Audio Quality Differs with transmit frequency?

[Ed/KB1HYS]

Ok, here's the deal. 
I've gotten back on the air with the Valiant, with a slightly opened up audio section, Mods are pretty much what I could do from the AMWINDOW page, with out rebuilding the whole audio section. Fatter coupling caps, clipper out and bypassed, the audio filter Choke and cap removed. Fed with a D104. Basic stuff I thought. When I run, I try to not load the final up full, but under run around 200mA or a little less depending on the loading but close.

I get "Good Audio" comments on 160meters.  I get, "Helpfull suggestions" on improving my audio on 75meters. ( I don't usually get on any higher bands on AM).

I don't understand, How can the audio be worse on a higher transmit freq?? When the basic operating parameters are pretty much the same.  I DO run a scope, and the modulated waveform looks ok. No Flat topping, and nothing trying to go negative. I am still under 100% modulated, (80-90% measured on the scope) even when lightly loading the final for reduced output, which is probably all I can expect from the almost stock modulation setup.

I want to understand what is going on with this very basic system before I make more changes to the Valiant, and before I get too deep into the homebrew rig.

[Steve - WB3HUZ]

Have the same people given you audio reports on both 160 and 75? If not, you may just be hitting two different crowds with two different "tastes" or "ears" for audio.

[w3jn]

200 mils is a bit light to load a Valiant.  I run mine about 300 mils.

Also you could be getting RF in your audio on 75 but not 160.  I am very familiar with this problem 

[W7XXX]

In my opinion Ed, it is the difference of the operators on 160 and 75. Here on the West coast 160 AM operators as a group tend to be more experienced and unprejuiced than the 75 meter groups. As mentioned before have the same folks give you a report and remember the difference in band conditions and receivers is a factor. If your signal looks good on your scope, then don't let these prejuiced 75 mtr AMers worry you. First they may get prejuiced if you say you are running a Valiant, then to admit to a D-104 may have prejuiced the 75 mtr. groups that will only praise pro processed audio with lots of lows. Ask the ones giving a report what their receiving set up is? Get a feel of what they conceive as good audio. Everyone has a different opinion. Also hearing loss is a factor. Many operators have never had a complete hearing test and have no idea what frequencies they may hear weakly.

Maybe they are using an Icom with the built in speaker ... lots of variables to consider before you start any more mods.

[The Slab Bacon]

Ed,
     200Ma is definately too light to load a valiant. I've ran mine for years around 275-290 mills, this gives around 100-105w outpoot.
Try loading it a little heavier and see what happens. Sometimes loading too lightly will have a dramatic effect on the freq response passed by the mod tranny.

Also like john said, you may have a little rf getting back into the audio. This could be from a "rf in the shack" problem, or something in the transmitter it self. Remember that the outpoot loading cap is pretty close to the audio section. When I do up a valiant, I often put a few more "rf dams" in the bottom of the chassis to help overcome the transmitter's lack of internal shielding. I also add a few extra bypass caps in strategic places as well.

Also keep in mind that when you open up the transmitters audio frequency response you open yourself up for some of these little monsters waiting to bite you in the butt.

                                          The Slab Bacon

[KF1Z]

Have the same people given you audio reports on both 160 and 75? If not, you may just be hitting two different crowds with two different "tastes" or "ears" for audio.

Yup, there's that, and the possibility of RF into the audio on one band, and not the other...

One of those cases where it would be much better to monitor your own signal... and not rely so much on reports.....

I mean, of course monitor the audio.....
Sounds like one of those cases where, sure, It LOOKS ok on a scope.... but the scope of course can't tell you what it sounds like....

And monitoring your own signal removes all of the other variables mentioned.....

[W3RSW]

I first thought the RF contanimation might be it, and you mentioned that you don't get on higher freqs. than 75.  So I'd try 40 metros or 20 to see if it gets worse.  If so, voila.

Also band conditions on 75/80 have been so crappy lately, I've noticed phase distortion on 75 but not on 160 on just about everybody's signal even INR's...     Sometimes a signal has no highs either and sounds muddy depending on location and distance separation of a station from me.  Perhaps most people you work on 160 are closer in and maybe even ground waved.

[W7XXX]

Monitoring your signal using headphones is great, but best yet is to have someone with a good receiver record you and play it back to you or set up a recorder on your audio monitor and listen for yourself. If both 160 and 75 sound the same, then it is band conditions or listener preference.

