Slight hum on my DENTRON CLIPPERTON L

[NC4XX]

 Hello,
   Name is Don and call sign is NC4XX.
   I'm have a problem with my Clipperton L, Nothing new has been doing this since I had it. I'm getting a slight hum when amp is keyed trough my audio. Sounds like a ac hum in the audio. I've had the Harbac modes done on this amp. Some have said to change wire in the transformer.
         Any help will be greatly appreciated!!
              Thanks, Don  NC4XX

[K7KBN]

Any reports of hum when you're just using the exciter barefoot?

[K8AXW]

Don:  Would you please backtrack and get instructions on how to "change the wire in the transformer please?"

[KD8GEH]

Hi,

Amp hum in audio is usually power supply related. As the other OM said, an amp will also amplify the exciter noise.

If you are not familiar with amps, be carefull or get some help from a friend with experience.

I would check the capacitors, diodes in the power supply first off.

73,

Dave KD8GEH

[W6OU]

From the title and text of your post I don't know which one of the following problems you are having:

1) Your transmitted signal has hum on it as reported from a distant receiving station.
2) Your amplifier transformer is humming during transmission.
3) Your receiver speaker outputs hum in the transmit mode.

Which one is it?

[KA5N]

Yeah!    And do you want to increase or decrease the hum?

Seriously it might be a mechanical hum caused by loose laminations on a transformer which
can often be cured by tightening the bolts on the transformer or wedging something
resilient under the transformer.

Allen

[KE3WD]

Sounds like a ac hum in the audio.

That sounds like a signal report to me. 


Need to find out if the same applies when barefoot first...



73

[W8JI]

Hello,
   Name is Don and call sign is NC4XX.
   I'm have a problem with my Clipperton L, Nothing new has been doing this since I had it. I'm getting a slight hum when amp is keyed trough my audio. Sounds like a ac hum in the audio. I've had the Harbac modes done on this amp. Some have said to change wire in the transformer.
         Any help will be greatly appreciated!!
              Thanks, Don  NC4XX

Don,

Every Clipperton L made by Dentron hums. They hummed when new, they hum when old.

The reason they hum is Dentron never used a center tap on the filament transformer. They feed the filament return from the bias and relay system from one side of the filament winding, instead of the center.

This puts about 20 volts peak to peak ripple on one side of all of the filaments, and this biases the filaments off and on. The result is an AC biased intermodulation product generator. One pin and one side of the filament has bias + (6.6 * 1.414 = 9.3 volts) positive, and then bias - (-6.6 * 1.414 = -9.3) on the other swing. That ripple effectively swings all the emission current to one area of the filament and back to the other side 60 times per second.

Back when the amp was first built, I asked the person who claimed to be the designer why he did a thing like that. He told me it was to keep the amp from oscillating on ten meters.

You might have another issue besides the built in factory hum generator, but every Clipperton manufactured, unless modified in the field, has some amount of built-in 60 Hz hum from the factory.

It also has too much filament voltage, not that reducing voltage will make Chinese tubes last any longer. If you correct the missing center tap, you might want to think about correcting the filament voltage. It really won't make the tubes last longer, but it will be more comforting to know 6.3 volt tubes are running closer to 6.3 volts.  :-)

73 Tom

[N6AJR]

NO, NO, NO,  You are ALL wrong. The only reason it hums is BECAUSE IT DOESN'T KNOW THE WORDS.....

[KB4QAA]

Obviously the red transformer wire needs replacing with green wire.

[K7ZRZ]

Don,

Tom has the right info on this.  After having had reports of the same audio related phenomenon, I started doing a little of my own investigation, by listening to my own signal on another receiver.  What I discovered was that when transmitting a very low wattage signal, the AC component on the carrier was most significant, and decreased to nothing upon raising the carrier power level - even just a little.  The same result was observed with SSB transmitted audio. The "fuzzyness" was most appearant on low speech, or on the low portions of the spoken words. Raising my voice and consequent output, caused the audio to clear up.

As my Idle Current level was quite low, even with good tubes in the amp, I decided to try lowering the bias and thereby raising the idle (quiescent) plate current.  That change made a very conciderable difference in the hummy/fuzzy amplified signal... cleaned it up a lot.

My filament voltage was generally between 6.9 and 7.1 volts.  I fixed that too with about 11' of #16 solid copper insulated wire in one of the filament leads... all neatly wound into a coil and tucked away inside.  The new Harbach power supply board has much shorter capacitors, and provided some space in that area for that coil.

[W9GB]

Dentron Clipperton L Notes
http://www.w2xc.com/Clipperton_Notes.htm

Ray Shatzel, W2XC proposed this solution for 60 Hz hum
due to lack of center-tap on filament secondary from transformer.
http://www.w2xc.com/Clipperton_L_Hum_Mod.htm

[K1TM]

I had not seen Ray's modification, but it looks like we did the same thing to a degree.  I placed the transformer on the center dividing wall, and moved the bias diode to there as well.  I added .01uf decoupling caps from each side of the secondary to ground right at the terminal strip where the filament choke connects to the transformer leads.  Moving the diode and placing the transformer on the wall removed a lot of wire and tightened up the circuit path.  I use a QSK system where an extra pair of contacts shorts a 47K resistor that is in series with the bias diode during transmit.  This makes for nice clean waveforms without switching discontinuities found in the original bias circuit, and no hum. 

There are other ways to do this mod like using 2 resistors in series from each side of the filament and then moving the bias connection to the middle of the divider formed by the resistors.  The Clipperton tends to trap heat, so that is not a great option, but it will work as well.