Yes, but the ground is not considered a current carrying conductor by the NEC, and should not be used as such (the risk is that an open ground somewhere near the panel could allow a downstream load returned to ground to make ALL the exposed metalwork connected to that ground hot).

For this reason the ground wire MUST NOT (Absent that weird thing with dryers),  be used as a circuit return for anything apart from fault current, that is what neutral is for. Just because the two are bonded at the main panel does not mean they serve the same purpose or are interchangeable.
Remember the ground is allowed to be implemented using things like metallic conduit as the conductor, and after a few years who knows how well the clamps are still making contact?

Slightly OT for the OP as he needs just 240V and ground so there is no neutral involved.

I guess the EU crowd are somewhat hotter on this stuff as our 230V to ground presents a much nastier shock to ground risk.

Regards, Dan.

Ahh, no!
There are (as I understand it) two common connectors for US 240V service, a three pin one which has two lives and an earth, and a 4 pin one which has two lives, neutral & earth.

(Dryers got an odd (and none too safe IMHO grandfather clause allowing the 120V control circuits to return to earth in the 3 pin variant)).

IF the amp just needs 240V, then it should be wired White/Black between the two lives, with the ground (green) to the grounded pin (which will measure as shorted to neutral, this does NOT make it a current carrying conductor per the NEC, that is the difference between neutral and ground, the neutral is current carrying the earth is not (normally!)).

If the amp needs 120-0-120 (4 wire), it is NEVER safe to common the neutral and earth inside the plug to allow the use of a 3 pin connector, you MUST get a 4 pin type installed, but the AL572 will have a 3 wire cord that can be easily and safely wired to a 3 pin 240V plug.
You will want to fit the appropriate fuses in both the primary fuse holders, and these should both be rated for breaking 240V.

Lets not talk about wild leg three phase (A weird US abomination).

For 240V service I would advocate a GFCI (suitably rated), 240V tickles way more then twice as much as 120V does).

Regards, Dan (Disclaimer - I have been drinking, and obviously am not a US licensed electrician).

Expensive parts, probably better to use 4 SD2933 with a magic T splitter/combiner, the sand works out much cheaper and it gives you a sane degraded mode of operation when it all goes wrong.

The SD2933 is ~$150, and good  for 300W balls out, so a pair on a amplifier module will give you 500W to the combiner for $300 on transistors, or ~1KW post combiner for $600 on transistors. Back it off to 800W or so and reliability (and IMD performance) will come up, and you could run to 4 sets for the cost of a single pair of 157's (~2KW all up, or 1KW of really clean, reliable power). 

Of course with that kind of thing the ferrite starts to cost, as does power and cooling, so the choice of sand may not be that huge a factor in overall amplifier cost, but I still say that 8 * SD2933 makes a better KW then 2 MRF157 for about the same money.

That RM amp has some 'interesting' features (VOX switching?), and the (granted, preliminary) data sheet is more then a little  thin on the details (IMD, Duty cycle, harmonics), but VOX is odd as it never works well on SSB, and I always figured it for a total CBism.

Merry Christmas all, Regards, Dan (Who has been drinking so if I have dropped one, that is why).

It does state that it will do full PEP for CW, so while it may hit a duty cycle limit somewhere, it would appear to be capable of 1200W for sustained operation at CW duty cycles.

Due to the very different thermal time constants between transistors and vacuum fets you will tend (all else being equal) to find that transistor stuff does not usually have a peak rating much in excess of its quasi CW rating. Basically an anode on a transmitting tube has a huge thermal mass and will take a short term kicking, where a transistor will not, so the transistor tends not to be able to be quietly 'uprated' for speech peaks, where a tube sometimes can be.

That PA  board looks almost like something Granburg would have cooked up.....

Regards, Dan.

Cooling typically (And not necessarily the cooling of the active device), and I would expect the amp to make PEP while remaining within whatever performance figures are given for IMD, which can actually be the limiting factor for PEP output in some cases.

It could be a power supply limit, but I suspect that simple over heating is a far bigger issue with trying to run key down at PEP.

