AL-82 on AM

[Edward Cain]

Question for anyone familiar with the Ameritron AL-82: What carrier power out can this linear produce "comfortably" on AM, assuming adequate drive power?

Thanks,
Ed

[AB2EZ]

Ed

My short answer to your question is 250 watts. Here is why:

For linear AM operation,  you should tune and load the amplifier to produce a peak power output of 1500 watts. The amplifier will be operating (when producing peak power) at about 67% efficiency*... so the electrical plate power input will be about 2250 watts, and the plate dissipation (both tubes) will be about 750 watts.

*Note that the theoretical maximum efficiency for class B operation is pi/4 x 100% = 78.5%, but a practical amplifier, running class AB will likely have a plate efficiency closer to 67%.

Then you should back off the input power to the amplifier to produce the target carrier level amplifier output power.

If, using the above procedure, you set the carrier level to 375 watts (i.e. carrier power = 1/4th the peak power, which allows only enough headroom for 100% positive peaks)... then the electrical input power will drop to 2250 watts / the square root of 4 = 1125 watts. 1125 watts of electrical input power - 375 watts of output power = 750 watts of plate dissipation (both tubes). In this case, the plate dissipation at carrier is equal to the plate dissipation at peak power.

[*If you redo this calculation assuming the theoretical maximum plate efficiency for a class B amplifier (78.5%), then the plate dissipation (both tubes), at carrier, will be 580 watts. But, achieving a plate efficiency that high is very optimistic]

Alternatively, using the above procedure, if you set the carrier level at 250 watts (i.e. carrier power = 1/6th of the peak power)... then the electrical input power will drop to 2250 watts / the square root of 6 = 918 watts. 918 watts of electrical plate input power - 250 watts of output power = 668 watts of dissipation (both tubes).

This (668 watts of plate dissipation for both tubes) is about the maximum carrier power you can use comfortably with a pair of 3-500Z's. The tubes will still get very hot on long AM transmissions.

Also, 918 watts of electrical power input at carrier is (obviously) less taxing on the power supply than 1125 watts of electrical power at carrier.

AM is very hard on tubes. The heating and cooling from long transmissions followed by periods of standby will take is toll in terms of cracking of the glass envelope near the plate cap, and (for the older tubes with the vertical fins of the plate) metal fatigue at the point where the plate structure attaches to the top of the tube.

If you ever change the tubes... buy the newer types with the ceramic (or whatever it is) plate structure. The older tubes with the vertical fin metal plates will fail (with a bang as the plate structure tilts over, and shorts to the rest of the elements) very quickly.

The above is based on my experience with a single tube 3-500Z amplifier from QRO Technologies. In that amplifier, I added an auxiliary fan on top of the cabinet, to draw air through the vent holes above the tube.  

On AM, I operated it with between 100 and 125 watts of output at carrier. I could watch the tube by using a small mirror mounted above the vent holes at 45 degrees. Even at 125 watts of carrier, the plates would get pretty red on long AM transmissions.

Stu

[W3GMS]

Question for anyone familiar with the Ameritron AL-82: What carrier power out can this linear produce "comfortably" on AM, assuming adequate drive power?

Thanks,
Ed

Ed,

I have an AL-82 and its been my main AM linear for the last 10 to 15 years or so.  I tune it up for maximum power while applying 100W of drive and can get about 1800W into a dummy load.  Then I switch to my Ranger which is modified to run the 6146 off the LV power supply.  I did replace the LV choke on the Ranger and have SS diodes on the supply.  I think I get somewhere between 18W  to 20W of drive on the AL-82.  I nominally get about 350 carrier output out of it and its been running that way for 10 to 15 years without any issues.  Same tubes as what was shipped in it new from the factory.  Mine was made in the time frame when Peter Dahl iron was standard on that amp.  I do have the blower motor set to high speed.

