Kenwood TL922 Amplifier simple question

[G7IVJ]

Gents,

Thanks for your help in advance

I have a Kenwood TL-922 amplifier, and recently I was told NOT to use this amp with AM, FM or DRM as it will cause the problems to the amp and cost a large amount to get fixed.

The amp has switchable CW and SSB

Please let me know your thoughts.

Thanks, Simon GIVJ

[AG3Y]

It is never a good idea to use any radio equipment at full power when running AM, FM or DRM or any of the other 100% duty cycle modes such as RTTY. Most of the equipment is designed to operate under SSB conditions which really only has about 35% average power as compared to the peak power.

So if you run a 100% duty cycle mode, it should be perfectly safe to run it at around 35% of its peak power capability. That would mean around 350 watts for a 1000 watt amplifier, etc.

You can use an amp in those modes, just don't crank up the power all the way.

BTW, if an amplifier is designed to run full power at 100% duty cycle, the advertising department will be sure to tell you that !

[WB2WIK]

Gents,

Thanks for your help in advance

I have a Kenwood TL-922 amplifier, and recently I was told NOT to use this amp with AM, FM or DRM as it will cause the problems to the amp and cost a large amount to get fixed.

The amp has switchable CW and SSB

Please let me know your thoughts.

Thanks, Simon GIVJ

Is the power limit still 400W in the U.K.? If so, you could certainly use the amp at 400W on FM or DRM. On AM, I wouldn't do that, but running 100W carrier power (400W PEP output with 100% modulation) should be okay.

[AB3FL]

Just run it in CW mode. It will work fine that way for any constant carrier mode. I think the manual even rates it for 30 minutes key down in CW mode


Tom - AB3FL

[WA7PRC]

Jim/AG3Y & Steve/WB2WIK gave the correct answers. Here's why. It's all about average dissipation power versus available cooling and (for AM) distortion. Since the human voice has a duty cycle of around 25%, you can run 1000W on SSB.

With AM, the carrier (100% of the time) is 50% of your power (both dissipation and output power). All things being equal, you need to run at reduced power or risk overheating the tubes. With FM and digital modes, you need to run even less power since your duty cycle is 100%.

Now you know why.

[W8JI]

There is a whole lot more to it than duty cycle.

On AM, the maximum possible carrier efficiency is 30%. Figure 25% efficiency to be safe. This means we have about three times the heat as we do carrier power.

The TL922 was designed when power limits were 1 KW DC input CW or average power. It really is only designed for about 600 watts average output power, or about 500 watts of TOTAL average heat from the tubes. This is why airflow is so low compared to the amnount required for full power from 2 tubes.

Allowing 500 watts of heat, maximum carrier power on AM in ICAS duty is about 150 watts. You might be able to get 175-200 out on carrier, but 150 is safest for the cooling they have.

Steady level FM and FSK digimodes can run more power if output is peaked at the lower drive level, but amplitude modulated digimodes have to run at reduced power like AM.

[K1ZJH]

Also, consider using a monitor scope of some sort to keep an eye on modulated waveform, just to verify there is no flat topping on the positive peaks when trying to run AM.

Pete

[AB3FL]

I just looked, it is 10 mins key down CW or RTTY in CW mode.

Just run it in CW mode. It will work fine that way for any constant carrier mode. I think the manual even rates it for 30 minutes key down in CW mode


Tom - AB3FL

[W0UZR]

With my 922 I run AM around 100 watts. That isn't much oomph on the tubes at all. But what I'm more worried about is the oomph on the power supply. If I had a Peter Dahl transformer in it, then I wouldn't worry about running 400 watts out. The 100/120 watts I ran on AM is plenty sufficiant to run and they can't tell much of a difference on the other end if I run more. So if they can't tell that much of a difference between 100 watts compared to 200, or 300 watts, then why cook my equipment?

[W8JI]

With my 922 I run AM around 100 watts. That isn't much oomph on the tubes at all. But what I'm more worried about is the oomph on the power supply. If I had a Peter Dahl transformer in it, then I wouldn't worry about running 400 watts out. The 100/120 watts I ran on AM is plenty sufficiant to run and they can't tell much of a difference on the other end if I run more. So if they can't tell that much of a difference between 100 watts compared to 200, or 300 watts, then why cook my equipment?

The power supply will take it fine. The real problem is tube heat. The seals will go over temperature rating in less than 1 minute at much more than 175 watts carrier on AM when the amp is properly tuned.