Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Sep Tue 18, 2018 5:16 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 27 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Wed 21, 2018 2:07 pm 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5615
The amateur move from AM with full carrier and both sidebands to only transmitting one sideband made a lot of sense from the resource utilization viewpoint in the 1960s with consistent heavy HF band utilization even when there weren't contests and operating activities. With the environment today there would have been less incentive for the move because outside of contests finding an open space to operate is rarely an issue.

I am not sure whether 2 meter FM operation should be viewed as sad or humorous with the huge number of repeaters that were installed. This large installed base required making sub-audible tone controlled access a necessity to avoid the issue of "keying" numerous repeaters but now many of those repeaters are silent except for periodically identifying themselves with even the no code types preferring cellular to 2 meters. I expect many of those will be abandoned out of neglect as the amateur population continues to age.

I would have loved to have been around to hear one of the popular contest weekends when AM was still king because the hetrodyne interference must have been phenomenal. When I first got involved in contesting in the mid 1970s it was pretty common to use the 1.5 khz filter in the Drake C line for 15 and 20 meter SSB contests because the band was so packed that the very restricted audio bandpass of this filter allowed pulling out stations that were otherwise unintelligible. Using a filter that was primarily intended for wide shift RTTY on SSB didn't result in comfortable listening but it was effective and resulted in intelligible voice communications once the passband control was set properly.

SSB certainly made legal limit voice operation more practical allowing a much smaller and lighter transmitter. Even with tube type technology a then legal limit Hallicrafters SR-2000 and power supply is pretty compact but a legal limit AM rig was anything but light or compact.

I am not at all interested in the extremely wide SSB audio groups that popped up in the last few years but providing selectable narrow (somewhere around 2.1 or 2.4 Khz. bandpass) or wider (maybe 3.1 Khz.) would be reasonable for SSB operation. Swan rigs with their wider than typical 2.7 Khz. SSB filters are known for their pleasing audio.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Wed 21, 2018 5:56 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 270
Jthorusen wrote:
Greetings to 10.7Megahertz and the Forum:

It might be a good idea to go back and review classes of operation and what they mean. The idea when amplifying audio for the purpose of modulating a transmitter is to produce audio of sufficient power to do the job with minimal distortion. How one goes about this is dependent purely on one's preferences. It is perfectly feasible to have a class A amplifier to provide the modulation power... and it would sound great. It is also quite wasteful of power and requires the largest investment in power supply, tubes and other components because of the low efficiency and the higher DC power required to overcome this inefficiency.

Classes Ab1, Ab2, etc. are modifications of the bias point for what is essentially a class A amplifier. They give up a certain amount of distortion in order to get better efficiency with the accompanying lower cost.

Class B amplifiers are biased at cutoff. Therefore, they can amplify only 1/2 half of a sine wave; the other half (which makes the grid even more negative) is lost. The resulting distortion is severe. However, by employing two tubes in a "push-pull" configuration, each tube amplifies 1/2 of the sine wave... one amplifies the positive going half while the other amplifies the negative going half. In this way, one can realize the efficiencies of class B (no idling current) and still have relatively low distortion. Note that one can bias a push-pull amplifier as any of the variants of class A if one wishes; but the trade-offs are distortion versus efficiency. Since most tubes are a bit non-linear just coming off of cut-off, a pure class B push-pull amplifier will have a bit more distortion than one that is biased so as to have some idling current.

Class C has the tube biased well beyond cutoff and cannot amplify a sine wave without severe distortion. The output of a class C amplifier is a series of pulses representing the most positive peaks of an input sine wave. Thus, a class C amplifier is wholly unsuitable for audio. However, when a class C amplifier has a resonant "tank circuit" as its output circuit, the "flywheel effect" of a resonant circuit will reconstruct the sine wave from a series of pulses, just as a child's swing describes a smooth motion even though an adult pushing the swing applies only brief pulses of power at only one part of the swing's travel.

Note that this effect occurs at a single frequency (or very narrow band of frequencies) where the tank circuit is resonant. Since we are transmitting RF at a single frequency, this works fine. Audio, however, represents a frequency variation of several octaves and cannot be reconstructed by a resonant circuit, so class C is strictly RF.