73,
Joe, GMS

   

[w1vtp]

It's worked for me.  I run 250 watts output after tuning as Stu has indicated.  Don't use so much anymore as I have a Collins 32V1 and a Class E xmtr for AM service

Al

[flintstone mop]

I have the AL 1500 and the Ameritron amps are really good and heavy. Power supplies have a lot scrote.
Fred

[Edward Cain]

Stu,
   Thanks for taking the time to explain your answer.


Joe and Al, thank you for your comments.

Fred,
   What power is your AL1500 capable of producing comfortably on AM with that 3CX1500A7?

Ed

[flintstone mop]

I have run the AL 1500 as high as 400 watts carrier and there were still positive modulation peaks of 130%, using the Mod monitor from Steve 'QIX. Radio Engineering Assoc.
18 watts of drive from the Flex SDR 1000 radio.
The AL1500 can put out peak RF of 2500 Watts.
I run it at 350 watts carrier and try not to go above 120% mod. The characteristics of the microphone or my voice gets about 95% negative.
Fred

[w1vtp]

Here's how my PEP works out given a 250 watt carrier and 150% positive peaks - not BTW unusual numbers with Flex Radio transmitters or class E transmitters.

Just note, that in the old days, EFJ, especially, deliberately mismatched the modulator with the class C load to limit the positive peaks to around 100% -The solution then was to put a high level splatter filter across the high level modulator output.  I have talked to some respected AMers who had to deal with modulation transformers mysteriously blowing up and the solution was to eliminate the splatter filter and correctly match the modulator to the class C amplifier.

Modern day design is different for boat anchor and SDR and class E where proper design allows for the asymmetrical voice that would easily exceed 130 to 150% positive peaks.  Add in the design 3 diode negative peak limiting and it is possible to have an excellent good fidelity AM signal that occupies a minimum channel width.  The old design goal of needing to have 1/2 the modulator power to modulate a class C amplifier is pretty much passé.  We need to carefully calculate the required peak levels in both the selection of modulator tubes, maximum capability of the modulation transformer..

Al

[ka1tdq]

I run a pair of 3-500's at 250 watts carrier, and my plates just start to get red on long transmissions. 

Jon
KA1TDQ

[W1AEX]

Hi Ed,

I've enjoyed reading this thread as I am an AL-82 owner who uses it often to run AM linear. It has a beast of a power supply and another plus is that it uses under-chassis forced air chimney cooling that pushes a lot air through the sockets and over the entire envelope of each 3-500. I typically run mine at 150 - 200 watts of carrier power and with the asymmetry of the SDR transceivers I have used with it the peak power reaches a little beyond 1000 watts. The plates of both of my 3-500s quickly turn red and on long transmissions they will get bright red and start moving to orange. If you flip the bias switch to CW they will run far cooler and will still be plenty linear in AM service, but they require more drive from the exciter to reach the same level of output.

Tom - W8JI who designed the AL-82 is in complete agreement with Stu that you should not exceed 250 watts of carrier in linear service with the amp. He gives his reasons why on his page that deals with AM linear service. To answer your question regarding the AL-1500 his tube chart's absolute maximum is 375 watts of carrier power with a single 8877 but he adds the caveat that if cooling is not optimal this power level would not be recommended.

http://www.w8ji.com/am_linear_amplifiers.htm

Ameritron mentions AM linear in its AL-80B manual where they recommend a carrier level of 200 watts, assuming the usual 1:4 carrier to PEP ratio. This always struck me as a very optimistic power rating given the fact that the tube only has a fan blowing on it from the side and there is also somewhat of a reputation that you may end up with solder dripping from the pins of your 3-500 if you actually tried it. I never ran more than 150 watts of carrier on AM when I had the AL-80B and count myself lucky that my Chinese 3-500ZG survived those transmissions. In retrospect, I wouldn't venture much beyond 100 watts of carrier on AM with that cooling setup.

At any rate, the AL-82 is a great choice for AM linear service and with those plates getting cherry during your AM transmissions I don't think the tubes will be getting gassy any time soon.

