math and electronics formulas.

Your tuning capacitor is probably variable from about 35 pF to about 365pf, if it's like most AM radios made in the last 60 or 70 years. That's the value of the physically larger section. So, at maximum capacity (plates fully meshed) it would tune that coil on eBay (also at maximum, 226 uH) to 554 kHz. At minimum capacity (estimated to be 35 pF) it would tune to 1.79 MHz -- almost to the Ham 160 meter band.

Most phono oscillators like this Knight didn't tune quite the full AM band because they used readily-available oscillator coils rather than ones made for this purpose, but it covers enough for most folks' purposes. But you will see, if you play with the math, that you can generate signals over a wide range by adjusting the coil and the capacitor values. The eBay coil, by the way, appears to be adjustable with an internal core which is usually turned with a plastic hex-head tool to set it exactly on frequency. Don't be tempted to use a metal hex wrench -- the core is made of a fairly fragile powdered iron mixture. Buy or borrow a nonmetallic adjusting tool (commonly called a "diddle stick") of the appropriate size to screw the core in or out as needed.

The coil you salvaged from the radio might work on at least part of the AM band too, even if it doesn't have a core. Some radios were designed to use non-adjustable coils with no core. So I'd give that a try first. As Chas said, if it doesn't oscillate, then swap the ends (A and B, or C and D) of one of the coils.

Oh, and the higher-resistance coil (which typically has more turns) should be considered the C and D coil. That's the only use for resistance measurement in this case. More turns = more inductance.

Whew! That's a lot to take in, I'll bet. Good luck with your next steps.

AJ2X
New Member
New Member

Number of posts : 10
Age : 72
Registration date : 2018-09-11

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Mon Sep 14, 2020 3:14 pm

Thanks for all that information, I think I understand it all. I have worked a fair amount with modern electronics, but I am still getting to know tube stuff. I am also only 18 so I am certainly still a newbie on all this.

I do have a plastic diddle stick, I got a giant box of over 200 tubes a TV parts from family of friends a while back, while most of the tubes are not that useful it has tons of old parts and tools.

for the tuning cap, how much difference would it make if the two sections were in parallel?, resulting in a max capacitance of around 465pF, closer to what I have seen for the spec of the one in the original kit. When I use the formula it gives me a frequency of about 490kHz, which actually seems a little below the AM band.

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by AJ2X on Mon Sep 14, 2020 7:43 pm

Sure, both sections in parallel will be fine.  With the wide tuning range, you might find the ease of frequency setting a bit lacking, and also stability.  Most phono oscillators used adjustable mica padder capacitors, with limited range, both for stability and for lower cost.  A limited frequency coverage is not much of a problem for these devices' intended use.

I remember one of the upperclassmen in engineering school telling about building a wide-range, high power (2 watts) oscillator to revenge-annoy someone in a nearby dorm room who played his (AM) radio too loudly.  The oscillator's plate supply was taken directly from the AC line.  When the offending radio-user played a station too loudly, they blanked it with the 60Hz buzz from the oscillator.  If he changed to another station, they followed to buzz that one too, until he gave up, and peace was restored.  Just one possible (not condoned) use for a wide-frequency-range AM oscillator. lol!

AJ2X
New Member
New Member

Number of posts : 10
Age : 72
Registration date : 2018-09-11

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Transmitter kit

Post by tommytinker on Tue Sep 15, 2020 12:23 am

I built this same Knight Kit transmitter in high school in the 60s. I had a RCA othophonic HIFI record player and a Mike connected to it .It had about 12 or 15 feet of antennae wire out a window to a tree.Icould put a couple of LP s on and drive my 50 Chevy to a drive in hamburger place a block away and listen to it the sound quality was amazing. That was the max it would transmit. Sometimes my brother would play DJ on it while I drove around the Block.Those were the days......

tommytinker
New Member
New Member

Number of posts : 4
Registration date : 2013-02-05

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Tue Sep 15, 2020 12:32 pm

Once I put the new coil in, what is the best way to see if it is oscillating properly? I only have a USB oscilloscope.

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Chas on Tue Sep 15, 2020 1:51 pm

Electr0Maker wrote:Once I put the new coil in, what is the best way to see if it is oscillating properly? I only have a USB oscilloscope.
I suppose the USB scope could be used but do NOT make a direct connection to the scope leads or computer... Create a small coil, say 3/4" diameter of #18 solid magnet wire or any other insulated wire that gauge. Attach the coil to the scope vertical connections.

Or, if a portable radio is available set the radio for 1mhz and tune the Knight through the range. The radio will reply with a whoop.

Electr0Maker
I am also only 18 so I am certainly still a newbie on all this.

I built both the Knight T-50 and the Ocean Hopper, at 13, as a Ham. 

-- Keep asking questions, the web is a valuable resource. Find some older handbooks too.

Fair warning, this transmitter will have a higher output than permitted. Keep the antenna short, if the signal goes outside the property there can be problems...

Unless you live on a ranch with 100's of acres Smile
Chas
Chas
Member
Member

Number of posts : 149
Age : 73
Registration date : 2008-07-24

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Tue Sep 15, 2020 2:49 pm

Ok, so it works, It does transmit audio with reasonable volume and clarity, but it only works when I touch the big rubber wheel on the tuning capacitor.

I hooked a coil to my scope and I can see the carrier frequency that varies when I tune the capacitor. When I don't tough the rubber wheel the amplitude is about 120mV rms, but when it is actually transmitting it drops to about 5mV rms, any ideas?

obviously I am capacitively coupling something somewhere, but I don't know what or how to fix it.

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Tue Sep 15, 2020 3:07 pm

After messing with it some more I have found that it doesn't stop working, the frequency just shifts upwards 5kHz or so. While it is working there is alot of distortion when I turn up the volume on the transmitter and the "whistle" is very loud, perhaps what I was seeing on the scope?

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Electr0Maker on Tue Sep 15, 2020 3:29 pm

After messing with it some MORE!(I promise I wont do this too many more times Smile ) I got it to sound really good, first I move the audio source away from the antenna and then turned the volume of the audio input up a bit so that the high gain in the transmitter didn't factor in so much. With that it sounded pretty good but it would still sometimes have interference depending on what was where, so I grounded the chassis, which looking at the schematic doesn't look like a good idea but it sounds like a good idea (get it Laughing ).

I am using an old Ipod for the audio, which may not help with some of the noise issues, but I was wondering if I should be careful what I plug this into. I don't know if there are any weird invisible voltage potentials anywhere or if it would be perfectly fine to plug this monstrosity into my PC. I am probably just worrying about nothing but you never know.

Electr0Maker
New Member
New Member

Number of posts : 9
Registration date : 2020-09-03

Back to top Go down

Recreation of a Knight kit broadcaster amplifier Empty Re: Recreation of a Knight kit broadcaster amplifier

Post by Sponsored content


Sponsored content


Back to top Go down

Back to top


 
Permissions in this forum:
You cannot reply to topics in this forum