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???

You cannot control a UAF on HF. Not only is there no bandwidth but most of the time even with kilowatts and beams there is no path. All control is via satellite.

I love hearing these Art Bell discussions.

Gentleman,

All two way radios wither part 97 or part 90 are made overseas. Even Motorola's. The US got out of the 2-way radio manufacturing a long time ago.

At to days shop labor rates of $175 per hour you will not save any money buying a more expensive radio thinking you wil be able to repair the unit. You only buy specifications and features.

I just thought of another thing to realize about noise blanker operation in many older and newer radios.

Older radio mostly have only a single level of noise blanker operation and when applied, like a hammer, can provide as much artifact as the noise they are removing.

New blanker circuits can also adjust the timing, width and depth of the blanking signal. When combined with a good DSP system they can not only make the noise disappear but will also aid the RF gain in setting the proper level for the incoming signal.

My current SUV has coil over plugs and a nonmetal hood. The ignition noise was very intense.  My IC-706MK II G and the FT-100D did a reasonable job of removing the ignition noise but it was far from perfect.  On advice from other hams using the IC-7000 I bought the radio at HRO and installed into the SUV.

The ignition interference disappeared completely. I was able to adjust the depth of the blanking for minimum effect on the recovered audio.  In addition the ANR (Automatic Noise Reduction) also reduced the background hiss withy out reducing the audio highs. I was dumbfounded how far the DSP in these little radios have come.

Bottom line is you can have mobile operation without turning your engine wiring into a shielded airplane ignition system

Some folks have problems with noise blankers and don’t realize that not all noise sources are removable. Pulse noise is relative easy to remove with the new DSP functions in the radio. This is because the noise source has a signature that repeats itself and has a relative long interim of audio between the narrow pulse width.  White Gaussian noise has no signature and is very broad in band width. AS a result it seems that the high frequency is reduced and add little to the recovered audio.

If your noise is from a specific source you can use RF interference noise cancellation but it requires sampling the RF source in some way. This is phase/amplitude inversion and can reduce the noise up to 30 dB. It is frequency sensitive and will have to be adjusted for each frequency.

Getting your antenna as high and far as possible from the noise source is also helpful. In addition using an antenna with a pattern null in the direction of the noise location will also reduce the noise level.

Having a amplifier that is not certificated by the FCC is not illeagle. Using it improperly is.

To the larger question ...does anyone in the FCC care? Only if someone higher up wants them to.

The ACOM 1000 uses a single tube. What is to ballance?

Everybody can have a bad day. Don’t let that color your technical considerations. In my opinion the 160-6 meter amplifier by Acom (1000) is one of the best bargains out there. Six meters is a very desirable band and having to buy only one amp for all the useful HF bands are a plus.

The cost of an amplifier is what it depreciates. ACOM  has a stellar reputation, the amplifier design is mature, many have been made and their resale is very good.

The 4CX800A7 (GU-74B) is very available and is also being made by the Chinese. This tube will be around for a long time.  The key to tube life is not trying to get the last 10% of power out of the tube.

As far as solid state amplifiers go remember they have a hard power output limit. Unlike tubes that go into power compression softly, transistors when they hit the rail voltage are very nonlinear. Also they are not tolerant of VSWR issues and will blow out instantly if the load is less than perfect.  If you use an antenna tuner to resonate the feed to your antenna, I would not use a solid state amplifier. One sleepy oppsey can be expensive.