The thing is you can get an amp with the same PEP and key down ratings, even at 100% duty cycle, but it will be big and heavy and expensive... I have a amp based on an ex military PA module from Redifon Mel that manages ~500W key down all day and manages 500W PEP (Cannot use it on air until my license upgrade comes through), BUT the heatsinks are a tunnel 12cm on a side, and 40cm long full of fins. By the time I had the rest of it together it is a 6U rack 50cm deep, and a two person lift.... 

Now for amateur service you will basically never use much of that continuous carrier capability (Even RTTY listens sometimes), so all that weight and blower noise is really redundant (except that I had the module), the contentious part is the question of what duty cycle do we really need and over what time period?

PEP only tells part of the story, ideally there would also be numbers given at various duty cycles (with cycle times given), but I suspect they are often depressingly low.

Thing is, if  you are only going to run SSB, then a 1500W PEP amp with a 300W @ 100% duty cycle rating is probably actually overkill in practice as you will not manage more then that with real modulation. There is no point in having more heatsink then that if SSB is all you are into. Of course if you put such an amp in FM or RTTY service, do not be surprised if it melts.
Yes, No, Maybe, it depends on the intended use of the amp.
For SSB service, yes , it makes for a far smaller and lighter amp.
For CW service, maybe, ~60% duty cycle during transmission is about as heavy as it gets, and you typically spend 50% of the time listening.
For FM/RTTY service, maybe, again you spend time listening, but the time constant needs to be longer, and No may be the safer answer.

Our use case is usually ICAS (Intermittent Commercial and Amateur Service) as opposed to CCS (Continuous Commercial Service) or Broadcast, and you can buy an amp that fits any use category, but expect severe sticker shock.

Any of the reputable manufacturers will specify allowable power for each of the major modes, and if you buy from a CB company, well....
Could be either, but at best it tells you something about their technical competence.....

Regards, Dan.

I don't know about full on stall, but I have seen 120mm axial fans making an awful lot of noise with remarkably little air movement.

There is at least one manufacturer of amps using external anode tubes over here that cools them with a single 120mm fan sucking air out of the anode compartment....
Probably just about enough for UK legal SSB (400W PEP), but I really would not want to try it in a RTTY contest while playing fast and loose with the license power limit (We all know people like that).

"Engineering" like that is why I tend to build my own gear or convert ex military/commercial kit.

High pitched whine is sometimes a sign that someone has perpetrated a rather crude PWM controller, you might find that a series resistor with the fan fixed on full speed will get you a happy medium.

Ah, you are confusing watts with joules, a watt is a rate of work, a joule is a unit of work such that 1 Watt = 1 Joule per second.
A watt is NOT a measure of energy used of created, it is a measure of the rate of energy flow (or creation, or consumption), you will notice that your comparison with BTUs stated 3.41 BTU in a watt-hour, not in a watt, but in a watt-hour, it is a vital distinction.

BTU is a unit of energy, so a little dimensional analysis should give you Power * Time = Watts * Hours which makes sense, thus the Watt must be a unit of power not energy.

And no, a dB is a measure of a power ratio, you have to give a reference (And a way of measuring it) for it to have any meaning.

Regards, Dan.

For PEP in the general case you integrate POWER over a single cycle of the RF, so integrate voltage squared over the cycle to give Power (V^2/R) then do some fiddling with roots and such to get power.

Incidentally the fact that average power over a cycle is the mean of the root of the integral of V^2 is where RMS voltage and RMS comes from (The Root of the Mean of the Square of the voltage).

However, in ham service any given cycle of the RF is essentially sinusoidal (Our modulation bandwidth is ALWAYS much smaller then the RF frequency), so it actually suffices in this special case to simply measure the peak voltage, and assume we are looking at a sine wave, so divide by sqrt 2, to get the RMS voltage that assumes a sine wave then figure the power from that.

The difference between PEP measurement and average measurement is in what happens next:
For PEP you have a very fast peak hold circuit that causes that single cycle peak level to drive the meter to the appropriate level and hold it there long enough to read.
For average you simply average the value over a few hundred ms and send it to the meter (Typically you actually have different attack and decay times, customers like to see the needle bounce).

The whole area gets somewhat confused especially as once you move away from PEP you get into a morass of different metering dynamics, and even PEP often does not mean quite what it says on the tin the meter came in.

Regards, Dan.

Not seeing it, Watts are NOT a unit of energy but of Power, and most (all?) meters have a integration time due to the way they work.