So, the class of operation of a modulator stage is completely independent of the class of operation of the stage it is modulating.... it depends only on amplifier configuration... a choice of the designer.

Regards,


"Class B amplifiers are biased at cutoff. Therefore, they can amplify only 1/2 half of a sine wave; the other half (which makes the grid even more negative) is lost. The resulting distortion is severe."

This is true and also the fact the class B modulators are driving a class C RF amp means the distortion isn't an issue, as you already explained above. Otherwise it would be. I know what you mean, though. It's stated that class AB1 amplifiers never draw grid current but the class AB2 amplifiers do draw grid current. I never really understood that till recently but found a chapter where it simplified it. The grid can be driven very positive (more so with class B) in such a way as it draws current from the cathode. Then, as you say, with very strong negative cycles the tube will be cut off.

For some reason, all of the modulator circuits described in my 1960 ARRL are class B and what struck me was they urged grid bias only through the C battery. Or just dry batteries. So, when I noticed a few circuits with the modulator tubes having resistance bias I started to look up some of the tubes used.

Just to get the best idea I can, I started to toy with how I'd put together a fairly simple RF transmitter, using grid modulation. I was thinking today the book may well overcomplicate some of this with in-depth theory. I felt there's an easier way to explain how grid modulation works but anyway.... Sure, I did the digits my own way to add the required value of watts to the plate circuit of the class C RF amp. I even did the transformer ratios. One thing too I'll share that struck me. And this is thought-provoking. If you added 50 watts modulation power (from the modulators to the class C amp with a plate dissipation of 100 watts), the multimeter won't register any change in the RF amp plate voltage. Given it's AC voltage imposed on DC.

Of course, there will hopefully come a time when I make something practical. For now, I find it helpful to just run through a few figures and circuits and think about how I'd possibly go about it. There's an interesting design for a very low watt modulator in my ARRL that uses a phase splitter as the driver for push pull modulators (and I think the tubes may be something like 2 AX11s???).

By the way, I'm not strictly a Ham enthusiast but I like to compare a lot of different aspects of radio. The ones I actually work with tend to be mostly antique AM LW SW receivers so pretty much never come across BFOs or spotters.


Top
 Profile  
 
 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Thu 22, 2018 2:06 am 
Member

Joined: Dec Sun 14, 2008 3:33 pm
Posts: 718
Location: Tokyo
>I would have loved to have been around to hear one of the popular contest weekends when AM was still king because the hetrodyne interference must have been phenomenal.<

Yes, it was. And you didn't have to wait for a contest weekend. Just about any evening on 80 or 40 in northern NJ could be bedlam. 80, especially. My carrier controlled screen modulated HT40 wasn't remotely up to the competition. So my friends and I used 20 meters for chatting. 20 was dead after dark back then: nice and quiet and local.

Last night, here in Tokyo, about 8pm, I did a sweep, starting at 160: surprised to hear one station, CW, calling CQ. Apparently gave up after 10 minutes or so. Surprised because there's usually nothing on 160.

80 meters? One very weak SSB'er. That's all.

40 meters? nothing at all, no CW, no SSB.

I never liked contests back in the 60s. But I love them now (for listening, that is). Very popular in Japan. The frenzy really takes me back. But after a few hours, it's over and the ham bands are like graveyards again.

Rob


Top
 Profile  
 
 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Thu 22, 2018 4:23 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 2967
Location: Lincoln City, OR
Greetings to 10.7Megaherz and the Forum:

Regarding the following quote:

Quote:
This is true and also the fact the class B modulators are driving a class C RF amp means the distortion isn't an issue, as you already explained above.


Careful here. Distortion is VERY MUCH an issue in any modulator. If you will examine the circuits for class B modulators, you will find that they are INVARIABLY push-pull requiring two tubes to deal with the distortion issue. You will never find a single-ended class B modulator circuit because of the inherent distortion.

The application (plate modulator or public address amplifier) has nothing to do with the class of operation and its attendant distortion. Many low to medium power discrete modulators of the 1950's and early 1960's had multiple taps on the output transformer, allowing them to be used as either plate modulators or audio power amplifiers.... for public address or hi-fi.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Thu 22, 2018 4:35 am 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5615
Rob,

I am sorry to hear that the ham bands are also vacant in Japan.

