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Author Topic: SB-200 grid current  (Read 9221 times)
KB4EN
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Posts: 8




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« on: July 30, 2004, 12:32:18 PM »

just bought used SB-200..  
with amp in line but not on all is fine swr 1.2:1 normal 100 watt output  
but
manual says 90 ma grid drive with exciter keying amp ( but no drive from exciter)  I see less than 10 ma..tried on both 40 and 20 mts same result
put about 50 watts from my kwm2 to it and the linear grid current goes negative (sigh)  am assuming my new amp  needs doctor  
plate voltage 2400 drops to ~2200...plate current is 230 ma when tuned max output when keyed
loading capacitor had little affect on out put  max  output show when loading  capacitor is at zero

any suggestions  help,,, prayers

been off the air for ten years  and forget all the little things hihi

Jim KB4EN
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WB2WIK
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Posts: 21837




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« Reply #1 on: July 30, 2004, 02:32:59 PM »

You're misstating what the manual actually says.

There should *not* be 90 mA "grid drive" when the amp is keyed but without RF drive, that's impossible.  The grid current should be *zero* under this condition.  The plate current (Ip) should rise to about 90mA or so (not terribly critical).  If it's substantially less than 90mA, this could indicate weak tubes.  If it's substantially more than 90mA, this could indicate a tube fault (internal to one of the tubes).  But without drive, the "grid drive" (grid current) shouldn't indicate anything.

If the grid current meter goes backwards when you drive the amplifier, something is definitely wrong, either with a tube or with the metering circuit.

You didn't say what, if any, RF output power the amplifier is producing.  Is it producing output when it's keyed and driven by a carrier?  If so, how much?

WB2WIK/6
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KB4EN
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Posts: 8




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« Reply #2 on: July 30, 2004, 05:09:51 PM »

You are correct...This thing has me flustered..The keyed amp without drive runs about 6ma PLATE current..
when I had 90 watts drive. the plate curret goes to 280 ma max when tuned for max output  but grid current goes negative.wattmeter shows 160 out at this point...

I am going to carefully check all connections, coax and connectors, ground, reseat tubes ( amp had 100 miles trip in front seat yesterday   yes make sure the caps have bled off first
 thanks much  will let you know what i find this evening

Would a bad tube or tubes casue this ??  the Ham I bought it from is good guy and swore it was working



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W8JI
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« Reply #3 on: July 30, 2004, 05:50:49 PM »

by KB4EN on July 30, 2004  Mail this to a friend!  
manual says 90 ma grid drive with exciter keying amp ( but no drive from exciter)>>>

You are reading the manual incorrectly, or you typed that wrong.

It is IMPOSSIBLE to have grid current without drive.

It could be 90 mA PLATE current you should see, but NOT grid current  with no drive!!

<<<I see less than 10 ma..tried on both 40 and 20 mts same result
put about 50 watts from my kwm2 to it and the linear grid current goes negative (sigh) am assuming my new amp needs doctor
plate voltage 2400 drops to ~2200...plate current is 230 ma when tuned max output when keyed
loading capacitor had little affect on out put max output show when loading capacitor is at zero>>>

Sounds like soft tubes and other components possibly bad. It is tough to find good 572B's now,they are mostly junk. 572's have a common tendency to arc and destroy components.

The first thing I'd do is add a fault resistor in the anode supply. It should be a 10-20 ohm pulse rated energy absorbing type resistor, like a 175P RCD resistor.

The second thing I'd do is look to be sure the supply does not have a diode from the negative rail to ground (this would be an add-on, Heath never used it). Such diodes are commonly added to protect metering systems.

When a tube arcs, the diodes short and protect the meters. When the diodes short, the supply rail is grounded and the grid meter reads negative with drive. Any short from the B- rail to chassis will cause the problem of negative grid current with drive.

You also could have an amp that's cobbled up inside.

As to the loading, that may be another issue entirely.
It sounds like you need to sort through other problems first.

73 Tom
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W8JI
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« Reply #4 on: July 30, 2004, 06:01:45 PM »

Bad tubes coud cause this, and 572's are notoriously problematic. You also could have circuit problems or a combination of the two.

Read the manual carefully, take some time, and start over. Be sure everything is connected properly, and be sure you have no ALC connection. The time to add ALC is after everything else is OK.

If you have only 8ma plate current at idle and the keying line is held properly low (I'm not sure how you could see a number that low on the meter and define it that well)then you probably do have have soft tubes.

Check R16 33 ohm resistor that goes to the ANT RELAY jack and be sure it has not increased in value.

You might actually ground the ANT RELAY jack without going through the cable to your rig. One bad thing about this amp is with a poor ground on the ANT RELAY jack center pin, it can have many of the problems you describe.

Also look at the 33 ohm grid resistors near the tube sockets. Be sure they are not open.

572B's now have a big tendency to fault from the anode to the grid. This is caused by Chinese manufacturers not pumping the tubes down well enough, and not testing them properly.

Good luck.

73 Tom

 
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W6TH
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Posts: 1




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« Reply #5 on: September 09, 2004, 08:49:09 PM »


With 90 watts drive, you should be reading close to 400 ma plate current, 2200 volts dc on the plates and an input of 880 watts, the power output should be half at 50 % efficiency or 440 watts output. Looks like you have a problem maybe not with the 572B tubes. Not yet anyway as check further.

Negative grid voltage indicates the grid resistors are open and at one time the filaments of the 572B tubes shorted to the grids causing the negative reading you now see on the grid meter.

These tubes you are now using may be good and the chap before you had a shorted tube which cause the failure and he replaced the tubes thinking that was the problem.  Check the grid resistors and you may find them exploded.

More to follow if you did the mentioned checking.

.: W6TH
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