Yes, load that Valiant up ... however it seems to me light loading would affect 160 more than 75, but I can't see this as the problem. As I understand your post, the 75 mtr AMers aren't saying you sound bad, but think you can use improvement. How are your monitoring with a scope, the trapezoidal, full envelope, or a tracer line? If monitoring the full envelope, you should be able to see RF on your signal.

It may be the 75 mtr bunch is trying to impress each other with their expertise by giving you suggestions. Ask for details ... do they hear hum, distortion, restriction, phase distortion, etc., make them explain.

I just reread your post ... try modulating 100% ... I think their complaint is audio is unmodulated. On 160 you must have better conditions that don't show your unmodulation.

[Ed/KB1HYS]

Ok,
Looks like what I need to do is get a decent monitor setup, right now I am just using a scope looking at the full envelope from a capacitive pickoff near the feed line,with no off air monitoring, so I guess I may be setting myself up there.

Next, will be to make sure there is no RF getting into the audio. Shields should be easy enough to put in around the audio section. I have tube shields around, should I put them on as well?

I'm using a K1JJ style balanced link fed tuner, to twin lead, and the shack ground is 3/4 inch braid to a common tie point (all items run to one point) that is connected to an 8' rod by ~8-10 ft of #6 stranded copper.

[flintstone mop]

Hi Ed
Usually the RFI headaches start at 80M. My antennas are 100 feet or more from the shack. 40M is a problem when I use the dipole which is 50 feet from the shack. RFI proofing (Ferrite cores,etc) is the only answer, grounding is great for lightning protection and ground loops.
Fred

[WD8BIL]

I wouldn't worry about the reports, Ed, unless they are from the same person using the same receiver and there's a major difference.

On air monitor your signal (using one of the countless methods) and determine for urself.

Here on the West coast 160 AM operators as a group tend to be more experienced and unprejuiced than the 75 meter groups.

 

[N3DRB The Derb]

East Coast! West Coast! yo, boi!

rat a tat tat, I like it like that!

Can we start some AM gangs? Since I got a brain tumor, I wanna be a member of the East Coast Crips as in crippled! You know what I'm sayin?

YEAH. East Coast AM crips rulez, homie! West Coast AM bloods sux!!!   

[WD8BIL]

Yo yo Derb ..... bro.... I'm holdin' the Midwest turf  here. Ya know what I'm sayin' ?
But yo sorri ass can hang at ma crib anytime yuz in town...... dig ?

[N3DRB The Derb]

roger rite on brotha....that dont confront me none...If I be in town , we'll get down!

(ok, I'll stop now, this is the tech forum)

[Ed/KB1HYS]

Ok,  My dipole (a 160m job) is 65-70 ft up directly over the house (1 story ranch) and does Zig-Zag through the trees over the place.
The feed line runs between the joists to the chimney. Then up following the chimney framing to the attic and out through the ridge vent...  96ft of feed line if that matters.

The feedline is the polyfilled 300 ohm feedline. Not the thin stuff but the thicker stuff with the solid poly dielectric and black covering.

Anyway ya think that the antenna is in a bad spot for RF in the shack?
I ran some RF safety software for RF exposure calcuations and that said safety wise I should be ok-fine. I thought that would also mean that radiated energy from the antenna wouldn't effect the shack equipment, guess I'm wrong on that one.

Should I just wrap all the power cords and mic cord through some ferrites since they aren't too expensive?? 

[Steve - WB3HUZ]

It's almost impossible to say if your antenna is in a bad spot for RFI. Just too many variable. I'd get some more reports, maybe some recordings, on both 75 and 160, both preferably from the same person. Try to verify if you even have a problem before you go "fixing" it.

[Steve - WB3HUZ]

Throw yo guns in the air!
Everyone's chillin,
ain't no killin'

roger rite on brotha....that dont confront me none...If I be in town , we'll get down!

(ok, I'll stop now, this is the tech forum)

[WD8BIL]

Ed... grab u a dummy load.... know what I be sayin'.
Tune u in on a receiver with muffs on... dig. Do sum jammin'
and listen up gud.

Then slide over to thother band and see ifn uz still righteous.
If so.... it ain't the maul bro !

