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Author Topic: SB-200 Grid Current?  (Read 5426 times)
W8JX
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Posts: 5793




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« Reply #15 on: April 09, 2011, 07:54:37 AM »

Dick I think that Cetron 572b's are the key to long tube life. 

I disagree. Many years ago I put a set of four Chinese 572's in my Dentron and they are still going strong 16 years latter. I can still get over 1000 watts out with 50 watts of drive.  Proper cooling is important to long tube life too. Chances are if you cannot hear you fan it is not moving enough air.
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K8AXW
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Posts: 3839




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« Reply #16 on: April 12, 2011, 05:11:05 PM »

AD4U:

Dick - OK, two stiff ones and a couple little white pills and I got back into this amp tuning stuff.  Made hard copy of your step-by-steps.  Here's the way it went.

Everything cool down to the LOAD control adjustment.  (Recall - preset to 10:00 o'clock position)

I adjusted the TUNE control for max. on the power meter

Adjusted the LOAD control (going from 10:00 down to 9:00 or from 3 towards 0 on the dial)  The lower I moved the LOAD control the more output I had until I reached 0 (zero on the dial)

Increased the drive, watching the power output and also the plate current.  As soon as I got the plate current up to around 450ma, I heard something arcing in the amp. 

I raised the LOAD control back up toward 10:00 stopping at 2 on the dial.  No arcing, readjusted the TUNE for maximum output. 

Final readings are:  Ip=490ma,  Ig=50ma,  Pwr Out=700W

Just for the hellovit, I then moved the LOAD control to 2.5 which lowered the Ig to 30ma.  Re-peaked the power output with the TUNE control.  Final power output was 650W.

I was under the impression that after the initial adjustment of the TUNE control for maximum power output that I would find a point where the power output would peak and then drop off with adjustment of the LOAD control.

If you reread what I have found, the power output continues to increase with the adjustment of the LOAD control until something arcs in the amp. 

This is the kind of stuff that gives me a pucker factor high enough to crush a bolt!



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AD4U
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Posts: 2164




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« Reply #17 on: April 12, 2011, 06:40:43 PM »

Are you learning to tune your amp up into a dummy load or some antenna with a good SWR?  Typically when operating into a 50 ohm dummy load, it is my experience that a SB-200 will peak out with the LOAD control around the 10-11 o'clock position.  When operating into a high SWR the position of the LOAD control cannot be predicted when tuning for max power.

Since the SB-200 was a kit and since they are around 40 years old and since gosh knows what they have been through over the years, there may be a problem (bent variable cap plate, wiring error, etc) inside your amp that is causing the arcing.  Of course there could be other reasons as well.  Nobody can tell without examining the amp.  The arcing is not normal.

Even though Heath advertised 100 watts input to drive a SB-200, IMO that is too much drive.  60 watts should drive a SB-200 to full output.  Trying to get more than 600 watts out of a SB-200 more than likely will result in a poor quality signal, over drive, and splatter. 

A properly operating SB-200 will draw around 500 mA cathode current, which you are getting.  A properly operating SB-200 will typically put out 550-600 watts on the lower bands and a bit less on the higher bands.  You are getting this.  As long as your grid current is in the white area of the meter, you are OK.  Ideally I guess you could continue to adjust the TUNE and LOAD controls over and over until you got 550-600 watts with a minimum of grid current, but the wear and tear on your tubes trying to find the "sweet spot" will probably negate any advantage you may find.

As I posted earlier I have two SB-200's.  I built the first one on my desk in 1970 while in college.  I bought the second one as a basket case and restored and modified it for 160, 40, 30, 17, and 24 meters.  I have never been too concerned with grid current as long as it stayed in the white area of the meter. The SB-200 I built in 1970 still puts out rated power with the original tubes.

Dick  AD4U
« Last Edit: April 12, 2011, 06:54:22 PM by AD4U » Logged
K8AXW
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Posts: 3839




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« Reply #18 on: April 12, 2011, 08:26:50 PM »

Dick,

I meant to add to my previous post that this was tuning into a dummy load.  Sorry.  At any rate, I can get my 80m half sloper down to a 1:1 SWR and always tune the antenna before putting the amp in the line.  On the higher bands, my beam antenna has an SWR of less than 1.5:1.  I do understand that the antenna can change loading parameters. 

As for the amp being 40 years old, etc., I can add that this one has had a bad life.  I bought it at a yard sale and right off ran into a problem with arcing in the TUNE cap.  Long story short, I came to the conclusion that the amp had been used on CB.

I got the TUNE cap cleaned up so it no longer arced and eventually found a like new "pull" on eBay. 

Thank you for your help.  I still haven't got the stones to try it on 80m yet, which is where I blew two wafers. 

73

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AD4U
Member

Posts: 2164




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« Reply #19 on: April 13, 2011, 05:34:27 AM »

Since you don't know the history of your SB-200, I would suggest that you go through the manual step by step and check each and every wire and component to make sure they are in the right places and that they are "good".  This should not take more than a couple of hours.

Other than that, I really don't have any more suggestions.  It seems as if you have covered all the bases.

Arcing and destroying band switch wafers is not normal or even typical.  It also sounds as if you are tuning and operating it properly.  "Something" is not right inside your amp.

Dick  AD4U
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K8AXW
Member

Posts: 3839




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« Reply #20 on: April 13, 2011, 07:12:12 AM »

Dick I basically did this after finding the TUNE cap arced.  However, I didn't go over "every wire."  Maybe that's the next step.

I think I'll also change it's name from "##%@$@$" to your suggestion, "Sum Ting Wong."

Thanks for your help Dick, and also the others on this forum that offered suggestions.

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