Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: AL-811h Low plate current reading  (Read 4278 times)
W4KPZ
Member

Posts: 16




Ignore
« on: May 31, 2010, 09:54:54 AM »

     I just bought a used Ameritron al-811h as my first amp after 20 years as a ham. After reading the tune up procedure and watching the you-tube video I decided to take it for a test run. First problem I encountered was when I switched from HV to Ip the meter was showing 75ma at idle. I did a search here and found out about the switch problem so I sprayed it with cleaner and now it goes to 0 as it is supposed to.

     It did not cure the second problem however. After I tune up the amp is putting out 600+ watts on cw key down as it should but my Plate Current is only showing about 450-475 ma on the meter. At no time does it go over this and it does not dip as I see on the videos. Under load my Grid Current is about 170ma and HV is 1300.

     Since 1300v x .450a = only 585 input x .7= 410 output watts and my meter shows 600-650 watts I think the problem is in the metering circuit not reading the full plate current. Am I wrong or can some point me as to where to look for the problem or bad component. Thank You
Logged
KA5N
Member

Posts: 4380




Ignore
« Reply #1 on: May 31, 2010, 10:32:26 AM »

What are you using to check the output power?  How accurate is the meter? 

Allen
Logged
W4KPZ
Member

Posts: 16




Ignore
« Reply #2 on: May 31, 2010, 10:41:05 AM »

What are you using to check the output power?  How accurate is the meter? 

Allen

A CN-101L Diawa meter, I realize it is not test equipment quality but I don't think it would be 200 watts off.
Logged
W4KPZ
Member

Posts: 16




Ignore
« Reply #3 on: May 31, 2010, 10:45:00 AM »

     I should state that there was a couple of times that when I first tuned it would show about 600ma then would jump around like a bad contact then go to the 450ma range and stay there.
Logged
AD7WB
Member

Posts: 29




Ignore
« Reply #4 on: May 31, 2010, 10:54:28 AM »

Are you testing into a dummy load? An inaccurate but somewhat functional plate current meter wouldn't cause the plate current not to dip. Sounds like the output is not tuning properly. Does this happen on all bands?

The erratic current and inability to get the dip makes me think poor bandswitch contact or poor plate capacitor rotor contact. Or bad connection or capacitor somewhere in the tank output.

73,

Lance
Logged
WX7G
Member

Posts: 5908




Ignore
« Reply #5 on: May 31, 2010, 11:05:45 AM »

With my AL-811 I never noticed a real plate current dip. Tune for maximum RF output (with drive set to deliver 600-800 watts).

One way to check the RF power meter is to tune for maximum output. If it goes much above 800 watts I suspect the meter is reading high.

Assuming that the RF power meter is accurate, tune for 600-800 watts and go on the air. I would not worry about the plate current reading. For 600 watts it can vary by over 50% depending on what band it's on. Also observe the tube plates while running the amp in a normal QSO. On CW they should glow not at all or just a dull red. On SSB I would not expect any color. I don't worry about the grid current. It has been reported that 811A grids do not fail in this amp.
« Last Edit: May 31, 2010, 11:07:47 AM by DAVE CUTHBERT » Logged
W7MJM
Member

Posts: 109




Ignore
« Reply #6 on: February 15, 2011, 09:36:58 AM »

I've also noticed that the plate current on my AL-811 never goes above 425 ma. Otherwise, it tunes up normally, with a definite output peak. I've observed this with 811A tubes in the amp, as well as with 572B tubes.

Plate voltage is 1600 volts no load and 1400 to 1500 volts full load.

On 20 meters, when drive is set to 70 watts PEP, my output appears to be 600 watts PEP (the mean of two different peak reading meters in my shack; one reads 500, the other reads 700), with SWR less than 1.5 to 1 and very little reflected power. Results are pretty much the same on other bands.

At that drive level, grid current is about 150 ma (key down, CW). But plate current (key down, CW) never exceeds 425 ma, at least according to the AL-811's internal meter, regardless of band. Perhaps it's higher on SSB peaks, but the internal meter is not fast enough to read those peaks.

Audio reports on SSB are excellent and I've had no reports of splatter. My signal is generally reported as being about one to two S-units louder with the amp on, compared to running barefoot at 150 watts output. So I'd say the amp is doing what it should. And it's been performing fine for years, the first 3 with 811A's installed and the last 3 with 572Bs.

So what would cause the low plate current reading?

WX7G reports that the plate current reading in these amps, when tuned for full output, can vary as much as 50%, depending on frequency. Any thoughts as to why? Is it just that the internal meter isn't registering peak current? But if so, why would the manual say that normal full output should occur with a CW/key down plate current reading of 550 ma and all I see is 425 ma, regardless of band.

