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Author Topic: AL-80A low output  (Read 2820 times)
AE6RF
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« on: October 20, 2008, 10:31:06 PM »

Greetings,

Back with the next question...

Starting with the AL-80A on 120 VAC. The HV reads 3150 V. When keyed and adjusted as per the manual the HV drops to 2800 V. Not too surprising based on wimpy 120 VAC.

However, when adjusted according to the manual, (into a dummy load) for CW (Ip = 400 ma) (Ig = 60 ma) I only see 200W on the amp PO and about 400W on the external wattmeter.

The difference between 200W and 400W doesn't bother me nearly as much as the difference between 400W and the amp's CW rating of 850W.

I can account for some of that with the wimpy input voltage... But shouldn't I be doing better than 200-400W? (That's with a drive of 25W).

Methinks I am not seeing enough grid current for the amount of plate current being used.

Help?

73 de Donald

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VR2AX
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« Reply #1 on: October 20, 2008, 10:55:04 PM »

Try increasing drive power and loading (LOAD control turned clockwise) gradually, to a max Ig of 110mA. Keep Ip at 400mA on resonance (dip). This should coincide with peak in Ig.

VR2AX
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W3LK
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« Reply #2 on: October 21, 2008, 07:18:29 AM »

How are you tuning up?

You are tuning up at about 90-100w and then reducing the drive to whatever output you are shooting for, aren't you.

If you are simply tuning up at 25w, you are seriously underloading the amp.

My 80A puts out 850w, steady carrier, with 70w drive, after tuning up with 90w drive.

73,

Lon - W3LK
Naugatuck, Connecticut
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WB2WIK
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« Reply #3 on: October 21, 2008, 08:26:47 AM »

>RE: AL-80A low output       Reply
by W3LK on October 21, 2008    Mail this to a friend!
How are you tuning up?

You are tuning up at about 90-100w and then reducing the drive to whatever output you are shooting for, aren't you.

If you are simply tuning up at 25w, you are seriously underloading the amp.

My 80A puts out 850w, steady carrier, with 70w drive, after tuning up with 90w drive.<

::My AL-80B puts out 1kW carrier power with ~70W drive on most bands.  However, once I've tuned it that way, I back the drive down to about 750W out and leave it that way (without re-tuning anything).  For "full" output power, Ig will vary depending upon the tube used, but in my case Ig has to be >100mA, and usually closer to 150mA for "full output."  60mA won't make it happen.

WB2WIK/6

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AE6RF
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« Reply #4 on: October 21, 2008, 12:24:51 PM »

Tuning up was done as instructed in the manual, then backing the drive down to get to 400 ma for CW.

The thing is that I need to back the drive all the way down to 25W in order to get Ip down to 400 ma. That only puts out 200-400W or so into a dummy load.

THAT'S what I'm asking about. Why would the Ip be way higher than it should be so that I have to back off so far.

73 de Donald
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W3LK
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« Reply #5 on: October 21, 2008, 12:32:43 PM »

<<The thing is that I need to back the drive all the way down to 25W in order to get Ip down to 400 ma. That only puts out 200-400W or so into a dummy load. >>

I'm not sure what you are doing when you are tuning up, but I have a feeling it's wrong.

73,

Lon - W3LK
Naugatuck, Connecticut
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AE6RF
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« Reply #6 on: October 21, 2008, 12:34:40 PM »

I am following the procedure published in the AL-80A manual.

73 de Donald
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WB2WIK
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« Reply #7 on: October 21, 2008, 01:01:46 PM »

I never pay any attention to Ip (plate current).  400mA is very low when you have Ep that's dropping down to 2800V: This is only 1120Wdc input.  Even at 60% efficiency, output power can't be more than 672W and it's likely to be less than that because at reduced drive the amp cannot run at peak efficiency.

I'd increase drive to 50-60W (it's actually rated 65W drive typ), peak TUNE and LOAD for maximum output power, keep an eye on grid current and if it's above 150mA, turn the LOAD control more clockwise to increase loading and that will drop the grid current lower without reducing output (much).

