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Author Topic: Sb-200 no plate current at idling  (Read 7423 times)
VE7SHM
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Posts: 51




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« on: April 01, 2013, 12:12:14 PM »

First Amp i have ever owned seems to work fine puts out 500 watts on 80 meters same on 40 with about 70 watts of drive but the manual states there should be about 90 ma idling ............Thanks
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WB2WIK
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« Reply #1 on: April 01, 2013, 01:35:51 PM »

When you "key" the amp and the relay clicks etc. but without any RF drive (like on SSB) the Ip meter reads "zero?"
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K6AER
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« Reply #2 on: April 01, 2013, 02:36:56 PM »

When you key the amplifier the cutoff bias is reduced (-145 Vdc to -7 Vdc ) and the tubes should draw some current (60-80 mA). This depends on the diode string or Zener that has been placed into the circuit.

Some meters will just barely move with such low current.
« Last Edit: April 01, 2013, 02:49:25 PM by K6AER » Logged
VE7SHM
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« Reply #3 on: April 01, 2013, 02:58:35 PM »

When you key the amplifier the cutoff bias is reduced (-145 Vdc to -7 Vdc ) and the tubes should draw some current (60-80 mA). This depends on the diode string or Zener that has been placed into the circuit.

Some meters will just barely move with such low current.
\

Thank you very much i did not realize i had to key the amp the manual just said with no driver output from the exciter ..........Now shows 85 +- ma with no drive in ssb mode idling ......thanks again my green horn is showing

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KG6YV
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« Reply #4 on: April 04, 2013, 08:30:49 AM »

Don't feel bad, your "rookie-ness" is why there are forums like this.

We all started somewhere... I loaded a 35 watt transmitter using 60 watt lightbulbs into an antenna without any idea
whether or not I had resonance and low SWR.  I saw an SWR bridge in an elmer's shack and said "what's that thing for"?

Greg
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WB4VDM
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« Reply #5 on: April 14, 2013, 03:57:25 AM »

Now I'm confused.  My SB 200 has no GRID  current at idling and exciting it with the FT 950 at 35 watts out in CW mode, the grid current reading goes negative.  That is, it dips below zero.  Plate current reads abt 300ma, and HV at idle abt 2300, during transmit drops to around 1800 best I can remember.  It has the Harbach mods, all of 'em.  Wouldn't think the Harbach mod which replaces the original 2K 7 watt  R18 with a 4.7K 5 watt would do that to the grid current, BWDIK.
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K2DC
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« Reply #6 on: April 14, 2013, 05:03:15 AM »

Negative grid current is usually indicative of a bad tube in a GG amp, but the SB-200 grids are only RF-grounded.  Something hinky in the bias supply, perhaps?  Bias should be around -2V when keyed, around -100V unkeyed or in standby.  Both the tubes and the bias should be checked.

73,

Don, K2DC
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KA5N
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« Reply #7 on: April 14, 2013, 05:13:55 AM »

You can't have grid current without grid drive.  You can't have plate current  without RF
drive.  Also the grid current can't reverse direction of flow.  Installing the Harbach mods
includes making a lot of wiring changes and my bet is that something is connected incorrectly.
Probably somewhere in the bias/grid circuit.  
So go over ALL the connections and make sure that they are correctly made and connected to the correct places.  If the meter is connected backwards grid current will flow but
would indicate downward instead of upward even though the amp would still work.  
Installing the Harbach mods is not a trival task for the inexperienced.  One has to trace down
all the existing connections (which may have errors left from the initial build by another
inexperienced ham) and then add new connections from the Harbach main board.  Maybe
a local ham can help spot the problem.  
Since the amp does operate, the funny meter reading is probably caused by a minor
mistake.    Good luck and 73  
Allen  KA5N

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K8AXW
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« Reply #8 on: April 14, 2013, 06:58:03 AM »

Check the grid resistors first.
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VE7SHM
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« Reply #9 on: April 14, 2013, 09:10:43 AM »

Now I'm confused.  My SB 200 has no GRID  current at idling and exciting it with the FT 950 at 35 watts out in CW mode, the grid current reading goes negative.  That is, it dips below zero.  Plate current reads abt 300ma, and HV at idle abt 2300, during transmit drops to around 1800 best I can remember.  It has the Harbach mods, all of 'em.  Wouldn't think the Harbach mod which replaces the original 2K 7 watt  R18 with a 4.7K 5 watt would do that to the grid current, BWDIK.

I have all the mods done to mine and am getting 80 ma at idling
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KA5N
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« Reply #10 on: April 14, 2013, 10:16:50 AM »

I think there is a confusion with the word (idling).  With power on and amp not keyed
there will be no plate current as the amp is in "standby" not "idling".  When keyed there
should be plate current of around 60 to 90 ma this is "idling current".  When RF drive is introduced the plate current will increase. In other words talk and current goes up and
 you have power output.
Allen KA5N
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W8JX
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« Reply #11 on: April 14, 2013, 10:22:59 AM »


I have all the mods done to mine and am getting 80 ma at idling

I would not get in a twist on 80 vs 90ma idle key down no signal as it is not rocket science and meter accuracy and/or line voltage can easily account for this too. I would worry more about high idle current than low. The higher the current the higher the zero signal plate dissipation.  
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VE7SHM
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Posts: 51




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« Reply #12 on: April 14, 2013, 11:03:53 AM »


I have all the mods done to mine and am getting 80 ma at idling

I would not get in a twist on 80 vs 90ma idle key down no signal as it is not rocket science and meter accuracy and/or line voltage can easily account for this too. I would worry more about high idle current than low. The higher the current the higher the zero signal plate dissipation.  

Oh i am fine were it is manual said 90 ma i get 80 ma and my amp is working great ..
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WB4VDM
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« Reply #13 on: April 14, 2013, 05:52:00 PM »

Well I should learn how to read.  For some strange reason I was looking for 90 ma on the grids and the manual clearly says it should be on the plate.  And thanks Alan for the idling explanation.  Make more sense now.  The Harbach mods were made before I got the amp and there's one wrong turn that I can find.  The R 18 resistor, 2K seven watts, was not replaced with a 4.7K 5 watt one for the new relay.  Methinks that may be causing the strange negative voltage I see on the meter when its keyed.  By negative I mean the needle goes below zero by a noticeable amount, never on the positive side of zero.  Will change it out tomorrow if I can find one locally.  Thanks for all the help.
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W0BTU
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« Reply #14 on: April 15, 2013, 02:57:58 PM »

FYI, here's a web page I found quite useful, dedicated to all things SB-200:
http://pa0fri.home.xs4all.nl/Lineairs/SB200/sb200eng.htm
I made many of the mods there to my own SB-200 amp.
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