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Author Topic: SB-200 power supply sag: how much is acceptable?  (Read 4839 times)
K3WQ
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« on: March 16, 2012, 05:04:50 AM »

I just finished refurbishing an SB-200 linear amplifier I purchased through eBay.  I replaced out-of-spec resistor and caps, touched up various solder joints, installed the Harbach power supply and soft start circuit, and installed a new set of 572B tubes.  It generally behaves as expected; however, I notice considerable sag on the high-voltage (HV) supply.  The idle voltage is 2300 to 2400 VDC, but when keyed (in CW) it drops to 1800 VDC, so the RF output power is lower than expected.

The unit is wired for 120 VAC, but I suspect my mains are not sufficiently “stiff” because the line voltage drops about 8 VAC when keyed. 

QUESTION: What amount of HV sag is considered acceptable? 

-Dave, K3WQ
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WX7G
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« Reply #1 on: March 16, 2012, 06:05:27 AM »

The AC line sag is responsible for 160 volts of the 600 volt sag you see.

So the question is "is 400 volts of sag is normal for an SB-200?"
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W8JX
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« Reply #2 on: March 16, 2012, 06:15:15 AM »

The line sag will cause a bigger drop than in theory because of efficiency looses. Before I would jump conclusions and look for a problem that may not be there, I would try to power it off 240 or a much heavier 120v circuit. Also the value of replacement CAP's and bleeders resistors plays a role here too.
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KA5N
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« Reply #3 on: March 16, 2012, 06:21:51 AM »

Since you used the Harbach power supply board that shouldn't cause a problem (I have two
SB200's one with a Harbach board and one original I see no difference in voltage between the
two).  I also run my SB200's off 120 vac circuit and have only about 100 vdc drop when keyed. 
So first suspect is the AC circuit powering the amp.  So beef it up or rewire for 220 VAC.

Allen
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K3WQ
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« Reply #4 on: March 16, 2012, 09:15:31 AM »

The AC line sag is responsible for 160 volts of the 600 volt sag you see.

So the question is "is 400 volts of sag is normal for an SB-200?"

Yes, that's another way of putting it.

I can certainly wire the unit for 240 VAC and (inconveniently) get that voltage to the shack.  However, before I attempt to do anything like that, I'd like a sanity check from anyone else running an SB-200 from a 120 VAC line.

I also run my SB200's off 120 vac circuit and have only about 100 vdc drop when keyed. 
So first suspect is the AC circuit powering the amp.  So beef it up or rewire for 220 VAC.

Allen

OK--someone gets only a 100 VDC drop when using a 120 VAC line.  I assume this is under a CW signal.  Very helpful.  Thanks!

I'm running an extension cord to a plug that, I believe, is on a fairly unencumbered circuit.  It's a beefy cord, but could still be the cause as it's probably close to 20 feet long.

-Dave, K3WQ
« Last Edit: March 16, 2012, 09:26:53 AM by K3WQ » Logged
W8JX
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« Reply #5 on: March 16, 2012, 10:41:26 AM »

My amp has been powered from separate a 15 amp 240v circuit for 18 years and I used 14 gauge to run it  and it draws more power than a SB-200 too. You do not really need 12 ga or better or a 20a breaker unless you have a amp that runs beyond legal limit.

BTW the 200 is still a very fine amp and better in my book that any 811 amp except maybe Collins 30L-1. Shame nobody made a modern version of it.
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WX7G
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« Reply #6 on: March 16, 2012, 11:08:53 AM »

So far only one person has told you how much his SB-200 sags. Is it really 100 volts?
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AD7VB
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« Reply #7 on: March 16, 2012, 11:20:44 AM »



The unit is wired for 120 VAC, but I suspect my mains are not sufficiently “stiff” because the line voltage drops about 8 VAC when keyed. 

QUESTION: What amount of HV sag is considered acceptable? 

-Dave, K3WQ


Well mine is wired for 120 and the drop is a little over 300 volts on the HV. It's plugged in right next to the CB panel. It also has the Harbach power supply board.

Hope that helps

73
Todd
AD7VB
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KA5N
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« Reply #8 on: March 16, 2012, 12:11:53 PM »

OK since there are doubting Thomases about I rechecked.  The Plate voltage is 2500 volts with
power on unkeyed.  When keyed (no rf drive) the voltage drops to 2400 Volts. 
On CW tuned to 500 watts output and key down the drop is from 2500 volts down to 2000 volts.  In other words a 500 volt sag at 500 watts output.  '
Remember when keyed with no RF drive the tubes draw about 60 ma.
There is often problems with the series resistor for the voltage metering circuit.  Since the
Harbach board has 14 1meg 1% resistors in series (new good ones) the voltage should be
pretty accurate. 
The original post wasn't really clear about whether the measurements were made with or
without RF drive.  With drive 300 volts drop is pretty much in line.  So maybe a glitch
in communication.
Allen
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KC9JCH
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« Reply #9 on: March 16, 2012, 03:23:49 PM »

My SB-200 is wired for 120 and HV reads 2400 no load, not keyed and 2200 keyed up and putting out 600 watts CW.  No mods, Ameritron ARB-704 keying interface.  Amp is on it's own circuit  Hope this helps, don't know what value is acceptable.  It's a fine old amp and does a nice job for not a lot of $$$.
KC9JCH
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K3WQ
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« Reply #10 on: March 16, 2012, 05:42:56 PM »

OK, it sounds like my amp's performance is nominal for an SB-200 wired for 120 VAC.  I'm sure some of you fired up your equipment to provide the data.  Thanks--much appreciated.

I'll be remodelling my kitchen this year and will have it rewired for 20A on a separate circuit.  While the electrician's at it, I may have him drop a 240 VAC line to my shop that doubles as my shack.

-Dave, K3WQ
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AB4ZT
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« Reply #11 on: March 16, 2012, 06:22:52 PM »

One more data point for you.  My SB-200 runs off a 120v circuit (not dedicated) and unkeyed HV is 2250V and keyed HV (600 W out) is 1850V.  I have the Harbach power supply.

73,

Richard

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KC2ZPK
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« Reply #12 on: March 18, 2012, 07:06:54 PM »

Quick question, what are your AC line voltages before and after keying the amp? And how far away from main panel?
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John
KC2ZPK.com - A work in progress
AD4U
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« Reply #13 on: March 19, 2012, 05:49:53 AM »

The SB 200 is rated at 1000 watts INPUT on CW.  Assuming 2400VDC no load voltage.  If it "sags" to 2000VDC at 500 mA on CW, then 2000V X 500mA = 1000 watts input.  This is right on the specs that Heathkit published for the amp.  Anything less is probably too much.  I would look at your AC line voltage sag as the problem.

Dick  AD4U
« Last Edit: March 19, 2012, 06:23:47 AM by AD4U » Logged
WX7G
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« Reply #14 on: March 19, 2012, 09:01:43 AM »

The line voltage sag is listed in the original post. It is 7%.
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