When my AL-80B is fully and properly tuned up, Ip runs 550mA at 2950V, Ig is 100-150mA, and output is 1kW.

You cannot run it there forever, but for tuning purposes this is fine.  SSB is very low duty cycle and the average Ip will be way lower, but still maintain 1kW PEP output.

WB2WIK/6
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VR2AX
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« Reply #8 on: October 21, 2008, 10:09:51 PM »

If you can not get Ip to dip below 400mA EXCEPT by reducing drive to 25 watts - assuming your LOAD control is already fully anticlockwise (ie max C2) - then you may have a problem in the amp.

Increasing loading (ie turning LOAD control clockwise, which reduces C2), causes Ip to rise (and Ig to reduce). Maybe you have too little C2 for the band in question (I assume these tests are into a 50 ohm resistive load).

Some amps (AL-80A?) use fixed padding capacitors to switch in extra C2, particularly on the LF bands (160-40). Is your issue band related, or is it present on the HF bands as well as LF Bands?

VR2AX
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WW5AA
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« Reply #9 on: October 22, 2008, 06:30:33 AM »

From what you described it sounds like the tube is going soft.

73 de Lindy
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WB2WIK
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« Reply #10 on: October 22, 2008, 09:25:18 AM »

>RE: AL-80A low output       Reply
by VR2AX on October 21, 2008    Mail this to a friend!
If you can not get Ip to dip below 400mA EXCEPT by reducing drive to 25 watts - assuming your LOAD control is already fully anticlockwise (ie max C2) - then you may have a problem in the amp.<

::That might be a good diagnosis, but not with the AL-80A.

The 400mA max Ip rating is from an old Eimac data sheet from the 1960s and is based on running the tube at full voltage, which is 4kV.  The AL-80A only runs it at <3.5kV, and in the case of this particular user, he's running <3.0 kV.  At 2700-2800V and 400mA, the amp doesn't run much power, nor should it.  The amp is rated 1kW PEP output power, which it will achieve, but not at 400mA Ip -- that's impossible.

The newer graphite anode tubes (3-500ZG) will easily operate at 500-550mA Ip. To achieve "full" power from the AL-80A, you need about 65W drive, which will absolutely drive it to >500mA Ip.  That's perfectly normal.

If the manual says to "dip" it to <400mA, that's silly because the amp will be underdriven under that condition.  That might be okay if the exciter can only run 25W output power maximum, but for a 100W rig (typical), to "load up" to only 400mA Ip is ridiculous and will never achieve 1kW PEP output power.

My tube is brand new and perfectly good and it takes about 550mA Ip at 2.9kV to run 1kW output power.  It will do that, fine.  Under that condition, Ig is about 150mA if the amp is peaked for MAX output power, but can be reduced to about 120mA if loaded a bit beyond peak (less LOAD C), and will then run about 950W output.  Perfect.

WB2WIK/6
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VR2AX
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« Reply #11 on: October 22, 2008, 05:40:09 PM »

That's interesting, clearly as you say it depends on the plate voltage.

The pair 3-500ZG in my L-4 run at only 2400V. Standing plate current is 200mA (ie 100mA per tube). The tubes which came from RFP are marked 115mA, but there is no indication of the voltage at which that was measured.

I cannot achieve Ip of 400mA per tube (ie 800mA total) except by using >100W of drive. I can just manage that with around 115W drive on 15 metres. My initial Ip dip, with 40-50W drive, will not exceed around 4000mA (ie 200mA per tube), at optimum position of C2.

I tend to keep more of a watch on the grid current, as it is easy to exceed the limit there, and blow tubes, regardless of plate voltage or drive levels.

VR2AX
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K4DPK
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« Reply #12 on: October 23, 2008, 07:11:45 PM »

I don't think anything is wrong except you're just not driving it hard enough.

25 in for 300 out is about 11 dB gain, close to expected under those conditions.

Drive it with 75 watts and tune for maximum out on the outboard wattmeter, then if you need to get the grid current down, drop the drive and/or load it a little more.

Phil C. Sr.
k4dpk
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