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Author Topic: 2x 3-500z vx single 3-500z  (Read 17530 times)
W8NIC
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Posts: 63




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« on: February 09, 2014, 11:50:03 AM »

Hello gents, here's a newbie question (im a newbie) why is it that an al-80b will do a KW while my SB-220 and Drake L-4B both have a pair of 3-500z tubes will only do 1000-1100 watts? (40 meter cw)
Antenna is a 40 meter dipole, tuner is a palstar and radio is an Icom 756 Pro2......
Thanks in advance for your input.

Larry w8nic
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N6AJR
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« Reply #1 on: February 09, 2014, 12:16:24 PM »

some of the output depends on the ability of the power supply to supply higher voltages..
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W8JX
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« Reply #2 on: February 09, 2014, 12:23:40 PM »

The SB220 and L4B are loafing at 1kw and AL80 is exceeding tube ratings to make a KW.  What is holding the big boys back is low plate voltage as the AL80 has a somewhat higher plate voltage too. If you were to boost plate voltage to 3500 or more the L4B or SB220 would easily make it to legal limit.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
N9AOP
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« Reply #3 on: February 10, 2014, 08:40:11 AM »

a pair of 3-500's will last a whole lot longer if they are run at around 1300w.  In fact, TenTec crafted their twin 3-500 Centurion amp with a power supply that would not let the amp do a full 1500 watts to conserve tube life.
Art
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WB2WIK
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« Reply #4 on: February 10, 2014, 09:37:32 AM »

When the SB-220 and L-4(B) were designed the legal amateur power limit in the U.S. was 1 kW DC input.  There was no reason to design for more power, since it wasn't legal anyway.

It was raised to 1500W PEP output since then.

The AL-82, with two 3-500ZGs, makes 1500W PEP output easily.  Much bigger power supply.
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WA8ZYT
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« Reply #5 on: February 10, 2014, 11:16:39 AM »

I don't think they cut the power back to conserve tube life. Those tubes are quite rugged. The plate voltage is lower and that is the main reason output is lower as others have said. Run the tubes up to 4000 volts and they will start to play nice.
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W1VT
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« Reply #6 on: February 10, 2014, 11:42:16 AM »


The AL-82, with two 3-500ZGs, makes 1500W PEP output easily.  Much bigger power supply.

The AL82 power transformer costs $800, half the cost of an AL80!  I'm not sure, but I think the AL80 power transformer costs just $180.

Zack W1VT
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W8JX
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« Reply #7 on: February 10, 2014, 11:45:32 AM »


The AL-82, with two 3-500ZGs, makes 1500W PEP output easily.  Much bigger power supply.

The AL82 power transformer costs $800, half the cost of an AL80!  I'm not sure, but I think the AL80 power transformer costs just $180.

Zack W1VT

Do not know the cost but AL80 while not having a AL82 class transformer, it is a decent one and much bigger one than AL811H
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KG6YV
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« Reply #8 on: February 10, 2014, 02:46:16 PM »

When the L4B and SB220 were marketed the legal limit for US amateurs was

2000PEP SSB INPUT
or -1000W CW OUTPUT.

So, that is what they were designed for.  The L4B runs only 2700V on the plates, a modern amplifier with 2 3-500Zs like my AL82 runs 3650V.  THe limiting factor on 3-500Zs for power ouput is plate current and grid current.  Lower plate supply voltage means less output, end of story.

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WB2WIK
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« Reply #9 on: February 11, 2014, 09:11:00 AM »

When the L4B and SB220 were marketed the legal limit for US amateurs was

2000PEP SSB INPUT
or -1000W CW OUTPUT.

So, that is what they were designed for. 


I don't think that's not true at all.

There was never a time that Part 97 discussed "SSB input" power, and there was never a time that 1000W CW output power was the legal limit.

Regs went straight from "1000W DC input power to the final stage" (with no mention of PEP) to "1500W PEP output power" (with no mention of DC input power).

I don't believe there was ever a time the regs specified anything in between those.

The "2000W PEP input SSB" phrase was a marketing thing coined by amateur radio equipment manufacturers, as I recall; it never appeared in the regulations, and SSB input power is something nobody measures as it's a complicated measurement.
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K4FX
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« Reply #10 on: February 13, 2014, 10:01:23 PM »

This has been discussed here many times. 1000w PEP is a lot of power for a single 3-500. I own an Ameritron AL-1500 and have owned the 80A and the 811H over the past 35 years of hamming. I like their amps, they are inexpensive and for the most part they work.

But face it. Ameritron exaggerates their ratings to sell amps. If anyone thinks 4 811's will run 800W PEP for any length of time, you better think again, 500 is more like it. The Collins 30L1 was a very similar amp with correct ratings.

73

Bill K4FX
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W8NIC
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« Reply #11 on: February 24, 2014, 02:05:36 PM »

When the L4B and SB220 were marketed the legal limit for US amateurs was

2000PEP SSB INPUT
or -1000W CW OUTPUT.

So, that is what they were designed for.  The L4B runs only 2700V on the plates, a modern amplifier with 2 3-500Zs like my AL82 runs 3650V.  THe limiting factor on 3-500Zs for power ouput is plate current and grid current.  Lower plate supply voltage means less output, end of story.


Thanks for all the answers guys.

73
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W8JI
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« Reply #12 on: February 24, 2014, 05:27:17 PM »

People just say things here. The facts are:

The SB220 and L4B were designed when the power rating was 1000 watts plate input power. In grounded grid, the driver plate power input had to be deducted from amplifier plate power input, so that typically made any grounded grid amplifier run less than 1000 watts maximum on the plate current and HV meters.

