Audio Testing - My Ignorance is Showing

[WA2OLZ]

I'm getting awful audio reports and don't know if it's the microphone, the transmitter or the operator.

I would like to buy an audio signal generator in order to properly setup the audio on my Valiant. I have a scope and an AM monitor (Clean-RF). My thought is to feed an audio signal to the Valiant and finally get the audio set as it should be. After that I will reconnect my microphone, currently a D-104), adjust the gain and go from there. At least, hopefully, I will be able to figure out where the problem originates.

I've been looking at inexpensive audio generators, mostly on EBay, and have confused myself even more. It seems these generators all have an output impedance of 600 ohms. Am I missing something? A high impedance transmitter mic input and a low impedance audio generator is just one more point of possible trouble. At least it would seem so.

It seems I am just confusing myself more as I go along. A guiding hand would be appreciated!

Thanks & 73
Jack

[N2DTS]

I do not think the mismatch will be a problem.
You can download free programs that turn a laptop into a signal generator.
I use NCH tone generator.
It does all kinds of amazing stuff...

[K1JJ]

Hi Jack,

This short, one page article will get you started. Basically, you want to make sure the Valiant itself will reproduce a decent looking AM audio waveform.  Learn what a perfect sine wave looks like and compare it against what the rig actually puts out.

http://amfone.net/ECSound/K1JJ1.htm

As you probably know, Valiants can be difficult to get set up sounding hi-fi.  If the output audio waveform doesn't look like a decent sine wave when swept, then you will need to get this corrected before adding new mics or external audio gear.

There are a lot of mods out there.  Possibly the best sounding (and easiest) method is to feed a solid state amplifier into an 8 ohm to 2K CT audio transformer and drive the grids of the modulators directly.

T

[flintstone mop]

Hey Tom
Good advice to get started on the Valiant. And very easy to do without major surgery.
Any way to shed the 6146 modulator tubes? Would that clear up any bad audio? I am not really sure what I have heard about them, but 6146's are not great performers for audio. They were designed for RF.
What are the real limitations of the Valiant? Mod tubes? Mic preamp audio driver? ( it is well known to bypass the limiter ) Mod transformer?
Fred

[K3ZS]

Is it on all bands?  Don't overlook RF feedback as the cause, especially if the audio generator does not show a problem.

[N2DTS]

I never had any luck getting 6146's to sound good as modulators, but I have heard rigs on the air using them that did sound good.
Its easy to plug in something different though.
Back in the day, it used to be fun hacking up rigs making improvements and trying different things, now days they are all collectors items...

[WA2OLZ]

Thanks a million for the replies and the great information!

I didn't know there were programs out there to convert my laptop to an audio generator. Absolutely logical, but never occurred to me! If I understand correctly, that method requires the audio from the computer be played through a speaker then picked up by the microphone. That still leaves the microphone a source of trouble. Connecting a hardware generator directly to the microphone input eliminates that issue and permits a true test of the transmitter. I can see where the software version would be a convenient aid for fine tuning after the rig itself is proven to be OK. Maybe I am all wet - don't hesitate to say so if I am!

Tom, that article you wrote is excellent; one I have stored in my favorites and will be a constant reference as I move onward. Thanks!

I have only tried the Valiant thus far on 40 and 80 meters, and mostly on 40. RF could well be an issue and one I will need to look at as I narrow down what is happening.

The Valiant is a K1LTI restoration. Chuck brought the rig back to its original configuration. My preference for now is to avoid any mods, but do understand the audio circuits could be improved. I do not use any clipping at all..   

It sounds as if my next steps are to acquire a hardware audio generator, connect it to the mic input line and scope input vs. output while checking for distortion, linearity and amplitude. I have a W2IHY 8-band eq on the way but will not be using it until I solve the basic issues. I’ll look at maximizing the audio quality then.

I guess I’m off to EBay or the for sale forums to find an audio generator!

I truly appreciate the advice and input. This group is great!

73
Jack
WA2OLZ

[WA2OLZ]

Hey - brain light bulb just dimly lit!

Is there any reason not to connect the speaker output of my laptop directly to the Valiant mic input?

Maybe that's what you meant in the first place! 

[K1JJ]

Linearity:

I think tetrodes get a bad rap when used as modulators because some ham commercial rigs do not regulate the screen AND grid supplies well. This is absolutely imperative or distortion will occur.   Or, regulate the screens and put in a 100K grid resistor to limit grid current - and run them AB1.  However, this will produce less power vs: class B triodes with grid current.

Also, some rigs drive the piss out of tetrodes in AB2, causing heavy grid current, which makes for a dirtier output.  AB1 is much cleaner.  (But running recommended screen current is important)

I'll bet driving the 6146 modulators triode-connected, as a substitute method  -  screen and grid tied together, would work for 6146s, but I have never tried it. But would the Valiant driver handle it?

I have proven to myself that tetrodes can be very linear during my 4-1000A modulator experiments. Running them as pure AB1 tetrodes with electronically regulated screen and grid supplies produced a cleaner audio than run as triodes, screen/grid driven.  

Of course, real class B triodes (like 811As, 833As)  are an easier deal and are the accepted norm for modulators. Building a clean triode driver is not an easy task. But tetrodes can sound as good and are easier to drive.   Remember that there are many tube audio amplifiers out there running tetrodes - as well as RF linear amplifers and ssb rigs. Many have IMD figures better than -30db 3rd, which is equivalent to 1% distortion or better.

Tetrodes need to be set up correctly with the proper load, negative feedback, regulated grid and screens and the drive levels kept reasonable to achieve great IMD results in both audio and RF service.  The Valiant 6146s as tetrodes could be run as cleanly, but the surrounding circuitry in that rig is the limiting factor. Since Jack is hesitant to do any serious mods, perhaps an outboard SS audio amp driving the 6146s as triodes might be the easiest way. I have heard a few Valiants using this outboard method with the stock mod xfmr - and they sounded pretty good.

T

[N2DTS]

Any time I tried using 6146's as modulators, I did regulate the screens and used a stiff bias supply.
VR tubes in place of the vacuum rectifiers is what I usually did.

I have heard them sound good on the air, I just never got mine to sound real good.
Since they are used as amplifiers in the output of many ssb rigs, they can not be that dirty.

Yes, just hook the laptop output to the mic input and adjust levels, do not use a speaker/microphone.
I have done it many times no problem, and the programs generate any sort of tone or waveform you want.
You can then O scope the various stages and see where the distortion happens or frequency rolls off.

[WA2OLZ]

Thank you, Dan. I will be making that connection now!

[K1JJ]

Looking at the Valiant schematic, I see that they use a pair of VR-105s in series to regulate the 6146 modulator screens at 210 V. This is a good thang...        

The grids appear to have no regulation - just thru a 15K resistor and 5K pot, so be sure NOT to draw grid current from modulator overdriving, or there will be bias fluctuation, thus distortion.

For big tubes with big screens, I prefer electronic regulation. But for small 6146 screens using low current,  the VR tubes should work FB.


http://www.mbzponton.org/n2awa/radio_Johnson_Viking_Valiant_schematic.jpg

T