The AM Forum

Well here we go again. I have a Apache that I think has a problem. It doesnt seem to swing what I expect it to. With a 100 watt carrier, it will swing on voice peaks to a little over 200 watts, actually thats with a whistle into the mic. The Apache has the audio mods, from the AM window site done. It has new 6146's and EL-34's, the mic is a D-104. I've tried readjusting the final bias, the mod bias, the clamp tube adjustment all with no improvement. Any suggestions would be appreciated, hopwefully I'm missing something simple, or am I just looking for more swing than I should be expecting? Hopefully the problem isnt there is a nut loose behind the mic ...hihi   I'm at the point of do I admit defeat and let the rig win or ???

Have you looked at the output waveform on a scope? ? ? ?

The actual level pot is behind (inside) the phone jack on the front panel (right side, iirc), so if it is still in the circuit you may be lacking gain?

The other thing is that 200 watts is 100% modulation of a 100w carrier. Take a look at the scope pattern, maybe with a sig generator or similar as input (with a dummy load) and see if you are hitting the baseline.

If you do not have the 3 diode limiter circuit installed modulating beyond 100% is a bad idea anyhow, fwiw.

I am not sure the stock Apache modulator scheme (even with modified input circuitry) has enough plate voltage to swing beyond 100%, at least not much beyond. To get past 100% you need the negative 3-diode clipper (or a similar means to prevent baseline clipping) and then the voltage to swing up past 2x the carrier's voltage which then has to be backed with the requisite current (wattage).

This is what gave birth to the "turbo mod" and the K1DEU variant that uses a boosted (1500vdc) B+ for the modulator tubes (which of course are changed).

The mod in the Valiant II that I run (I got it modified) uses a pair of sweep tubes as modulators because they can handle more B+ and swing a ton more current than can a pair of 807s or EL34s... and they do.

So, look at what the scope says to start...

Also make sure that you are not loading the final too high!! (too many mils)

              _-_-WBear2GCR

6146's should do well under modulation.

The things I would look at are:

Grid drive/bias...for good modulation, a lot of bias works best, that usualy means a lot of grid drive.
Almost all of the 'ham' transmitters I ever had needed more grid drive/grid leak bias to modulate well.
 

The 6146 screen voltage setup, how is it 'self modulating'?

The modulator power and mod trans ratio.

Brett

I dont think anything's wrong with the rig. if your using a rms wattmeter; 100 watts of munky swing while also putting out a 100 watt carrier is FB for that transmitter. You gotta look at it on a scope. there's just no other real way to know for sure.

Uh, 400 watts pep would be 100% modulation of a 100 watt carrier if everything is right....

Brett

Am I wrong, or is 200 watts rms under modulation of a 100 watt carrier way more than you normaly see?

I never get anything like that on any rig from what I notice.

Seems to me like that would be way overmodulating.
I also think its totaly bogus, the only way to tell anything is with a peak reading watt meter, mod monitor or scope.

Brett

he's not gonna see that on a rms wattmeter. Needs a scope. Thats a pretty healthy swing on a rms reading meter.

It's 400 pep because the peak voltage is doubled over the carrier's voltage, yes?...

... the scope knows...  ;D

I was expecting closer to the 400 pep, I've checked it with the wattmeter on my Palstar tuner, in the peak reading position, and also with my P-3000 meter. Just asking because I was expectingto see more. My 32V-1 with a 100 watt or so carrier would show 395- 401 with a whistle to the mic. Thanks for all of the input ..I need all the help I can get  ;D   been shocked to many times I guess ...

On a peak reading watt meter I always get 4x or more than the carrier power.
On an rms meter, I dont see much increase with voice.
Never noticed a sine wave, maybe 25% more?

Brett

I never had an Apache, but the DX100 used to do 4x.

400 watts pep is good for a 32V3!
All the ones I had would top out at 300 stock....

Brett

I have an apache.  With 100W of carrier peak is about 400W, maybe a little more.  I have done the W3SCC mods and the timtron turbo mod

Tom - AB3FL

Also it runs the HV at close to 850V since the line voltage is high here (125V)

 The scope is the best way to know for sure.  

The W3SCC audio mods are done, and i have the info on the Tron's turbo mod but I havent done it yet ..dont want to till I get this problem figured out. I'm going to look closer at the clamp circuit on the finals. I'll check that out tonight.

you really need to look at the waveform on a scope.  with a meter, my apache only showed peaks around 200W, but a scope shows 400W or even a little more.  Put the meter in the mod position and read the current while you are talking.  It should be about 125ma and if you whistle into the mic or turn the gain up, it should easily pin the meter.  If this looks good, just get it on the air.  Most of the guys here will be able to tell if it is working correctly by listening to your signal.  Some of them even have scopes on the receive path and can tell you your modulation percentage...

Tom - AB3FL

The turbo mod will increase the modulation peaks. Like the DX-100, the Apache mod transformer impedance ratio was purposely selected as a mismatch. This mismatch would limit the modulation to something less than 100 percent by limiting and then clipping the peaks. The turbo mod changes the tap on the secondary to yield a better match. So, not only will you have more modulation, you won't splatter as much.