Applying a DVM (ohm position) across the pins will also show a short.

What happened to the tubes in the amplifier?

There is no room inside the TL-922 for a roller inductor. In addition the input tuning would also be off. Most MARS frequencies are very close to the ham bands. What is the frequency you are trying to use.

If you are very far from the ham bans the resonance of the HV choke can also be a problem.

The band switch is very fragile in the TL-922 along with the padder caps. I would go with a amplifier with more current reserve like a Drake, Alpha ORO or a new amplifier.

Follow the coax from the output connector.

The new Alpha amps don't go much over the legal limit. My 8410 would only do about 1900 watts. My buddies 9500 was about the same.

As for 8877 degradation, the tube will produce 1500 watts easily for 35 years. Key is not to exceed the grid current rating.

I would perfer to have an amplifier with at least 2-3 dB of head room. IMD will be much better with complex modulation.

Despite popular wisdom most 911 turbos just follow the car in front of them.

If the amplifier is oscillating. It will be drawing more than the idle current when you are not driving it.
Besides the bypass capacitors the others have mentioned, look for the following.

Is you bias set equally and is the current draw upon keying set to high.

Look for bad grounding both on the circuit board and in the input and output transformers.

Make sure the current is equal on you push pull devices.

Without an input pad your amplifier might have a bit too much gain. An input pad would stabilize the amplifier a lot.

MAKE SURE THE COMPONENTS ARE WIRED IN THE RIGHT SPOT.

You did not mention if this problem just started or has the amplifier always exhibited this problem?

“When i  measured the HLA300 not  with 2 tones using white noise loading using a IC706 and  HLA300 the IMD was disaster at fully output. The IC706 was putting out between 20 and 25 watts for about 300 to 400 watts across all the bands. Thats  about the manufacturers ratings and where most hams would be running them not at 200 watts. Why would spend money on a 400 watt amplifier and run it at 200 watts?  No normal ham would do this. Its a nice feel good argument for hams who buy this CB splatter garbage  and then run the amp at full power. Everyday on the ham bands we hear these crap boxes spewing out splatter and  occupying 20 khz of bandwidth with splatter  when they drive to the full rated output.”

The HLA-300 uses 4 SD1446 80 watt transistors. Why would you try to get 300-400 watts out of an amplifier that is in compression at 300 watts out?

What was your Amplifier Vcc voltage during the test?

What was the occupied bandwidth from the driving  transceiver?

How did you mesure that bandwidth?

What instrument did you measure the bandwidth with?

I still don’t see any hard numbers.

Wither using a bifilar wound transformer to DC feed a push pull amplifier or using a center tap transformer will neither add nor reduce the IMD of the amplifier. The linearity of the device and how it is being bias has a lot more addition to the IMD being produced.  Beyond that, the magnetics of the frequency being passed through the matching network can affect the efficiency.  Because of the broad band design goal, harmonics are reduced with the addition of low pass or diplexer filters for out of band energy.

Low pass filters have no effect on IMD distortion other than to reduce energy contained in harmonics.

In a classical push/pull design (class AB) the DC being fed through the impedance matching output transformer is very acceptable and in most Cases is the preferred DC feed method. All output stages in 99% of modern transceivers is class AB in a push pull transformer configuration.

I find it interesting that there has been such concern about amplifier IMD when the conventional transceiver in most cases is no better than 30 dB second to third. This in general is 10-20 dB worse than the amplifier they are driving.  Now before the IMD police jump in here I am quoting 2nd to 3rd for it is a fixed and settable number that can be compared and measured in all designs.  Yes there are higher order products but they tend to be less and 2nd to 3rd is very measureable and when the separation is higher (dB)  the higher order products also tend to also be less.

Even order harmonic output (2nd, 4th, 6th etc.) in push pull is generally canceled and there for provide another engineering point to use push pull amplifiers.  

Only in very high output amplifier does the transformer physics become problematic with efficiency’s and heat dissipation.  These amplifiers tend to be single ended in design.

Again going back to the original thread there are many designs that will perform quite well to take a typical 5 watt QRP rig to the 100-200 watt level.

One final note: many expensive amplifiers will produce a high order of IMD and harmonics when improperly driven beyond its design levels.  This includes expensive transceivers as well.

I have a gold label S-38 if you are interested. I was used very little in Harry S. Truman's travel trailer.

What are the design flaws with the OM Power amplifiers?

It is best to ship the transformer separately if you can. If the shipper drops the box with the amplifier and transformer connected, the kinetic energy of the transformer can bend the chassis or worse.

The tube can be shipped in the amplifier but pull it from the socket and bubble wrap the tube to cushion against shock. Shipping the tube in the socket can loosen the socket pins and place undue vibration on the tube elements.

Zenki,

My three main spactrum analyzers are:

HP 8593C
Agilent ESM-4402B
Agilent 89441B

What are you using?

The MRF-247 is a 70 watt class C transistor with about 6 dB of gain. It is used for FM mobile amplifiers and even when biased for linear operation makes for a very poor class AB transistor. A pair of these devices run in Class AB would only be able to provide about 100 watts out and the IMD would be no better than 20 dB second to third.

The device design on the MRF-247 is from about 1993.

When you key the amplifier the cutoff bias is reduced (-145 Vdc to -7 Vdc ) and the tubes should draw some current (60-80 mA). This depends on the diode string or Zener that has been placed into the circuit.

Some meters will just barely move with such low current.

What happens when you store a tube upside down???