[The Slab Bacon]

<snip>
"The feedline is the polyfilled 300 ohm feedline. Not the thin stuff but the thicker stuff with the solid poly dielectric and black covering."
<snip>


That stuff might be your problem, I dont think it is capable of carrying much power. Also try changing the length of the feedline. another 10' one way or the other might make a hell of a difference.

                                                the Slab Bacon

[WA3VJB]

YEAH. East Coast AM crips rulez, homie!

I am moving and rolling my upraised hand with several fingers canted at an awkward angle, in support of your contention.

Am I in ?

[N3DRB The Derb]

you gotta lose the stiff on yo left.

[Tom W2ILA]

Ed,

Load up to 275-300 mils as recommended, check for RF and get on the air.  For a quickie monitor ground your rx antenna input and listen on phones, back off rf gain.
Next operate a lot and don't ask for reports. 
Unsolicited reports from my valiant on 75 range from "sounds great, clear" to "very distorted and pinched, why dont you modify it".  Bottom line is you will get lots of reports from lots of people even without asking.  After a month or two of enjoying operating the rig, then decide how you want to take it next.
Most here in the northeast think a Valiant needs to be bulldozed, audio replaced with a pair of 811A's, RF deck replaced with a quad of sweep tubes and the mod iron replaced with broadcast pieces and connected with clip leads.  Then you need lots of state of the art processing gear to make that sound right.
There is always room for improvement but don't let the first few reports make your decisions. 
I'll look for you on 75.  I'm the one with the distorted, pinched Valiant. 73 Tom

[WQ9E]

Ed,

I am a little late getting into the discussion but don't rely too much on the Valiant meter readings, particularly if it was a kit built unit.  The meter shunts are very short pieces of nichrome wire and it was up to the builder to end up with exactly the right length of wire, after tinning and soldering, for the shunt value to be correct. In many cases it wasn't, the good news is that most builders erred on the high side and thus the Valiant was running less input than the meter indicated and not more thus avoiding cooking something.  However, if your meter is reading high and you are running lower loading intentionally then you could be way off.  I replaced my shunts with "real" precision resistors from Mouser, in one unit the meter indicated 450 mils with an actual current of 315 ma, interestingly enough this was within 5% of the expected error based upon measuring the shunt using my Wheatstone bridge so the basic meter movement is pretty accurate.

Furthermore, Johnson issued a bulletin stating that it is acceptable in the AM mode to load to an indicated 360 ma plate current since in AM the indicated current also includes the roughly 30 ma drawn by the regulator tubes (P.2 of factory bulletin issued Oct. '58).

Also, some early Valiants were built with a 1000 or 1200 PF output coupling cap which is too small for 160, this was changed to 2000 PF in later units which provides output on 160 roughly equivalent to that on 80 and 40.

I have a couple of Valiants and I had one as a novice in 1974.  Both are basically stock except for SS replacements for the 866A rectifiers.  With the clipper control set at full clockwise (counter-intuitive, but this is minimum clipping) then the clipper circuit is effectively doing nothing and there are times at the power level of the Valiant when being able to dial in a little clipping is the difference between getting through and no contact). 

73, Rodger WQ9E

[Ed/KB1HYS]

Talking into a dummy load and listening in the headphones, it sounds ok on 80 (audio is a little restricted, but it sounds ok to me I don;t have a golden ear however)  On 160 it's about the same.   I did hear a "powerline" buzz in the reciever headphones, on both bands, but that could have been junk from the HVPS(I was using a cheap "Boombox" type receiver as it also has a cassette deck built in and I can record "off the air"  If I can get the thing to listen and not get overloaded when I actually transmit).  Going into the dummy load, I had to get it close to the load and that put it close to the transmitter also.  So on the audio, I think it's Good Enough, for now anyway.

Roger --  I replaced the loading caps with small door knobs from RF parts or somewhere.  I think I went with the original values (or close) and I remember getting a 1200pf unit.  I think I will sub in a bigger one when I get the chance. I replaced the 866s with 3B28's as I had a couple.  I have also bypassed the clipper altogther, though the circuit is still resident in the rig. 

and confession time,  when I was loading it up, I was reading the WRONG scale on the meter... so I was actually well over 350mA instead of the 200 or so I was thinking I had... (watching my finals dissipate... )  Dumb...