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 Post subject: A simple modification for the 6888 Transmitter
PostPosted: Jun Fri 07, 2013 8:22 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2384
Location: Saskatoon
I mentioned in another thread, that I had put together a 6888 transmitter using some parts sent by Bill Hamre. In amongst the parts were a few 1N4148 diodes. These are small signal diodes, and I immediately thought of a use for them. I previously built a Controlled Carrier AM transmitter that made use of the grid to cathode rectifier effect to make a diode clamp which provided the controlled carrier effect. That isn't possible on the suppressor grid of a 6888, but an external diode will accomplish the same thing, a good use for the 1N4148!

Why controlled carrier?
I found that this gives increased immunity to overloading/clipping and undermodulation from audio sources that are at different levels. Effectively, it does the same thing as a compressor.

Here is the modified transmitter:

Attachment:
6888ccXmtr02.PNG
6888ccXmtr02.PNG [ 30.92 KiB | Viewed 3162 times ]


Only four components have been added in two different locations (highlighted in yellow).

A) Diode clamp:
- 1N4148 diode
- 0.1uF capacitor
- 1 Meg. resistor

B) Bias Adjustment
- 270 ohm resistor

For the diode clamp, the time constant is set by the combination of the 1 Meg resistor, and the 0.1uF cap which gives a time constant of 100ms and works well with all the receivers that I've tested it on. These date from the mid 1930's to the 1990's, and all have an AVC time constant in the same ballpark as that of the diode clamp. (Very old receivers without AVC probably won't like controlled carrier.)

I found that in controlled carrier mode I needed to adjust the bias to about 7.5 volts to get the best range of modulation. The original transmitter circuit has the cathode bias fixed at a constant 5.1 volts due to zener, which also supplies the regulated voltage for the oscillator. I couldn't increase the zener voltage because it would fry the oscillator, so I added a 270 ohm resistor between the zener and the tube cathode. The oscillator still gets 5.1 volts, but the suppressor grid is now biased to about -7.5 volts relative to the cathode.

Here is a video showing a comparison of the two modes:
http://electronbunker.ca/DLtemp/6888CC07.mov
Note, the audio in the clip is as received on my Sony hifi receiver (8kHz IF bandwidth on AM).

It's a very short video, and you really need to spend more time listening in order to make a proper judgement. I wired up a DPDT switch to the diode clamp and bias resistor so that I could instantly switch from CC mode back to standard AM. Anyone who's built the original 6888 transmitter, may want to try out this modification and see what they think. It certainly solves the problem of having to constantly tweak the audio level (or having to put up with perennial undermodulation to avoid occasional clipping).

Anyway, have fun!


Last edited by BobWeaver on Jun Sat 08, 2013 3:48 am, edited 1 time in total.

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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Jun Fri 07, 2013 8:44 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2384
Location: Saskatoon
BTW, This shows how the DPDT switch is hooked up, in order to switch between normal AM and CC mode:
Attachment:
6888ccXmtr03.PNG
6888ccXmtr03.PNG [ 33.26 KiB | Viewed 3151 times ]

Switch is shown in normal AM mode position.


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Jun Fri 07, 2013 12:45 pm 
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Posts: 14106
Kudos Bob...

Nice to see the difference in what a closely matched coil will produce... I've known for sometime the tank circuit will pass more signal than the Pi net, but unless one is shooting for max range the Pi works very well... I'm still a big fan of your tapped toroid, been just what my VFO 6GY6 Tx needed... Can set it up for 1080 as easily as I can for 1500, works for that transmitter as it has osc adj from the front panel...

Maybe I'll set up my 6888 with a tank and check range... Generally I transmit from inside my garage but it has a aluminum vapor barrier under the vinyl siding that blocks the signal... I'd like to use a short tower I have(only 7ft) to mount a antenna but I'd have to feed it with coax but it has so much capacitance much of the signal will be absorbed...




Tom


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Jun Sat 08, 2013 6:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2384
Location: Saskatoon
Hi Tom,
Thanks for the comments. I think you meant to reply to this other topic that I started at the same time:
viewtopic.php?f=12&t=224490

I agree that the 6888 puts out a very powerful signal. Though I don't think I've had any problems with it overloading any of my receivers.

Anyway, if you get bored sometime, you may want to try the above mentioned controlled carrier modification on your 6888 transmitter to see what you think. The unmodulated carrier level drops quite a bit, and may work better with some receivers.