For a meter to read a truly accurate PEP, it needs to know the frequency in use so the integration time can be set to match the period of a cycle, everyone else just uses an integration time that is short compared to a period of the modulation, but long compared to a cycle of the RF or, (simpler for the case where significant harmonic energy is not present), just implements a peak hold, then scales for a sine wave from the peak RF voltage.
This works as long as the RF waveform closely approximates a sine wave, which is essentially always the case in amateur service, and all Tom appears to be discussing is meter dynamics.

I really don't see how quoting PEP output in dB would help, particularly as a 1Kw amp would come out as (Depending on how weaselly marketing were feeling) +30dBW, +60dBm, +90dBu, and a two Kw amp would be +33,+63,+93, somehow I don't see the legal limit manufacturers being happy about being only 12dB (two S units) over a basic barefoot radio.....
Besides most of us can trivially convert back and forward between dBW and linear Watts.

Regards, Dan.

     

Not quite,
the RMS Current or RMS Voltage is the DC voltage or current having the same heating effect as the signal voltage or current, note that the term RMS applies to the voltage or current, not the POWER, which is simply average power over whatever integration time was used.

It is correct to speak of an average power over a specified integration time, which for PEP is a single cycle at the transmit frequency, and for average modulation level is long compared to the lowest modulating frequency.

Actually for PEP it is the highest such value occurring during a transmission.

It is worth noting that the average power resulting from some arbitrary signal being applied to a load is precisely the same as V(rms)^2/R as you would expect. 

Regards, Dan.

There is some really poor cooling and acoustic design out there, I think a lot of the "designers", particularly at the budget end of the market fit whatever can be had most cheaply, and may or may not look up the fan curves and get the manometer out to check back pressures.   

Fans running in blade stall are far more common then they should be, and you get silly things like fans hard up against heatsinks with no space for the flow to mix and decouple from the blades, loads of noise that way. A few inches can make all the difference and can actually increase airflow significantly.

External anode tubes tend to high back pressures, and usually need a radial blower rather then an axial fan to make sufficient pressure, they do not always get them (Radial blowers are expensive compared to a PC cooling fan). 

For real quiet you need large diameter fans and ducts so both blade speed and airflow speeds are low, but again that costs in both equipment size (Including heatsink size) and fan cost.

Be careful of attempts to muffle fans, particularly on the intake side, you can raise the backpressure easily, and that can hurt both cooling and noise levels. Lined ducts and designing plenum chambers into the equipment is helpful, but not something that is easy to retrofit.

In general when designing equipment it is better to have the fan blow air into the chassis rather then work as an extractor, most are less prone to stall if the intake pressure is as high as possible.
If the layout allows it, burying the fan inside can be a valid trick, between the power supply and RF bays is often reasonable, and the PSU bay can serve as a plenum for reducing intake noise, again not something easy to retrofit.

Regards, Dan.

Part of it is the push to ever smaller boxes, which means more backpressure, which means louder fan.

Part of it is the move to more solid state kit that needs to run the heatsinks cooler then the glass fets did, which means more airflow and a louder fan.

Part of it is 'value' engineering and the use of fans which cannot sustain the backpressure required and stall the blades resulting in lots of airflow noise (PC style Axial fans where a snail blower is really required), plotting the pressure curve and doing the graph plotting to check the operating point falls somewhere sensible on the pressure/flow curve for the candidate fan is apparently too much like actual engineering.

Regards, Dan.

Ah, that is the secret sauce added by the marketing department.

You see you rate the amplifier to the nearest kW (see note in white type on white background in picofont)......

Regards, Dan.

That used to be a very standard trick in low power AM Broadcast back  before everyone went all up EER, and actually you can do it so the effect on IMD is minimal.... Of course the electrically very short aerials with only ~10Khz bandwidth helped!
It was generally known as class H.

AM has no PM component (unlike SSB) so a simple pin diode attenuator at the input, power sensor at the output and most of the AM IMD went away as long as the supply ramped more slowly then the resulting loop bandwidth. I have designed several like that for various people.

Regards, Dan.

Well kind of, except that in reality the speakers are almost always the bottleneck!
All engineering is trading off the compromises, but speakers are basically a box of compromises.

Regards, Dan.