Rob W1AEX  

[ka1tdq]

Not to beat a dead horse, but I took note of Rob's mention that the plates on his AL-82 get red quickly and turn orange on long transmissions.  I can talk for a good 2 minutes and my plates are about half-red.  We're both running 250 watts carrier, so I'm guessing that it's just engineering of the different amps.

I have a diode string in mine which biases the cathode at around 7.5 volts (as I remember).  I guessed on the pi output coil but it's around 5uH for 40 meters, and the Loading cap is fully meshed (around 1800pf on the output, again as I remember).  And I think I'm running about 4000 volts on the plate (not sure because I don't measure it). 

Jon
KA1TDQ

[WD5JKO]

The plates of both of my 3-500s quickly turn red and on long transmissions they will get bright red and start moving to orange.

   I was thinking that the thermal time constant of the 3-500Z plate is relatively fast; perhaps showing some reddish orange in 10-15 seconds when the Pd is 400 watts or so. Now, if the tube starts getting brighter over a long transmission, then something potentially bad is occurring.

   For AM linear operation, full modulation or heavily processed audio should lessen the visible plate dissipation. Whistling into the Mic and achieving 100% modulation should double the efficiency of the tube as the peak power is increased 4X. So if the tubes are a blushing, that visible plate should dim considerable with a sustained modulation note, or a long Yaaaaaa.

   I think what might be happening is the the filament choke is getting hot. The permeability of the core will drop, and therefore the inductance will drop. On the lower bands this will present a problem. This is a common problem on some vintage linears. The Gonset GSB-201 has this issue, and I've seen folks talk about the filament choke on the Heath linears (SB-200 and possibly the SB-220).

   The Dentron MLA-2500 suffers from the fixed padding capacitors on the pi-net are not good RF current carrying capacitors, and the capacitance changes quickly as they warm up. This causes the PI-Net to go out of tune with a long winded transmission. The consequence is reduced efficiency, reduced output, reduced gain, and an increase in the tube plate dissipation. Better Cooling with the Dentron helps minimize the problem, as does running low duty cycle modes such as CW and SSB.

   Perhaps the Ameritron AL-80 suffers a bit just like it's ancestors mentioned?

Jim
Wd5JKO

[w1vtp]

I have stated in this thread elsewhere that the old "rule of thumb" of a 4:1 ratio between the Peak power and the carrier just not jive.  Given the asymmetry of the human voice / design goals the ratio is more like 6:1.  This is important because unnecessary bandwidth will occur if excessive flat topping becomes part of the design goal for the old rule of thumb.

This can become an especially noxious problem if excessive flat topping happens because there just is not enough head room with the transmitter - be it high level modulation or low level modulation using a linear.  

I run an AL-82 and have run (on a dummy load) fairly long tests between dead carrier at 1500 watts and then running AM with the Flex 5000.  That 6:1 ratio jives with my experience.

Now.  If one were to deliberately design an AM station such that the absolutely highest positive peaks were to be 100% with the resultant flattopping with attempts of higher modulation levels and then to employ high level filtering then bandwidth problems might not occur.  I just do not think we should be designing AM stations with those parameters.

Al

[ka4koe]

6:1 seems about right. I don't run more than 150W carrier on my SB220. Folks tell me it sounds best at this power level and no more.

[WD8BIL]

The Dentron MLA-2500 suffers from the fixed padding capacitors on the pi-net are not good RF current carrying capacitors, and the capacitance changes quickly as they warm up. This causes the PI-Net to go out of tune with a long winded transmission. The consequence is reduced efficiency, reduced output, reduced gain, and an increase in the tube plate dissipation. Better Cooling with the Dentron helps minimize the problem, as does running low duty cycle modes such as CW and SSB.

I've been running the 2500B for 15 years on AM with 375W carrier most of the time and have not seen any problems. Can I get a witness??
 
Interesting!

