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Author Topic: SB-200 low output  (Read 2054 times)
K4MCW
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Posts: 2




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« on: April 05, 2013, 11:53:05 AM »

I do not have a dummy load so I'm tuning into a G5RV with a tuner (please forgive me) with a Yaesu 857D after installing the soft key mod. This is what I'm seeing and would appreciate some input on what the problem may be. 1st, The HV idle is 2400 and drops to it's lowest of 2000 on 15 meters and 2100 on the remaining bands. 2nd, I'm driving with 75 watts. 3rd, 10 meters: grid 120ma, plate 350ma, FM key 190 watts, SSB modulation 150 watts. 15 meters: grid 0ma, plate 390ma, FM key 230 watts, SSB modulation 100 watts. 20 meters: grid 20ma, plate 95ma, FM key 40 watts, SSB modulation 20 watts and high SWR. 40 meters: not working. 80 meters: grid 110ma, plate 380ma, FM key 285 watts, SSB modulation 170 watts. These are also the same results I had when keying this amp with a Kenwood TS-820S before i did the soft key mod. The tubes I bought new, but, they are made in China. Thanks Mac
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AB4ZT
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Posts: 172




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« Reply #1 on: April 05, 2013, 12:15:40 PM »

First - get a dummy load, it's really not optional equipment, especially with a linear amplifier. I've used an inexpensive dry dummy load for 20 years, and the power rating is plenty for an SB-200. Just pay attention to the de-rating curve.

Second - check the SWR into the amp.  Depending on the radio if it is much more than 1:1 the radio will start reducing power and you won't get the input power you need.  If the SWR is high you will need to adjust the tuned inputs.  This is where I would start.

Also, unless you have a real peak reading meter, you won't get an accurate PEP measurement on SSB.  FM will because it is a continuous carrier, but be careful about keying FM too long into an SB-200.  It is not made for long stretches of 100% duty-cycle modes like FM and RTTY at full output.

73,

Richard, AB4ZT

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W1QJ
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Posts: 1444




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« Reply #2 on: April 06, 2013, 03:20:49 AM »

Please outline for us the procedure you are using to load up the amp.  Antennas for each band?
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K4MCW
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Posts: 2




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« Reply #3 on: April 06, 2013, 06:20:31 AM »

Richard

I'm using a MFJ-962C tuner into a G5RV and can tune the SWR down to 1:1 or 1:1.5 on all bands and on 10 meters I use direct into a AR-10 with 1:1. The SWR reading on the SB-200 meter also reads 1:1.5 or below. I will get a dummy load. I have removed each of the tubes and they show 150 and 125 watts separately.

W1QJ

First I tune the radio through the 962C into the G5RV without the amp on, then I tune the amp by peaking relative power with tune and load and verify this by switching to plate and see if it will dip with tune. The amp is never keyed longer than 4 or 5 seconds and with a 10 to 15 second pause in between
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KA5N
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Posts: 4380




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« Reply #4 on: April 06, 2013, 06:47:36 AM »

A properly working SB200 correctly tuned will put out a maximum of 500 to 600 watts into
a dummy load  (or correctly matched antenna).  Your high voltage sag seems excessive.
I think your tubes are soft and perhaps the filter caps and matching resistors need changing.
If your amp is operating off a 115-125 volt circuit is the line voltage sagging when the amp
is driven?  If so you may have to install a dedicated line for the amp.  If the rectifier diodes
are original units, I would replace them for more modern units  (1N007 or 3 amp  IN5408).
A check of the bias supply and amp keying is in order.  Remember these amps are long in
tooth and who knows what abuse they have suffered over the years.  I have two SB200's
that I have refurbished and they both easily put out 500 plus watts without blinking.
Good Luck
Allen KA5N
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K8AXW
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Posts: 3752




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« Reply #5 on: April 06, 2013, 08:47:14 AM »

MCW:  The readings you're getting don't make any sense at all.  For example:

You say the plate voltage is 2100 on most bands.  So working with that number:

80m - 2100 E/p times .380a I/p + 798w input power.  at 60% efficiency you should be getting 478w output.  Your readings are FM 285w and SSB 20w (If I assume you mean 200w that would still be an incorrect number)

Running the same on 10m, your so called output readings are also incorrect.

I suggest you key the amp using the CW option of your rig instead of FM and consider the power meter, whatever it is, might be giving your wrong readings. 

The best way to find out if your power meter is anything close, use the math for calculating the power input of your amp and then deduct 40-50% to give you the output power.

4ZT gave excellent suggestions to trouble shoot this amp and his suggestion on the dummy antenna is right on.  It's not an accessory.  It's a necessity!

Also, I agree with 5N that your tubes might be soft, or since they're Chinese, something else.



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AD4U
Member

Posts: 2157




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« Reply #6 on: April 06, 2013, 12:05:07 PM »

75 watts is enough power to drive an SB-200 to full output of 500-600 watts on SSB and CW.  When operating from a stiff 240V power source an SB-200 should have 2400V high voltage no load and around 2100V at rated input.  This is normal "sag" in the HV.

A properly operating SB-200 should show 500mA plate current at max rated drive with 500-600 watts output on bands up through 15M.  10M may show a bit less.  Grid current on all bands should stay in the white area of the meter. 

Anything less and you are looking at several possibilities.  We will assume the amp is wired correctly and that no components have failed.

The first is probably soft tubes.  The second can be improper input matching.  A 1:1 input SWR was not nearly as important when this amp was designed as it is today.  Another possibility may be improper tuning on your part or a wattmeter that is not very accurate.

As others have posted and as you have promised............get a dummy load.  It will make other hams happier and it will make your tuning and trouble shooting much easier.

Dick  AD4U



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