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 Post subject: 6888 transmitter.
PostPosted: Jul Sat 20, 2019 11:21 pm 
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Joined: May Mon 27, 2019 3:18 am
Posts: 49
i built the 6888 transmitter circuit board from catahoula tech. having a few problems. using any kind of ac power supply results in annoying interference. both the recommended 6 volt power supply for the board and my bluetooth receiver. if i run the board off a battery and use my phone directly plugged into it no interference but the sound quality is lacking. how far does it have to be from the radio. radio is an rca radiola 82 that sounds perfect on a broadcast station. using a 6 foot wire antenna on the radio and about a foot and a half of wire from the transmitter. transmitter is just the bare board no box. transmitter is in another room about 15 feet away behind 2 walls. sounds the same in the same room a few feet away. what do i do to fix this.


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 Post subject: Re: 6888 transmitter.
PostPosted: Jul Sun 21, 2019 5:54 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2372
Location: Saskatoon
Not familiar with the Catahoula Tech board. Does it use a switching type power supply? If so, that would be the problem.

Also, what kind of antenna matching adjustment does the transmitter have?

Edit:
I found a schematic online:
Attachment:
Xmtr6888a.png
Xmtr6888a.png [ 30.54 KiB | Viewed 2289 times ]

From the component values for the C5/L2 plate tank it's going to be a very inefficient match to virtually any reasonable antenna, especially one as short as 18". I would try a much longer antenna, like 10 feet, and if possible, increase the size of the inductor L2, to about 470 or 680 µH.


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 Post subject: Re: 6888 transmitter.
PostPosted: Jul Sun 21, 2019 2:21 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 126
Location: Texas, U.S.A.
From previous study of this type of 6888 transmitter circuit, I have the following recommendations on modifying the Catahoula transmitter. These mods will improve the sound quality, increase the RF output and reduce the susceptibility of the circuit to spurious modulation that may be a contributor to the interference to which you refer. I don't have time right now to address other possible power supply related interference issues, since the 6 volt battery works okay I'd say use that for the time being. I'm sure that other forum participants can provide useful help.

1. Make R6, cathode bias resistor, 1.8k ohms.
2. Make C3, cathode bias bypass capacitor, zero uF (remove any capacitor from this location).
3. Make R5, grid resistor, around 50k to 100k.
4. Make R1, plate load resistor, 75k (68k to 82k will work if the 75k is not on hand).
5. Do not install C10, the optional 100 pF fixed tank capacitor.
6. Install C5, the variable tank capacitor - make it a 50 pF variable, 300 volt rating preferred, may get by 250 V. (100 pF variable will work if 50 pF not available, may be touchier to adjust). Physical size needs to be compatible with the existing circuit board, mica or teflon dielectric most likely candidate, or small air variable. The mica/screw trimmer type is suitable and commonly found second-hand. https://www.mouser.com/ds/2/384/d86717_ ... 334523.pdf
Any wire interconnecting leads used for C5 to PCB shoud be as short as possible and do not twist leads together.
7. Make L2, the tank coil, 270 uH (microhenrys). This is for a 10 foot whip (wire) antenna as suggested by Mr. Weaver, and with a 50 pF variable for C5 will permit tuning for antennas having capacitance from around 29 pF up to around 69 pF, assuming 25 pF minimum tank capacitance (C5 minimum plus stray). Suggested parts for L2: Bourns RLB0913-271K or Coilcraft RFC0807B-274KE.
Or L2 can be made a higher inductance variable and the antenna capacitance alone used as the tank capacitance per Mr. Weaver's suggestion. See July 23 post re Coilcraft parts.
8. Add a mica or ceramic capacitor of around 2000 to 5000 picofarads (pF), 200 volt rating or more, between B+ and the cathode (pin 5) of the 6888. Suggested part: TDK FG24C0G2W222JNT06. It can be placed on the back side of the board.
9. Make R2, 6888 grid resistor, 220k.
10. Make C8, low pass filter capacitor, 330 pF ceramic.
11. Add a 330 pF ceramic across R5 (for RF bypass), same type as C8.
12. Add a 1000 to 2200 pF ceramic across C1 (RF bypass). Suggested part: TDK FG24C0G2W222JNT06.
13. Add a ground wire from the ground end of J1 to the AC power grounding conductor.

Looking at the Catahoula documentation, it looks like the C10 and C17 reference designators are swapped between the schematic diagram and the circuit board. On the board layout they show, C10 goes to the antenna and C17 is the optional 100pF fixed tank capacitor. The reference designators used in this list of changes are the schematic diagram reference designators.

Eric LaGess
WB5HDF


Last edited by infzqi on Jul Sat 27, 2019 1:53 pm, edited 14 times in total.

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 Post subject: Re: 6888 transmitter.
PostPosted: Jul Sun 21, 2019 3:36 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17899
Location: Utica, NY 13502 (USA)
I was delighted to see that Catahoula gave ARF members credit for the original circuit design.

http://www.catahoulatech.com/products/K ... erData.pdf

I suspect their method of generating B+ using a switch-mode design could be the cause of interference when using a 6-volt wall-wart instead of a battery. The wall-wart would need to be a high current, regulated type. Otherwise, inadequate filtering from a non-regulated wall-wart may cause hum and noise to ride on the B+ rail.

I would be inclined to look at the wall-wart output and B+ rail with a scope to see what is there.

Attachment:
Catahoula Power Supply.JPG
Catahoula Power Supply.JPG [ 96.96 KiB | Viewed 2269 times ]


As with other low-power transmitters used within a house, re-radiation from the house wiring can cause hum interference.

Dave


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 Post subject: Re: 6888 transmitter.
PostPosted: Jul Mon 22, 2019 10:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2372
Location: Saskatoon
To clarify a bit more on the effect of the values of C5, C10 and L2: the short 18" antenna will look like a capacitor in parallel with the plate tank. The RF current is divided among the capacitors in proportion to their capacitance values. An 18" antenna operating at 1000 kHz will have a capacitance of about 5 pF. With C5 at its minimum setting of 50 pF and C10 at 100 pF, you have a total of 150 pF of non-antenna capacitance. All together, including the antenna the total tank capacitance is 155 pF. So C5 and C10 will take 150/155 or 96.8% of the RF current, while the antenna takes 5/150 or 3.2% of the RF current. So, only 3.2% of the signal goes to the antenna. The rest is wasted in the tank capacitors and doesn't get radiated. That's assuming that C5 is set to it's minimum value, the best possible scenario. In reality, to resonate the 100 µH inductor to 1000 kHz, you need a total capacitance of 253 pF. Subtracting the antenna capacitance of 5 pF you get 248 pF for the C10 & C5 combination, which means that C5 is adjusted to almost maximum, and you now have 248/253 or 98% being lost in the non-antenna capacitance, and only 2% getting to the antenna. The required resonating capacitance is so close to requiring C5 to be maxed out that, due to component tolerances, you may find that it's not even possible to peak such a short antenna.

Ideally, you would use no tank capacitors at all, and use just an adjustable plate inductor that can be adjusted to resonance with just the antenna capacitance. Then, all RF would get to the antenna (but not necessarily radiated; see note below). Since adjustable inductors having a suitable range are hard to find, the next best option is to use the largest value inductor and smallest trimmer capacitor that will resonate the antenna. To resonate an 18" antenna having a capacitance of 5 pF at 1000 kHz, you would need a bit more than 5 mH. This poses further problems, because a 5 mH inductor would likely have a self-capacitance in the same range as the antenna capacitance, and RF output is lost again. So basically, trying to get any range from an 18" antenna will be difficult. Going to a 10 foot antenna increases the antenna capacitance to about 24 pF at 1000 kHz, which has a much better chance of getting a decent share of the RF current.

Note:
In the above discussion, I talk about the RF current getting to the antenna. This is the only current that CAN be be radiated as an actual radio signal, but even then, only a small percentage of that RF current getting to the antenna WILL be radiated. The rest will be lost to antenna conductor resistance, ground resistance, and other miscellaneous losses. This is because the amount of power actually radiated is proportional to the antenna's radiation resistance divided by all resistive losses. The radiation resistance of an 18" antenna at 1000 kHz will be only a few milliohms compared to other antenna and ground losses which will be at least in the tens of milliohms, and could be as high as a few ohms. So considering that only a small fraction of the RF current that gets to the antenna will be radiated, you want to make sure that you at least get as much of the RF current to the antenna in the first place, and not dissipate most of it in the plate tank capacitors.


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 Post subject: Re: 6888 transmitter.
PostPosted: Jul Tue 23, 2019 7:01 am 
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Joined: May Mon 27, 2019 3:18 am
Posts: 49
ok. i think i understand. i will try a longer antenna. if that doesnt work i will try some of the component modifications. if i still get interference would a 6 volt transformer with a bridge rectifier be better than a wall wart. i think the wall wart is switch mode. i also get the interference with a bluetooth reciever on the input. how can improve that.


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 Post subject: Re: 6888 transmitter.
PostPosted: Jul Tue 23, 2019 4:45 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 126
Location: Texas, U.S.A.
Yes, do use the longer antenna. More inductance than the 100 microhenrys shown on the schematic for L2 will be needed in any case. Ideally, the capacitance of the particular antenna installed in the desired location would be measured by means of a capacitance meter or other method. Measure between the end of the antenna that will connect to the transmitter and the "GND" end of J1, with the power source connected but NOT turned on (not plugged in if no power switch is provided). Then the antenna capacitance and 1 MHz frequency can be used to calculate the required tuning inductance (coil L2 in this case). http://www.1728.org/resfreq.htm
I see a handheld capacitance meter listed on eBay, not a kit, for less than $15.

For L2, the tank coil, the Coilcraft "SLOT TEN" series is available. https://www.coilcraft.com/slot10.cfm#
Looks like Coilcraft is small quantity customer friendly, no minimum order quantity and prices are reasonable.

At 1 MHz, using the SLOT TEN-2-18 for L2 (418 to 927 uH), not installing C5 and C10 (per Mr. Weaver's recommendation) and assuming circuit stray capacitance of 15 pF would allow tuning antennas having capacitance from roughly 12 pF to 45 pF

At 1 MHz, using the SLOT TEN-2-16 for L2 (198 to 426 uH), not installing C5 and C10 and assuming circuit stray capacitance of 15 pF would allow tuning antennas having capacitance from roughly 44 pF to 113 pF

It would be a good idea to check with Coilcraft or make measurements of the self-resonant frequency vs. inductance of these coils, because the self-resonant frequency of L2 at the required inductance needs to be greater than the operating frequency.

Yes, a 6 volt dc supply as you suggest that uses a transformer at 50 to 60 hertz AC sinusoidal power frequency and a bridge rectifier will most likely produce much less spurious radio frequency noise than most switching type power supplies ("wall wart" type) that are commonly available. Looks like Catahoula specifies 2 amps needed.

I am not able to answer the bluetooth receiver noise question at this time, don't have enough information.

Connecting the board ground to the equipment grounding conductor of the AC power system is best practice and may reduce noise (see previous post listing recommended changes).

Eric LaGess
WB5HDF


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 Post subject: Re: 6888 transmitter.
PostPosted: Aug Fri 02, 2019 3:46 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2372
Location: Saskatoon
I made this chart today while updating my antenna matching webpage. It should help in determining the correct plate inductor value.
Attachment:
LoadingL_ft.png
LoadingL_ft.png [ 50.49 KiB | Viewed 1888 times ]

Pick the curve that matches your operating frequency, and then look up the optimum inductance for your antenna length. This will give you the best match using the minimum possible variable capacitor setting.
If your antenna length and/or transmitting frequency falls outside of the range of curves then it's a good sign that you're not going to get much, if any, signal radiated.


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 Post subject: Re: 6888 transmitter.
PostPosted: Aug Fri 02, 2019 3:48 am 
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Joined: Aug Wed 26, 2009 1:50 am
Posts: 1508
Location: Mesquite NV 89027 (from Coventry, UK)
Bob, thanks for that chart. Very useful.


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 Post subject: Re: 6888 transmitter.
PostPosted: Aug Fri 02, 2019 6:38 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 126
Location: Texas, U.S.A.
The loading inductor chart is very informative, thank you Mr. Weaver.

