The power out of any solid state device is a function of the supply voltage. The  4 SD1446 devices in the HLA-300V are rated at 80 watts at 12.5 VDC. Initial engineering on the device would suggest that 300 watts is well within the design goal for a 300 watt amplifier. I don’t know where the 550 watts that everyone is suggesting as output wattage came from and maybe it is being referenced as input power draw.

The power out of any solid state device is in large part a function of the supply voltage. The SD1446 devices in the HLA-300V are rated at 80 watts at 12.5 VDC. Wattage out is a function of voltage squared divided by the impedance. In doing the math, just increasing the voltage by a couple of volts from a Vcc of 12.5 volts to 14.5 VDC will increase the power by 27%. Now this takes into account that the device dissipation is not exceeded and device voltage breakdown does not occur. No free lunch here. IMD of any amplifier is a function of the linearity of the device and the magnetics of the splitters and combiners.

As for using the device for CW that would not be a problem and you can use class”C” amplifier very well in that application.

I went to look at the test results for the HLA-150 that Tom (W8JI) tested and agree with his IMD tests. It is interesting that at 115 watts he felt the IMD (29 dB, 2nd/3rd) was acceptable. I did not read if he adjusted the bias for best results and it is possible that the unit might have had a production anomaly.  Interpolating his results to the HLA-300 would indicate that the amplifier (HLA-300V) would have similar results with 230 watts out. I have found in running my amplifier at about 270 watts PEP as indicated on the Alpha 4510, that the signal is clean and very acceptable both confirmed with on air results and by instrumentation monitoring.

There is no FCC regulation for amplifier IMD.  Higher is better but as it has been pointed out many times before most HF transceivers are much worse than the amplifier they drive.  Tom used a mix of signals with 300 Hz resolution. I wonder what the mix degradation would have been like with a wider mix of frequencies.

Bottom line is the little amplifier is a good bang for the buck and will serve you well if you realize the limitation of the design and not try to get the last 10% wattage out. Supply voltage of 12-14 volts is a hard voltage for linear amplifier design and most would like to see 24 volts as a starting point.  The impedance transformation with higher supply voltage is much easier to design linear HF amplifiers with.

I might as well wade into this mess.

The HLA-300V amplifier uses 4 ea.  SD 1446 transistors. They are good for 80 watts out at 12.5 VDC.  With higher Vcc voltage around 14.4 volts DC you can see as much as 345 watts out.  I use the amplifier in conjunction with my IC-7000. The transceiver runs a bit hot so reducing the power out to 20 watts makes the radio run cooler and the amplifier puts out a very clean 270 watts into 50 ohms. The amplifier is auto band switching and runs very well off of a 55 amp DPS power supply.

Over the weekend I connected the transceiver up to the 4 element SteppIR at 100 feet and played on 20 and 17 meters. All the while I was looking at the signal bandwidth on my Agilent 4402B spectrum analyzer. The signal was very clean. If I turned the drive up to 30 watts the power output went to almost 400 watts but the signal got wide and the second harmonic grew rapidly. Turning the power output back down to under 300 watts produced a very clean signal.

In each case I ask the ham on the other end to tune up and down the band to see if my signal was out of order.  I came up clean every time. Any amplifier can be overdriven. I have had Alphas, QRO’s, Palstar’s, ACOM’S and many others and in each case they, with improper drive, could be made to splatter.

It is not the amp but the operator who in the quest for the last watt overdrives the amplifier.  I might add that the transceivers can be overdriven as well and that is where most of the band splatter comes from.

Depending the construction variables the initial tune and load setting will vary from amplifier.

Start with 5 watts and adjust the setting for maximum output while keeping the grid current under 30 mills.

Increase the drive noting the tune and load settings as you tune for maximum power out.

This will get you into the ball park.

Tune quickly and adjust the settings in under 15 seconds.

Once you have set the tune and load settings for best efficiency for power out you can reduce the drive to set desired power out on each band.

On each band carefully observe the grid current and keep it under 50 mills.

Read W8JI web page on tuning an amplifier.

The IMD levels on most transceivers is better at its rated output than at lower output levels.

IMD in a typical HF transceiver can be much higher than levels in an class AB HF amplifier by as much as 15-25 dB.

Driving your amplifier with 60 watts instead of 40 watts will yield possible measurable IMD differences but most hams will never hear it on the air.

The main reasion for the post is to point out that lower drive levels does not produce higher amplifier output.

Unless you are sure your transceiver /amplifier is not hot keying in the full break in mode I would use the memory keyer in the simi break in mode.

Drive on legal limit amplifiers.

Product Review Section

 “OM 2500HF owned less than two months. The gain on the amp is reduced to meet FCC 15db. To run legal limit it takes 60 watts of drive from exciter. Amps that I have owned in past (Acom, Henry, Alpha) 35 40 watts of drive is more than needed. When emailed OM they would give me the mod to return gain to WORLD model but, were very busy right now. Your guess is as good as mine when that will be. Why a 2, poor support with I get to you when I got time!

Very well built amp, poor instruction manual!”

I spotted this in the product review section and it should be noted that all 1500 watt linear HF amplifiers are now required to have gain less than 15dB as per the FCC. Now I know many of you are thinking if I could drive may amplifier with 40 watts to get 1500 watts PEP out then driving the amplifier with 80 watts would produce 3000 watts PEP. For you new hams that is not the case. Amplifier wattage out is a function of tube power dissipation, anode voltage, power supply current capacity and other component ratings.

Increased gain will not increase the wattage out beyond maximum design capacity.

Many amplifiers have the bias set in conjunction with the design gain for minimum distortion and harmonic output. Shorting the cathode resistor to increase the amplifier gain can bring a whole host of problems.

You are better running the amplifier at higher drive levels for the drive transceiver generally has the lowest distortion (IMD) at the transceivers higher output capacity. On a 100 watt transceiver the IMD will generally be lower at 90 watts than 30 watts.

I understand the manufacture being hesitant at providing assistance in modification of the amplifier for they could have their FCC certification in jeopardy. Modification of a commercial amplifier requires a complete understanding of all the electrical interactions if you don't completely understand all of the electricals you can end up with the quiescent idle current too high of worse.

Trust me Steve is not Zinke. Steve is proud of his call and has been a ham for too long to go chasing windmills.

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.