[W2VW]

Ran my 2500 @ 125 watts carrier on 10 meters all season 3 years back.

Stopped at any sign of grid current but had the original tube type with just an extra exhaust fan.

Dunno about the padders being weak but have seen that plenty elsewhere.

[WD5JKO]

I've been running the 2500B for 15 years on AM with 375W carrier most of the time and have not seen any problems. Can I get a witness??

See Post 16 - 18 here:
http://www.radiobanter.com/showthread.php?t=94835&page=2
Apparently everyone of those amps was like a box of chocolates...depending on the parts that could be salvaged at the nearby surplus houses.

Ran my 2500 @ 125 watts carrier on 10 meters all season 3 years back.
Dunno about the padders being weak but have seen that plenty elsewhere.
Stopped at any sign of grid current but had the original tube type with just an extra exhaust fan.

The fixed padding caps were for 160-80-40 meters. Ten meters should have been just the air variables.

Back to point, does the AL-82 use NPO type fixed padding capacitors rated for the circulating current in the tank, or do they use the types with a negative drift coefficient such as N5600? This last type is best for bypass use, and not the best choice for use in a high Q resonant circuit.

Jim
Wd5JKO

[AB2EZ]

Jim

I agree with most of your comments/observations below... and, in particular, your observation that a change in the resonant frequency of the tank circuit, due to heating of the padding capacitors, will cause the efficiency of the output stage to drop... and the plate dissipation to rise. This is a variation of the "Chernobyl effect".

I do not agree with this comment (below)... but I do agree that modulating +/- 100% will lower the average plate dissipation:

"For AM linear operation, full modulation or heavily processed audio should lessen the visible plate dissipation. Whistling into the Mic and achieving 100% modulation should double the efficiency of the tube as the peak power is increased 4X. So if the tubes are a blushing, that visible plate should dim considerable with a sustained modulation note, or a long Yaaaaaa."

It is true that if the amplifier is loaded at peak power... then the efficiency will be only half as much at carrier (v. the efficiency at peak output power), if the carrier output power level is 1/4 of the peak output power level.

But, the plate dissipation will be roughly the same.

Example (class B operation):

1500 watts with 67% plate efficiency => electrical plate input power is 2250 watts, and the plate dissipation is 750 watts

Reduce RF drive to cut the average plate current in half

Electrical plate input power drops by a factor of 2 => 1125 watts
RF output power drops by a factor of 4 => 375 watts
Plate dissipation stays the same => 1125 watts - 375 watts = 750 watts


What will happen, if you modulate, is that the output power will rise to (for example) 100% of the peak power, and fall to 0% of the peak power. The plate dissipation can be calculated at various levels of modulation:

With 67% maximum plate efficiency, class B operation:


1500 watts RF output power: 750 watts of plate dissipation (100% positive modulation)
1215 watts of RF output power: 810 watts of plate dissipation (80% positive modulation)
844 watts of RF output power: 844 watts of plate dissipation (50% positive modulation)
375 watts of RF output power: 750 watts of plate dissipation (carrier level)
93.8 watts of RF output power: 469 watts of plate dissipation (50% negative modulation)
15 watts of RF output power: 210 watts of plate dissipation (80% negative modulation)
0 watts of RF output power: 0 watts of plate dissipation (100% negative modulation)

Assuming sine wave audio modulation (for example), the average dissipation (averaged over each audio cycle) will be lower than at carrier. You can see this be averaging the dissipation at corresponding positive and negative modulation indices. For example, averaging the plate dissipation at 80% positive modulation with the plate dissipation at 80% negative modulation yields 510 watts of average plate dissipation.


Stu

 

The plates of both of my 3-500s quickly turn red and on long transmissions they will get bright red and start moving to orange.

   I was thinking that the thermal time constant of the 3-500Z plate is relatively fast; perhaps showing some reddish orange in 10-15 seconds when the Pd is 400 watts or so. Now, if the tube starts getting brighter over a long transmission, then something potentially bad is occurring.