In 1975 I remember searching for a clear spot on the lower end of 15 meter CW during morning openings to Europe on weekdays and it was a big incentive to get my extra class ticket. Now instead of searching for a space I am searching for a station. I aligned my Yaesu FT-736R last weekend and today I had it sitting in my upstairs office tuned to what used to be the main repeater (I guess it still is) located 25 miles away. I was processing photo images for close to three hours this morning during the peak "drive time" and after and the only activity was the repeater going through its ID process. I was never a heavy user of 2M FM but it used to have a lot of activity.

While I was a novice I worked quite a few of the JA hams with their fishing fleet who were running with their low power license class. They would be some of the few stations heard in the old 15 meter novice band (21.1-21.2 Mhz.) when it began closing for the night.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Fri 23, 2018 6:39 pm 
Member
User avatar

Joined: Jan Mon 22, 2018 5:59 pm
Posts: 270
Jthorusen wrote:
Greetings to 10.7Megaherz and the Forum:

Regarding the following quote:

Quote:
This is true and also the fact the class B modulators are driving a class C RF amp means the distortion isn't an issue, as you already explained above.


Careful here. Distortion is VERY MUCH an issue in any modulator. If you will examine the circuits for class B modulators, you will find that they are INVARIABLY push-pull requiring two tubes to deal with the distortion issue. You will never find a single-ended class B modulator circuit because of the inherent distortion.

The application (plate modulator or public address amplifier) has nothing to do with the class of operation and its attendant distortion. Many low to medium power discrete modulators of the 1950's and early 1960's had multiple taps on the output transformer, allowing them to be used as either plate modulators or audio power amplifiers.... for public address or hi-fi.

Regards,


I am still pondering the basic circuits given in my 1960 ARRL book, which is now starting to fall apart at the cover as it's an old book. Now, the circuit I briefly viewed last night has the modulator push-pull tubes classed as AB1 but the voltage for the modulator tubes is externally supplied and pretty high. Likewise the screens are externally supplied via a couple of those gaseous regulator tubes. The driver and AF amplifier tube were supplied at lower voltage.
Although I don't actively work with assembly of HAM radio equipment, I do find parts of the information are useful at a more general level. For example, I was intrigued to find a very neat way to bias grids in a push-pull circuit. This was simply through the use of a selenium rectifier where negative voltage was allowed to build up at the - side of an HT capacitor and sent between a variable resistance and the chassis ground. Once the correct bias setting was found, it would be directed to the grids of the output tubes to establish the bias. It's easier to find this in the book itself than follow my explanations.
By the way FM reactance modulation is mentioned too but this appears to be limited by loss to the carrier watts. Given not much space is dedicated to FM, I imagine not many Hams used it (when they had SSB). I leave you a pic of a radio I have in storage and then found it's dated 1947. I've not done anything to it yet as I'm snowed under but it should be fun to fix up one day. Koster Brandes L.T.D. CR20 manufactured in Great Britain (assumed date1947). Tubes: 6K86, 6K876, 6Q76, 6V6G, 6X5. IF is 465 Kilohertz.


Attachments:
vra1.jpg
vra1.jpg [ 32 KiB | Viewed 244 times ]
Top
 Profile  
 
 Post subject: Re: SSB Explanation in AARL
PostPosted: Feb Sat 24, 2018 2:18 am 
Member

Joined: Dec Sun 14, 2008 3:33 pm
Posts: 718
Location: Tokyo
Again, the ARRL SSB handbooks I mentioned do have, right at the beginning, good introductions to SSB theory and practice. The technical articles that follow are naturally dated but still fun to read.

Rodger, I should have noted that what's absent from the frenzy of contests here in Japan is the heterodyne interference. It's funny how you can end up missing something that was once so annoying. I never thought I would miss Radio Moscow, and I may end up saying that about CRI.

Rob


Top
 Profile  
 
Post New Topic Post Reply  [ 27 posts ]  Moderator: Sandy Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB