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Author Topic: SB-200 How Low will it go?  (Read 25525 times)
K0XS
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« on: October 25, 2014, 06:19:03 PM »

I have access to a Heathkit SB-200 and I’m wondering what the lowest power output that it can be driven to is, in the original condition.
There are lots of maximum numbers but I’ve not seen any minimums.  I realize that the driving source would need to be in the 1 to 10 watt range.

I’m thinking about acquiring, restoring a SB-200 and attempting to drive it to only about 100 watts.
I’m sure that I can find a multitude of ways to lower the power output but I am thinking that I would like to leave it as original as possible to ultimately preserve higher power levels.

Any suggestions would be appreciated.

TNX
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W0BTU
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« Reply #1 on: October 25, 2014, 06:41:51 PM »

I'm not quite sure what you're asking. What are you going to drive it with? a QRP rig?
When I want to run 100 watts, I switch off my SB-200 and just run my IC-765 transceiver at max output.
The SB-200's output power is about 7 or 8 times the input power, with new 572Bs.
« Last Edit: October 25, 2014, 06:45:14 PM by W0BTU » Logged

WB6BYU
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« Reply #2 on: October 25, 2014, 07:10:05 PM »

If the amplifier is truly linear, you can reduce the output power as far as you want and it still
works.  That's what happens during a speech waveform anyway - the power varies from full
at the peaks to zero between words.

If the amp isn't perfectly linear, you may find a point where there is too much distortion on
SSB to be useful (though not likely on CW).  But as long as you are running the stock bias
you should be able to turn your drive down to where you get just a watt or two output.
(Assuming you have sufficient carrier rejection in your transmitter, of course.)
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W0BTU
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« Reply #3 on: October 25, 2014, 07:22:38 PM »

But as long as you are running the stock bias you should be able to turn your drive down to where you get just a watt or two output.

That was probably a better answer that I gave him. But doing that would be a waste of energy and tube life. An amplifier is meant to make us louder, and running it way less than what it's capable of is pointless.
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W1QJ
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« Reply #4 on: October 26, 2014, 05:23:23 AM »

The problem is the design value of the tank circuit will not be reached with such low drive.  The efficiency will suffer and I doubt the amp will load properly making it "out of tune"  If this will have a negative effect in the long run, I am not sure but I can tell you it was not designed to run like that.  Your tubes may run hot maybe.  You would have to try it and see.  Anyone who owns one of these amps knows that it does not load up properly on 80 meters unless at least 40 watts of drive is used.
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W3RSW
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« Reply #5 on: October 26, 2014, 06:21:16 AM »

Only the efficiency will be a problem. As mentioned, if an amplifier is linear at much higher power levels, all lower levels should be linear whether the amp is retuned for the lower level or not if it was properly tuned for the higher power in the first place.

  If The amp is retuned for pep output derived from the lower drive level, gain will be greater than the amps nominal high power rating.  
Gain at low drive levels approaches 18 db, at high drive levels 15 db. I see no reason a SB200 would be much different.  

Why do this? In addition to the original poster's wish, one might want to run 50 watts carrier with plenty of headroom in AM service. ...so would tune for 200 to 300 watts pep, instead of the amps rated power of 600 watts pep or so.
« Last Edit: October 26, 2014, 07:22:19 AM by W3RSW » Logged

Rick, W3RSW
W3RSW
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« Reply #6 on: October 26, 2014, 07:27:18 AM »

W3RSW data:
Quote
Repeating my recent data in another forum.

At the 5 and ten watt input level, high power GG amps may run almost 18 db gain.

Following from K3, LP100a wattmeter,, Alpha 89, actual k3 gen. 2 tone pep measurements, 238vac line.
7200 kHz, 13 April, 2014, w3rsw

K3 nominal,    Lp100a in,  Alpha out,   Gain db, 10 log (out/in).
 ( non lab precision, Rounding and sig. fig. overstatement but indicative anyway )

         Both Lp100a
W.      W        W.       db

5,      5.87,    361,    17.9
10.   10.9.     654.     17.8
15.   17.6.     856.     16.9
20.   22.1.    1102.    17.0
25.   26.3.    1215.    16.7
30.   33.4.    1365.    16.1
35.   37.6.    1489.    16.0
40.   42.5.    1560.    15.7

The nominal 15.0 db gain pt. Isn't matched until the 55/60 watt input point on the Alpha.
Output is a point best left to the student.  Grin

Tune and load were optimized at each power output point., (as if the output power was all that was desired or on hand for a given input.) -- looks like the 15 watt input point was a little off gain curve.
Of course the tube dissapations won't be much lower than if you were running a full bore KW plate input on your SB-200.
Go for it.
-Rick

« Last Edit: October 26, 2014, 07:36:12 AM by W3RSW » Logged

Rick, W3RSW
W8JX
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« Reply #7 on: October 26, 2014, 08:06:44 AM »

W3RSW data:
Quote
Repeating my recent data in another forum.

At the 5 and ten watt input level, high power GG amps may run almost 18 db gain.

Following from K3, LP100a wattmeter,, Alpha 89, actual k3 gen. 2 tone pep measurements, 238vac line.
7200 kHz, 13 April, 2014, w3rsw

K3 nominal,    Lp100a in,  Alpha out,   Gain db, 10 log (out/in).
 ( non lab precision, Rounding and sig. fig. overstatement but indicative anyway )

         Both Lp100a
W.      W        W.       db

5,      5.87,    361,    17.9
10.   10.9.     654.     17.8
15.   17.6.     856.     16.9
20.   22.1.    1102.    17.0
25.   26.3.    1215.    16.7
30.   33.4.    1365.    16.1
35.   37.6.    1489.    16.0
40.   42.5.    1560.    15.7

The nominal 15.0 db gain pt. Isn't matched until the 55/60 watt input point on the Alpha.
Output is a point best left to the student.  Grin

Tune and load were optimized at each power output point., (as if the output power was all that was desired or on hand for a given input.) -- looks like the 15 watt input point was a little off gain curve.
Of course the tube dissapations won't be much lower than if you were running a full bore KW plate input on your SB-200.
Go for it.
-Rick



It is true with a SS amp but when I reduce drive on amp so does plate current. Efficiency may drop some but tubes will run cooler. Idle current stays same though. Unlike a 811 amp where idle current is near max disapation rating a 572 has a lot of headroom.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
K3VAT
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« Reply #8 on: October 26, 2014, 12:34:29 PM »


<snip> ... But doing that would be a waste of energy and tube life. <snip>

Mike, regarding your note above: "waste ... of tube life" is there some technical reference that I can confirm this?  I'm confused on how running the amplifier at low drive levels would reduce tube life.

73, Rich, K3VAT
 
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KH6AQ
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« Reply #9 on: October 26, 2014, 01:00:42 PM »

As mentioned, if tuned for maximum gain at QRP drive the pi-network will not be working in it's intended impedance range. The way around this is to increase the loading to where it would be at full power, and adjust TUNE for max power out for the QRP input power. For increased loading, turn the LOAD control for decreased capacitance.
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K3VAT
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« Reply #10 on: October 26, 2014, 04:23:38 PM »

As mentioned, if tuned for maximum gain at QRP drive the pi-network will not be working in it's intended impedance range. ... <snip>

thanks Dave, given a linear amp with a fairly robust pi-network, doesn't it also depend on what band(s) that you operate?  In other words, driving one's amp on the lowbands with QRP may be markedly different, tank-impedance-wise than running it on 10M?   More 'work-around' may be required on certain bands? 
73, Rich, K3VAT
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W0BTU
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« Reply #11 on: October 26, 2014, 05:12:22 PM »


<snip> ... But doing that would be a waste of energy and tube life. <snip>

Mike, regarding your note above: "waste ... of tube life" is there some technical reference that I can confirm this?  I'm confused on how running the amplifier at low drive levels would reduce tube life.

73, Rich, K3VAT
 

It's not that low drive per se damages the tubes (although the tubes might very well dissipate less power at a certain drive level when the plate current moves above idle a certain amount, since the efficiency is zero with no input). It's mainly that tubes have a finite life, and every time you turn it on, you have the Miller-Larson effect shortening the filament life. See http://www.w0btu.com/miller-larson_effect.html.

The OP didn't say what he was going to drive it with. Since most transceivers put out 100 watts, that's why I said what I did in my first reply. He might as well just run the rig and leave the amp off unless he needs it. Does that make sense?
« Last Edit: October 26, 2014, 05:25:21 PM by W0BTU » Logged

K0XS
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Posts: 9




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« Reply #12 on: October 26, 2014, 06:02:45 PM »

Time to re group.
I’m thinking about acquiring, restoring a SB-200 and attempting to drive it to only about 100 watts.
I’m sure that I can find a multitude of ways to lower the power output but I am thinking that I would like to leave it as original as possible to ultimately preserve higher power levels.

Any suggestions would be appreciated.

TNX

Obviously my drive level would be in the 1 to 20 watt range.

I'm looking at options such as:
2 original 572B's
2 811B's with propetr plate voltage
Can't see that one of either tube would work due to the reactance of the output network

Comments?
TNX
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W0BTU
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« Reply #13 on: October 26, 2014, 06:33:48 PM »

I've never heard of an 811B. Do you mean 811A?
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W3RSW
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Posts: 606




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« Reply #14 on: October 27, 2014, 07:03:25 AM »

The SB 200 is a venerable amp and probably deserves to be restored.
If it were mine or could be reasonably obtained from others, I'd simply restore it to its original specifications first, then play with decreasing input levels. If I thought I might keep it despite outcome, I'd install the Harbach mods, new filter caps, etc.

Forget all the wailing and gnashing of teeth over whether it's energy efficient to do so, tube aging as a lesser or underutilized child, etc. -- Just try it at reduced levels.  Worst case, you'll have a nice amp for sale or to keep at its original or slightly under-rating specs., in your case, say 600 Pep instead of max'd out 800 pep.

When I run an amp at reduced levels, say only 1200 watts from an amp designed for 1500, I inject a two-tone signal(low distortion and well matched in level) and tune / load for max. peaks with sharp sine wave peaks at the max. expected for that input rating with an oscilloscope.  This always yields better efficiency than tuning, loading at the amps 1500 watt level.

Of course, You can go ahead and inject enough signal to tune/load the amp for a max. 1500 watt level, then back off the input for your new desired output as others suggested but at somewhat reduced efficiency.  Note that all signal inputs below or up to reduced output will be linear in both cases.

In the Alpha 89 data presented, I retuned and reloaded for each input level's amplified output for the maximum undistorted peaks and output levels shown on the scope. The LP100a, traceable standard wattmeter tells the tale, showing the two tone pep output rating for each drive level.

I assure you when I tuned for 1500 first, then simply injected reduced input while not retuning and reloading for that reduced input rating,  the output was less than when I retuned and reloaded for that reduced input but still linear in both cases.

I think obtaining a scope is essential in amplifier experimentation and really shows, if nothing else, gross flat-topping, max'd out gain, etc. 

If a scope and decent wattmeter's not available, then the tune/load for max. Design of the amp using best manufacturers recommendations for such a case, then with reduced input for all other drive conditions may be the way you have to go to guarentee linearity.

Go for it!




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Rick, W3RSW
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