   For AM linear operation, full modulation or heavily processed audio should lessen the visible plate dissipation. Whistling into the Mic and achieving 100% modulation should double the efficiency of the tube as the peak power is increased 4X. So if the tubes are a blushing, that visible plate should dim considerable with a sustained modulation note, or a long Yaaaaaa.

   I think what might be happening is the the filament choke is getting hot. The permeability of the core will drop, and therefore the inductance will drop. On the lower bands this will present a problem. This is a common problem on some vintage linears. The Gonset GSB-201 has this issue, and I've seen folks talk about the filament choke on the Heath linears (SB-200 and possibly the SB-220).

   The Dentron MLA-2500 suffers from the fixed padding capacitors on the pi-net are not good RF current carrying capacitors, and the capacitance changes quickly as they warm up. This causes the PI-Net to go out of tune with a long winded transmission. The consequence is reduced efficiency, reduced output, reduced gain, and an increase in the tube plate dissipation. Better Cooling with the Dentron helps minimize the problem, as does running low duty cycle modes such as CW and SSB.

   Perhaps the Ameritron AL-80 suffers a bit just like it's ancestors mentioned?

Jim
Wd5JKO

[WD5JKO]

Example (class B operation):

1500 watts with 67% plate efficiency => electrical plate input power is 2250 watts, and the plate dissipation is 750 watts

Reduce RF drive to cut the average plate current in half

Electrical plate input power drops by a factor of 2 => 1125 watts
RF output power drops by a factor of 4 => 375 watts
Plate dissipation stays the same => 1125 watts - 375 watts = 750 watts
Stu

 Yes, I agree with your scenario above.

But what happens when we modulate from there?

The DC INPUT stays the same, but with 100% sine wave modulation the RF output increases 50%. The carrier stays the same as read on a spectrum analyzer. We produce two sidebands where each are 6 db down from the carrier. So for the example above we have a 375w carrier, and two sidebands each at 82.5 watts. Add them up and we have 495 watts which is what we read on a non peak reading watt meter...a 50% increase in RF output.

So where does the extra power come from? The DC input stays the same. The only answer I can come up with is the increased efficiency during modulation lessens the tubes plate dissipation. AM linear operation is a form of efficiency modulation where increasing efficiency during modulation is the source of the extra power necessary to produce the two sidebands.

My conclusion may not be 100% correct, but for sure, in any form of efficiency modulation, and that includes grid modulation (control, screen, or suppressor), the tube resting plate dissipation lessens as the modulation is increased. I am assuming the tube is tuned for full power (4X) first, and then the drive backed off to 1/4th (linear), or the grid (grid modulation) is biased back for 1/4th the output power.

Jim
Wd5JKO

[AB2EZ]

Jim

Yes... I agree with your latest post.

See my revision to my earlier post for more details

Stu

[WD5JKO]

Those that tune their linear amplifiers for peak power at 100% modulation will be getting a resting carrier efficiency in the low 30% range; sometimes a bit less. Here the resting carrier is 1/4th the PEP at 100%. This is just the way it is. When we want 150% positive peaks to go through the amplifier, then we need to set the resting carrier to 1/6th the PEP. What people may not realize is that doing this makes the efficiency drop into the low 20% region.

If we bias the RF tube closer to cutoff (AM only), the efficiency improves about 5%, and the output modulation is higher then the input (about 5%). The trick here is since the gain decreases, you must increase the excitation to get the same output as before. This causes some distortion, but since we can back off the exciter audio level, the distortion of the exciter drops such that overall the distortion might not change much. A 5% change in efficiency might not seem like a lot, but going from 20 to 25% efficiency is actually a 25% increase. Some call this a class BC linear amplifier. The positive peaks improve also.

The image below shows a graphical analysis of what happens as we bias a linear amplifier closer to class C.

Jim
Wd5JKO