The AM Forum

All:

Here's a snapshot of what comes out of my Collins 32V1 / AL82 combo.  Positive peaks are correct in the display.  The instantaneous nature of the display does not show the negative peaks correctly but I was keeping the negs... at ≈ 95%.  The spectrum display is my Flex 5000 that allows me to not only monitor my frequency quite accurately but also keeps track of the sideband component.  I think the Collins station is quite clean.

The primary of the 32V1 has a split winding that can be configured either in parallel or in series.  The low power setting (primary in series) is perfect for driving a linear.  I like the way the old girl is performing.

Administrators can move this post or not.  I wasn't sure where it belonged except to say that I feel this kind of monitoring fulfills part 97 about an amateur radio station properly monitoring emissions according to good engineering practice.  There are other ways of monitoring sideband components but the Flex makes it very graphic.  Steve's (QIX) monitor needs no comment except to say it's an excellent way to monitor one's level of modulation.

Al

Hard to tell what is going on in the screen shot, other then the modulation looks like plenty.

I got my 32v3 working well, its an incomplete rig, no cabinet, no driver transformer, so I drive the grids of a pair of KT90 tubes directly, works very well.

I run it on the 700 volt tap at 180ma and about 90 watts out, it does 400 watts pep.

I ran into a problem though, I had no push to talk, I have a switch on the mic that I had hooked up to 2 different radios, but not the 32v, so last night I built a real ptt setup, a 24 volt 4pole double throw relay that is powered by a wall wart, in a rack mount box that has 4 contacts out for push to talk, so now its only one pair of wires to the mic switch, and have all three transmitters hooked up with contacts for one more.

Since the 32v is a small rig, it does not have step start which I find nice.
Great rigs the 32v series....

Plenty enough for me.  High % is fine because looking at the spectral display, there is no evidence of distortion products.  No evidence of distortion products also means that my linear is not flat topping.  That should make me a good RF neighbor.  Look for you on 40 metros.  Iffen you ever get interested in a driver transformer for that 32V... a Hammond 124B should do the trick - expensive tho'.

Al

Yes, I could come up with something for the grids, but I don't even have any 807's, I think I included them as spares when I sold the other 32V's I had.

I also thought about a pair of 4x150a modulators, at that power level, they would need no cooling at all.
700 volts on the plates, about 350 on the screens, plenty of power out in AB1.

The KT90's do well, but the KT88's I tried sounded very distorted for some reason.

 

Really ?       Check the data sheet for that family...  

As I recall the whole series (4x150, 4x250 as well as
the cx tubes) stated that air must be active on lighting
the filament...     And, by the way, the 6 volt filaments
on those tubes is just that, 6 VOLTS, not 6.3 !

Of course I could br wrong, it's been a loooong time
since I last read that data sheet.....

They work well with a .1 ohm resistor in series at 6.3 volts.

I never tried them with NO air, but I don't think they would have a problem at low power.
6 volts at 2 amps is 12 watts....not very hot...

It would be easy to cool them in that little box with a cpu fan in the removable cover...

Al
What is the modulating audio freq? The Flex screen shows like a low audio freq? 50hz? The REA monitor 'scope trace doesn't tell me anything.
Fred

That is an instantaneous display.  Yes, there seems a fair amount of LF power but most HIFI stations male voices show that sort of signature.  Note the neg... peak.  I know that I was hitting the negs close to 90 - 95%.  As you look at the display, you will see a sort of rippling effect as different frequencies are sampled at any instant.  Another important point: I do NOT use the averaging mode.  There is really NO information to gleaned from using averaging.  A hint is to note the positive is literally off scale yet the sidebands are showing -20 dBc which translates to way less than 100% modulation.

I yacked way too long.  Hope that helps

Al

It is MUCH easier to tell the bandwidth using the waterfall display.

Wish I had bought one of those REA modulation meters before I got laid off.

Waterfalls..
Can anyone point out a comprehensive free one(s) that work with analog audio/sound cards?
Hadn't thought about using one for that though! great idea if I can dig out an old PC and dig us out a sound card interface. No SDR present..

Just mounted an HP specan in a rack, a 140 or 141, a round analog storage type whichever. well, didn't want to let its input get near the TX!

Sorry to read about layoff. I might be in that mix in July. The "boss" says he cannot sustain losses of $1,000.00/month. The gummint is not compensating him for the rising Diesel fuel costs...
sorry Off Topic.

Al, I'll look into the REA settings. I did not know there were averaging and peak settings for the display.
Fred

In my opinion your display yields very little insight into the modulation character of your 32V1.

I have a state-of-the-art lab for FFT and continuous spectrum analysis .. but at the end of the day

I want to see this on the scope from my 32V3 -

dan W7NGA

I look at the o scope, the mod monitor, the sdr display, and the pep watt meter to tell me what a transmitter is doing.

The 32V3 looks good, 90 watts carrier, 400 watts pep at 95% neg modulation per the scope and mod monitor.

If you really want to know how something is doing, dump a tone into the rig at 99% modulation and look at the sdr display of the harmonics...

Time domain, Frequency domain, etc.  I think the point we can take away from this thread is that the output is being monitored by acceptable methods.  I put my Tektronix 2445 back on the shelf once I installed the modulation monitor because I noted that the scope wasn't picking up on the stuff that was transient in nature.

These are all good methods as long as we are aware of certain limitations and can interpret the display.  Yours certainly looks good.  But stations that have to be told that they are barely modulating or are beating the p!$$ out of their modulation are probably not doing anything to monitor things.


Al

very true ... I am convinced the majority of AM stations I work have no means of critically assessing their signal.

dan W7NGA

True, but not everyone has room and money for a O scope, a mod monitor, an sdr.

Appropriate scopes are available for $50 or less at most ham tests. A soft rock costs $20. A low end SDR goes for $100 or less. There are no longer any excuses.