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Jun Sat 08, 2013 6:03 pm 
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Yeah Bob I was replying to the other thread... Looked at it late and I was tired, then by the time I got on the board next morning you's posted this one...

Anyway I'm going to work with the tank circuit a bit but will prob be on a 6GY6 Tx, I'm so pleased with the construction of my 6888 in the intercom case, I really don't want to change anything... It's the smallest tube transmitter I have and was just dumb luck the chassis I used fits so nicely(well after I hacked approx a 1/3 off it), depth is absoultly perfect...

BobWeaver wrote:
I
I found that in controlled carrier mode I needed to adjust the bias to about 7.5 volts to get the best range of modulation. The original transmitter circuit has the cathode bias fixed at a constant 5.1 volts due to zener, which also supplies the regulated voltage for the oscillator. I couldn't increase the zener voltage because it would fry the oscillator, so I added a 270 ohm resistor between the zener and the tube cathode. The oscillator still gets 5.1 volts, but the suppressor grid is now biased to about -7.5 volts relative to the cathode.


LOL I'd found same but limited bias to approx 6.5v in my experiments... The osc in my 6888 is supplied by a dedicated P/S so all I had to do was fiddle with the bias resistor... The 6GY6 I mentioned earlier has a pot in the cathode circuit to tweak bias...

BTW at the same bias and drive, these Tx will sound a little different using a sq wave vs sine...


Tom


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Jun Sat 08, 2013 7:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: aston, pa, usa
I'll send any member a 1N4148 who would like to try this. I'll try to scronge up the other parts too. If I have to reorder the 1N4148 they are only 2 cents each.

_________________
"All glory is fleeting" - George Patton
KB3QNN 73


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Jun Mon 10, 2013 11:50 pm 
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Location: Warner Robins, GA
35Z5 wrote:
Anyway I'm going to work with the tank circuit a bit but will prob be on a 6GY6 Tx, I'm so pleased with the construction of my 6888 in the intercom case, I really don't want to change anything... It's the smallest tube transmitter I have and was just dumb luck the chassis I used fits so nicely(well after I hacked approx a 1/3 off it), depth is absoultly perfect...Tom


If you do it on a 6GY6 let me know what the results are as if there is a great improvement over the PI net I'll mod mine.


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Feb Wed 26, 2020 1:23 am 
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Joined: Jan Sun 26, 2020 3:39 pm
Posts: 11
Bob,

I adapted your controlled carrier modifications that you proposed to Norm Leal's 6888 transmitter, applying those mods to Jon Stanley's 2-tube variant of that transmitter. The schematic illustrating the mods is attached. I found the 270 ohm cathode bias resistor value too high and lowered it to 180 ohms. I also added an active voltage bias to the 1N4148 clamper to compensate for its forward voltage drop in order to have the entire audio signal to the 6888 grid be a positive voltage. The bias is generated in a manner similar to achieving hte 5.1 V for the XTAL oscillator; i.e. a 1N4001 at the cathode of the 6SQ7. C3 is now populated with a 680 pF to reduce some of the hash observed at the cathode. R6 in Jon's original circuit is replaced with a 1N4001 to bias the clamper. R2 is changed from 100K to 1M.

Attachment:
File comment: Controlled Carrier Version of 6888 Transmitter
6888-original-3.png
6888-original-3.png [ 266.22 KiB | Viewed 574 times ]


Best regards,

Mike
AB1AW


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Mar Sun 15, 2020 2:52 pm 
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Joined: Jan Sun 26, 2020 3:39 pm
Posts: 11
I am creating a PCB for this 6888 transmitter circuit with the carrier control modification. It is based off the design from Catahoula Tech.


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Mar Sun 15, 2020 7:38 pm 
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Location: Mesquite NV 89027 (from Coventry, UK)
ab1aw wrote:
I am creating a PCB for this 6888 transmitter circuit with the carrier control modification. It is based off the design from Catahoula Tech.

That's good to hear. If I might make one suggestion: use a stereo input jack, not mono as Jon's did.


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 Post subject: Re: A simple modification for the 6888 Transmitter
PostPosted: Mar Sun 15, 2020 8:32 pm 
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Joined: Jan Sun 26, 2020 3:39 pm
Posts: 11
Yes, stereo rather than mono. Also replacing the large trimmer cap with a smaller version to make more room for other components on board. Trying to maintain same board size of 4" x 2".


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