[AMRadio] AM Usage with Linear AMPS


Brian Carling bcarling at cfl.rr.com
Mon Nov 22 22:55:22 EST 2004


Aren't there a lot of commercially built amateur radio linear 
amplifiers and RF finals that exceeded he manufacturers MAX 
Pdiss rathings over the years, by adding a COOLING FAN to take 
away the extra heat?

One thinks of MANY sweep tubes rated for so-called Pdiss of 16 
watts and 20 watts that are regularly run at hundreds of watts of 
RF! The tubes don't seem to melt.

I think of the ubiquitous 6JS6C with a rating of 30 watts.
Yaesu ran a pair of these at 260 watts input in their FT101 series.
Many guys ran them at 260 watts p.e.p. on SSB and 260 watts CW.
Assuming 65% efficiency, you have 35% of 260 watts going into 
the plates. That is to say 91 watts split between the two tubes.
UH oh - POP! But no, they didn't.

Then when you throttle that FT-101 back to 40 watts input on AM
and go to your 30% efficiency (if it is) then you are actually 
putting only 20 watts carrier per tube which is SAFER.

Is that correct?

How about some of the othre rigs that rated their 2 sweep tube 
finals for 560 Watts or even 800 watts!?

Then youhave peak Pdisses of 140 to 200 watts between the two 
sweep tubes, most of which are rated for no more than 40 watts.

On 22 Nov 2004 at 21:29, John Coleman, ARS WA5BXO wrote:

> I've got to get in on this HIHI.
> 
> 	Some readers may not fully understand what you guys or talking
> about.
> So I will try to confuse it all some more. HIHI
> 
> 	Plate dissipation is the heat that the plate of a tube is
> emitting.
> The manufacture of the tube has what is known as max plate dissipation
> limits.  The 811A triode has a limit of about 65 watts.  This means if
> the tube is in a circuit where it is putting out a steady carrier and
> this circuit is 75% efficient at is max level this would be 4 X 65 input
> power (Ep X Ip) and 3 X 65 output power (as measured by a RF Watt meter
> with no reflected power).  This would be 260 watts input and 195 watts
> output with 65 watts wasted in heat of the plate.  This would not be bad
> for CW.
> 
> I don't think 75% efficiency is an achievable figure in a good linear
> operation even on peaks.  As the drive level is reduced so is the output
> level (linearly we hope) but the plate current will not drop as fast.
> As a result the efficiency drops and it is possible for the plate
> dissipation to be greater even though the plate current is less.  It is
> not a good idea to operate at the point where the plate dissipation is
> at its highest.  
> 
> In any linear amplifier audio or otherwise, proper design would be, to
> not exceed the plate dissipation at any time.  How ever we all know that
> plate dissipation limits can be exceeded for a short duration provided
> that there is an equal or more amount of time that has a much less plate
> dissipation than the rating and that the time intervals or close enough
> to keep the overall temperature down on the tube.  Exceeding Plate
> dissipation causes excessive secondary emissions and internal grid
> leakage.  It is also the biggest cause of vacuum loss because of leaking
> seals at the anode connection.  Poor vacuum causes more grid leakage and
> emission loss due to corroded heaters.  Over temp on a tube is not good
> even for a short period of time.  
> 
> 	In a class B circuit with no input or output the plate
> dissipation must be below the limit.    
> 
> 	If the input is varying as it is with SSB or audio then the tube
> will pass through an area of much higher dissipation.  This area is
> generally (but not always) somewhere near the 50% mark of peak drive and
> output.  This means, the tube could actually be hot with just a carrier
> and, then cool down when the input is modulated with a tone.  
> 
> 	As I recall 811s do not (or just barely) show color at 65 watts
> dissipation.  This would be static 2000V and 32 ma or 1500 volts and
> 43ma.  
> 
> 	Good practice would be to set bias some what below current for
> 65 watts dissipation.  As input carrier is introduced a output carrier
> will also be created.  It should be a linear progression until top end
> is met.  During this progression the tube will go through various levels
> of efficiency and plate dissipation.  Some of the places will be very
> close to or exceeding the limit of 65 watts.  
> 
> 	A point of best operation would be where the output is 25 % or
> less of the peak output POWER level (OR 50% of the RF output VOLTAGE)
> and the plate dissipation (input power - output power) is less that 65
> watts per 811A.
> 
> Good Luck to all
> John WA5BXO            
> 
> 
> 
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