The AM Forum

Still messing with my homebrew transmitter (see other posts, 807 modulated by 6CA7's).  See the scope photos below.

Driving it with a 400Hz sine wave input. I do not have a generator that does triangle waves. Top trace is the high/tube side of the modulator transformer secondary. So the bottom of the wave is approaching zero plate volts, while the top is approaching twice B+. Don't look at the display to guess the voltage; uncalibrated 100:1 probe in use. The lower trace is the RF output, tapped off the dummy load.

How hard should I drive it? I have external digital audio processing that I can brick wall limit at whatever output level I set, so the drive is going to be equalized, compressed, and absolutely limited.  It's more a question of when to stop....

Finally, any comment about the hiccup in the mod voltage when over 100%? Is that the transformer kicking back ? or??

And lastly, again for those who've had more time with stuff like this:   It seems when I first fire the thing up, tune it, it tunes fine, but I need to re-tune in a few minutes...like something shifts after finishing warming up. Should I just deal, or is this a sign of a problem?

Ed

Looks like you're doing fine. Middle picture looks like 100% modulation. Bottom is overmodulation; shows carrier pinched off, meaning your audio level is too high. Back it down. So how does it sound?  If you have no audible distortion, you're good to go. You say you need to touch up the tuning--do you mean frequency drift, or you need to touch up the plate tuning and loading? Either way, component values on tube rigs change a bit as they warm up. Should be stable after thirty minutes or so.

Looking at the photos from your previous post (http://amfone.net/Amforum/index.php?topic=45228.0) I'm a little concerned about the small, red switch used to short some turns of the output coil for 80M. It may not be able to handle the RF voltage across it on 80M or the RF current through it on 40M. Maybe this is the cause of the tuning drift? Hard to know, but either way it may be an issue.

Also, make sure that the frame of C15 (the output loading capacitor) is properly grounded. I see you have a painted chassis which may not be giving you a good connection there. Might be best to run a ground strap between the tune and load caps.


Don

It doesn't show in the pics, but I used a Dremel with a grinding point to remove paint under every screw or part I wanted to be grounded, including the three screws (with lockwashers) that hold that variable to the chassis. The coax braid is also connected at both ends, to the cap frame and the N connector. I ran a strap on the tune cap because it's mounted to the front panel and other than being screwed together, the panel and chassis aren't specifically bonded.

Ed

The only time that has ever happened to me is when something was getting way too hot.

By warmed up, do you mean, as in several minutes or longer of transmit mode with tone modulation as shown in the scope images?

If you run it long enough to de-tune, and don't touch it up, then un-key and wait 10 minutes, and key up again, is it tuned correctly again?

Since it's all new design and under 1st test, and also has some old parts in the mix, you might have to compare hot/cold checks on several things besides just the RF parts in order to eliminate them.

Things happen like power transformer windings getting hot well before heat is felt on the can, or some supply voltage changing and affecting the operating point of the tube and its desired load, so you have to retune.

Are the heatsinked devices all doing OK on temperature?

Is there a previously used or oldie power resistor in there, or a resistor that might accidentally be a bit small for the watts?

Is the 220pF Mica job C13 on the side of the plate tuning cap getting warm?

Just thinking outside the box as the whole thing looks like it should work perfectly.