This limited output power to about 600 watts carrier. At this level the anodes would dissipate a TOTAL (for two tubes) of 400 watts plus some heat from grid and filament, but about 200 watts per tube. Most kilowatt amps actually initially used 250 watt tubes, but the 3-400Z and later 3-500Z were actually cheaper to build than a 4-125 or 4-250.

The airflow system in the 220 and L4B (and similar amps like the Raytrak) were downsized to handle around 600 watts total dissipation, or 300 watts per tube, with rated seal temperatures. This is why sockets overheat and base pins melt when people hammer their SB220's, thinking that 2 tubes equals a kilowatt output CW.

The 220 was a good design, the power transformer, cooling, tank, and everything was all right at the edge with enough margin to last when operated at ratings.

The FCC rating for SSB, prior to 1982, was the same as CW. The panel meters had to always indicate less than 1000 watts DC plate power on the very maximum swing, and with grounded grid PA's the full plate input power of the stage driving the PA had to be deducted from the allowed input. If you had a T4X that ran 150mA at 700 volts, you were allowed 895 watts plate input power on the meters.

The SB220 and all amplifiers of that era were designed to the law, because the FCC was pretty strict at that time. This is why the SB220 has a CW Tune voltage of ~2100 volts under load, and a SSB voltage of about 1.4 times that value. 

With some conditions of voice, where processing was light, that might be 2kW PEP or it might be 4 kW PEP, but the meter power maximum was always 1000 watts or less. Since that was the law, that's what was planned.

The limitations in the 220 are not the number on the tube, they are airflow, power supply current, voltage, and heat, and tank circuit.

That law was proposed to change in the mid to late 1970's, and in 1982 it actually changed to PEP output on all modes.

Now for tube life. People commonly have a real misplaced notion about tube life. I'm not sure where it comes from, maybe egg beaters or gasoline engines. The picture I get is they think the filament or grid or something "wears out" from high speed running, maybe some sort of friction that wears away on parts like rings on a cylinder wall.

Tubes do not work that way.

The cathode emits all the electrons needed to run whatever peak emission, and that is based on temperature. That is filament voltage. If you ran the tube 20 years at zero plate current, the emission would go away almost as fast as running it near the emission limit for the same voltages. The limit to reducing voltage is how much peak emission is needed, and how low it can go without poisoning the emitter. In amateur service on thoriated tungsten tubes, you will never see the life change. It will be lost in the noise of other failures, and this is especially true with import tubes.

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

The other elements are damaged by heat. They have a thermal limit that is a function of time and the dissipation over that time. This is why Collins and Heath and a dozen other companies all run 811 tubes at greater than rated dissipation. A Collins 30L1, for example, was rated under the same kilowatt input laws. That's the same as an SB220. The 30L1 tubes were 65 watts rated dissipation, for a total of 260 watts. Because Morse CW duty cycle was 65% or so, they assumed dissipation was safe at 1/.65 * 260 = 400 watts. This would allow 600 watts output. The tubes were rated to be convection cooled, so any air was just to keep the cabinet cool.

This was called an IVS and IAS rating, and despite what forum tube wear-experts might think, Heath and Collins and others were perfectly right thinking that way. So we have kilowatt input amps with as low as 260 watt dissipation tubes, but suddenly 500 watts with adequate airflow is too low for 1200 watts peak input under the same duty cycle.

The actual bottom line on tubes is most tubes today fail from two reasons:

1.) They are junk when manufactured

2.) They get overheated from too much dissipation when tuning, or from mistuning, and the anodes fail

Neither of the two major failure problems relate to how conservatively the tube is run when properly tuned. One relates to entirely on how the tube was built. There is no way to test that out, or run that out. It has to be corrected at the assembly point and problems eliminated there.

The other failure almost always relates to how long the operator takes to tune or load the amplifier, and how he tunes it. If he runs 200 watts with the amp mistuned, the tubes might dissipate 1000 watts. If he tunes it right, the tubes might dissipate 400 watts with 800 watts out.

Overall, there is almost nothing anyone can do to extend tube life other than not melting an anode down.

Metal Oxide emitters are another issue. They can be depleted by excessive emission current and generally are easily damaged by high grid current. Grid damage can be cumulative, as can cathode stripping. The grid can migrate gold by elevated temperatures and poison the tube, and the cathode can strip if run too cold or if HV is too high. If you don't suck so many electrons away from the cathode to deplete the charge and allow positive ions to reach the cathode, and you don't overheat or run the grid at elevated current, there isn't a thing you can do with a oxide cathode tube either. If you ran an 8877 at 300 watts or 1500 watts, any life change would be lost in the noise of random failures.

There is one more important thing. 3-500Z's degass by anode temperature. It has to show a little color or the tube will not getter. RCA, for example, used a 400 watt dissipation driver tube, similar to a 3-500Z construction but a tetrode, in an AM transmitter. The driver tubes had a very high field failure rate. At Eimac's suggestion, they dropped back to a tube that ran with the anode red, and the problem went away.


73 Tom


   

 

On SSB, the

 Also, 3-500Z tubes were about $50 
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W8JX
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« Reply #13 on: February 24, 2014, 06:23:35 PM »

The dissipation of 3-500 is when it is in a envelope with air blown up thru socket (to cool pins too) and over envelope. This is the proper way to cool a 3-500z, not with a fan just blowing across it. This is why you see socket and pin failures when you push sb 220's and similar amps beyond design ratings. Sadly though this is were many cut costs and skip proper cooling.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
G3RZP
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« Reply #14 on: February 25, 2014, 12:02:16 AM »

Tom,

Didn't the FCC specify the meter time constant too?
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