Eric LaGess
WB5HDF


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 Post subject: Re: 6888 transmitter.
PostPosted: Aug Mon 05, 2019 2:33 am 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 126
Location: Texas, U.S.A.
Example power output and tank Q conditions for 6888 transmitter circuit shown above, using the antenna alone as entire tank capacitance (C5 and C10 not installed).

Assuming 81 ohms total tank resistance including 3 meter vertical monopole antenna radiation resistance, ground and tank coil resistance and assuming 150 volts B+ supply. Using 530 uH L2 tank inductance at 1400 kHZ, from Antenna Loading Inductor chart above. Hopefully I haven't made any serious blunders.

1. 150 V x 0.9 x 0.707 = 95 V RMS across tank.

2. Inductive reactance of 530 uH inductor: 2 pi F L = 6.28x1400000x0.000530 = 4660 ohms.

3. 95/4660 = 20 mA tank circulating current.

4. Power = Isquared R = 20 mA squared x 81 ohms = 34 mW total power dissipated in antenna, ground and tank coil resistance.

5. 34 mW into tank from tube at 95 Vrms. Current = power / voltage = 34 mW/95 V = 357 uA

6. 95 vrms divided by 357 uA = 266,106 ohms resonant tank impedance

7. Tank volt-amperes = circulating current x RMS across tank. 95 VRMS x 20 mA = 1.9 VA

8. Tank loaded Q = VA/Load Power = 1.9/0.034 = 56

(Reference for 1 through 8: B. W. Griffith, Radio-Electronic Transmission Fundamentals, 1962)

9. 3 meter vertical monopole radiation resistance at 1400 kHz is roughly 0.08 ohm. 20 mA squared x 0.08 = 32 uW radiated.

10. Tank bandwidth = Center frequency/Q = 1400 kHz/56 = 25,000 Hz

11. The loaded Q is higher than standard transmitter practice, but for this very low power level the circulating current is still quite small. The bandwidth should be wide enough for good audio frequency transmission.

Edited to correct math error in RMS tank voltage calculation.

Eric LaGess
WB5HDF


Last edited by infzqi on Aug Sun 11, 2019 3:04 pm, edited 6 times in total.

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 Post subject: Re: 6888 transmitter.
PostPosted: Aug Tue 06, 2019 3:04 pm 
Member

Joined: Nov Mon 06, 2017 2:35 pm
Posts: 126
Location: Texas, U.S.A.
I'd be inclined to try this PI tank network for a new part 15 tube transmitter design, because it can provide the desired Q of around 6 to 12 and should provide improved harmonic suppression over the tank shown in the schematic posted earlier in this topic and over the L network. The values shown are for 1400 kHz and assuming a 25 pF antenna (3 meter monopole) and 100 ohms loss resistance.
The antenna alone is used as the Rl plus jXl block (the antenna capacitance is actually minus jXl load reactance and the coil resistance, ground resistance and antenna radiation resistance will be Rl). The plate is the Rs plus jXs. A ceramic or mica dc blocking capacitor of around 5000 - 10000 pF would be placed between the plate and this network. The B+ is supplied to the plate through an RF choke at least 2 mH such as a Delevan 2500R-42J (Mouser 807-2500R-42J). Zero source reactance is shown in the example but with the 5000 pF coupling capacitance at 1400 kHz, minus 23 ohms (-23) should be entered. I would try one of the Coilcraft "SLOT TEN" coils for the inductance, L.
Impedance transformation network calculator, J. Wetherell: https://home.sandiego.edu/~ekim/e194rfs ... cher2.html
Attachment:
1.4_MHz_PI__01.jpg
1.4_MHz_PI__01.jpg [ 37.98 KiB | Viewed 1710 times ]

Attachment:
1.4_MHz_PI__02.jpg
1.4_MHz_PI__02.jpg [ 39.58 KiB | Viewed 1710 times ]

Attachment:
6888_TRANSMITTER_IMPROVED_TANK_02.jpg
6888_TRANSMITTER_IMPROVED_TANK_02.jpg [ 101 KiB | Viewed 1592 times ]

Eric LaGess
WB5HDF
...


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