73,
Martin
W7MJM


Logged
N4ATS
Member

Posts: 797




Ignore
« Reply #7 on: February 15, 2011, 10:01:46 AM »

In the piles and I mean piles of Ameritrons I have fixed , Before you do anything , check D-16 (on the schematic and for most Ameritron amps is D-16). It is a 1N4001 (sometimes larger current). In 99% of the time , you will find it is shorted. It is located right behind the meter next to a diode array. It sits all by itself. You MUST lift one end to test it. (typical Diode Check with your DMM). I'll bet its shorted.

www.n4ats.com
Logged
W7MJM
Member

Posts: 109




Ignore
« Reply #8 on: February 15, 2011, 03:14:26 PM »

...check D-16 (on the schematic and for most Ameritron amps is D-16). It is a 1N4001 (sometimes larger current)...

Thanks for the tip. In the AL-811 manual it lists D-16, the diode connected to the meter, as a 1N4007. Looks like RadioShack sells a mixed bag of 25 rectifier diodes, including some 1N4007's, for $2.99. Cheap enough.

What, by the way, does the last digit in the diode number signify? I see there are 1N4001's, 1N4002's, etc.

73,
Martin
W7MJM
Logged
K7GLM
Member

Posts: 51




Ignore
« Reply #9 on: February 15, 2011, 03:29:05 PM »

The last digit represents the peak inverse voltage rating for the diode.

1 = 50v
2 = 100v
3 = 200v
4 = 400v
5 = 600v
6 = 800v
7 = 1000v

There is also a change in the junction capacitance, if that's of interest.
Logged
W7MJM
Member

Posts: 109




Ignore
« Reply #10 on: February 15, 2011, 06:08:30 PM »

Thanks K7GLM. Just picked up Radioshack's $2.99 bag-o-diodes; I peeked through the plastic to make sure there were some 1N4007's among the 25 in the sack. I'll soon find out if it's a bad D16 diode that's causing low plate current readings on my AL-811's internal meter. 73, Martin, W7MJM
Logged
W7MJM
Member

Posts: 109




Ignore
« Reply #11 on: February 15, 2011, 09:02:57 PM »

I removed D16 and tested it. Sure enough, it was shorted. I replaced it with a new 1N4007 diode; one that tested good.

Now, when the amp is tuned to full output, the plate current reads a bit higher, maybe 450 ma instead of 400 to 425. The grid current reads lower; 100 ma instead of the previous 150ma. PEP output, as measured on my external peak reading watt meters, is the same as it was before, in the neighborhood of 600 watts, mas o menos. All this is at a drive level of about 70 watts.

One other thing I noticed is that when keying the rig on SSB, with no modulation (and therefore no drive), the grid current stays at zero, like it's supposed to. Previously, I'd register some grid current, maybe 30 ma or so, under such zero drive conditions. This is consistent with what W8JI writes on his website about D16's effect on grid current readings with zero drive.

I'm not sure why the plate and grid current readings are significantly lower than what the AL-811 manual suggests they should be; the manual says they should be 550ma and 150 ma respectively at full output. Perhaps it has something to do with the fact that I'm running 572Bs instead of 811As?

If anyone can explain this to me, I'd appreciate it. Bottom line, however, is that everything appears to be working, signal reports are good and, as they say, "If it ain't broke, don't fix it."

Having said that, I'm glad I replaced the shorted meter protection diode!

73,
Martin
W7MJM

Logged
N4ATS
Member

Posts: 797




Ignore
« Reply #12 on: February 16, 2011, 04:41:26 AM »

I am willing to bet that 75% of the Ameritron users have that diode shorted and don't know it , getting use to the symptoms you describe.

As soon as I get an Ameritron in for repair , I pop the covers and replace that diode before I do anything to the amp , 99% of them have that diode shorted.
Logged
WX7G
Member

Posts: 5908




Ignore
« Reply #13 on: February 16, 2011, 07:43:38 AM »

It is makes rated power output don't worry about 'low' plate and grid current.
Logged
W7MJM
Member

Posts: 109




Ignore
« Reply #14 on: February 16, 2011, 09:17:46 AM »

It is makes rated power output don't worry about 'low' plate and grid current.

I agree. No worries.

But I am curious as to why there should be such variation from the printed specifications. Variation in tube characteristics? Changes in the values of other components? Inaccurate documentation?

Or perhaps I should blame it on the extended sunspot cycle minimum.  Wink  Looks like that's starting to change with solar flux up to 113!

73,
Martin
W7MJM
"Being a Gemini, I don't believe in astrology."
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!