Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: An SB-200 question - high SWR  (Read 7176 times)
WA6NEQ
Member

Posts: 33




Ignore
« on: September 07, 2010, 06:13:02 PM »

I recently bought an SB-200 on ebay (maybe that was my first mistake ?) that was said to be in good operating condition. However, I can only seem to drive it to about 400W output on any band. I decided to measure the SWR between my exciter and the RF input to the amp. The bench setup is as follows:

Output of exciter connects to input of SWR bridge/wattmeter.
Output of SWR bridge connects to input of amp.
Output of amp connects to input of wattmeter.
Output of wattmeter connects to Cantenna dummy load.

Here's my procedure:

First, unplug RCA plug from relay jack on amp. Next, turn on exciter. Turn on the amp. Key the exciter in CW mode at full power (100W). Measure SWR (about 1.1:1).
Turn down power to about 10W out.
Plug in the RCA jack so the amp will key.
Key the amp and exciter. Adjust the TUNE and LOAD controls. Increase exciter drive until max power out, tweaking TUNE and LOAD along the way.
Measure SWR.
I have found that the SWR is generally around 2.4:1 or higher which has limited my exciter’s ability to drive the amp to about 75W input.
In doing some research, I saw where someone suggested that weak tubes could result in higher than normal SWR. Is this generally thought to be true?
Any ideas on other causes?
Of course the seller described this amp as being in good operating condition.
Thanks !
Logged
K2QPN
Member

Posts: 70




Ignore
« Reply #1 on: September 07, 2010, 07:39:58 PM »

I have an SB-200 that I built in 1968. Still working. My SB-200 has pretty good input SWR on all bands. It does not require a tuner. The SB-200 even works on 12 (10M position) and 17 (15 meter position) meters. It should work.

Is the SWR bad on all bands? Is the band switch knob pointing to the correct band? A close inspection of the rear band switch might be useful. You could check the input coil resonance with a grid dip meter. If you need a soft copy of the manual....I have one. Keep in mind that that this was a kit and may never have worked properly. Keep plugging - it is worth fixing.

Good luck and 73, Bob K2QPN
Logged
WA6NEQ
Member

Posts: 33




Ignore
« Reply #2 on: September 07, 2010, 08:26:44 PM »

Thanks for the comments Bob. Yes, it happens on all bands. The bandswitch is in the proper position. I tried cleaning the bandswitch with DeOxit but really couldn't get to the inside wafer very easily (my bottle has the long thin metal tube). I didn't try removing the screws and pulling it out a bit. I might have to do that. Tweaking the input coils had a minor effect on the output power.
I don't have a grid dip meter to use.

A thought just occurred to me. I do have a g-g homebrew amp that uses four 811A's. I suppose I could replace two of them with the two 572B's and see if the output drops off. That might give me a clue about the tubes, maybe not.

I have tried this with two exciters, an Icom-730 and a Drake TR-4C. Both produce the same result.
Logged
KA5N
Member

Posts: 4380




Ignore
« Reply #3 on: September 08, 2010, 08:15:49 AM »

Try tuning a different way.  First off about 60 watts input is all you need to drive the SB200 with good tubes.  Input 60 watts and tune the tune control for maximum output.  Then adjust load for maximum output.  This should take just a few seconds.  You should get around 500-600 watts output.  Check input SWR.  If less than 2:1 you are OK.  You may be able to adjust the input circuits for lower SWR.
I have (as have others) found that the input circuit is not really optimum especially for 17 and 12 meters (using the 15 and 10 meter switch positions).  I have designed mods and installed them on two
SB200's which give very good SWR's on 20, 17,15,12 and 10 meters.  This mod involves changing the number of turns on two coils and adding two compression variable caps.  40 and 80 meters are generally OK as is, but can be improved by changing to a Pi net circuit for each.

The other thing is that I would not buy a used amplifier and just start using it without going over the circuit throughly.  In both my SB200's I replaced electrolytic capacitors.  I had to change several resistors that that changed value in the bias and grid circuit etc., etc.  These amplifiers are getting to be around 40 years old and no matter what the previous owner said, "they ain't perfect!"

With a few new parts you can put these amps back in shape and they will last for many more years.


Allen
Logged
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #4 on: September 08, 2010, 09:24:06 AM »

Thanks for the comments Bob. Yes, it happens on all bands. The bandswitch is in the proper position. I tried cleaning the bandswitch with DeOxit but really couldn't get to the inside wafer very easily (my bottle has the long thin metal tube). I didn't try removing the screws and pulling it out a bit. I might have to do that. Tweaking the input coils had a minor effect on the output power.
I don't have a grid dip meter to use.

A thought just occurred to me. I do have a g-g homebrew amp that uses four 811A's. I suppose I could replace two of them with the two 572B's and see if the output drops off. That might give me a clue about the tubes, maybe not.

I have tried this with two exciters, an Icom-730 and a Drake TR-4C. Both produce the same result.


A grid dip meter will not tell you a thing that is useful.

Weak tubes can cause high SWR.

Contact cleaner on the switch is a waste of time, and can even be harmful on the HV RF sections.

Input SWR varies with drive power level and tuning of the tank in a grounded grid amp

2:1 is too high, and may even cause unnecessary splattering. 1.5:1 is probably a good upper limit.

You did not mention what type of exciter you have.

The proper way to adjust the input is with a small 390 ohm resistor from each tube filament pin to ground at  each tube (4 resistors total) and the tubes in place, but the amp not plugged in. You will have to physically close the relay. Use an antenna analyzer to view the SWR.

Just a warning, you could have one dead or nearly dead emission tube (or a problem in the grid circuit of one tube) and have a 2:1 SWR but still get 400 watts out. You could have two half-gone tubes also. It could be something else, but for it to be bad on all bands equally I would look at the tubes. I would not change a bunch of things just to change them on a whim, or because they are old. It is very unlikely you have a component problem other than tubes, or something making one tube dead, or some really hosed up bias on one or both tubes.

73 Tom







Logged
WA6NEQ
Member

Posts: 33




Ignore
« Reply #5 on: September 08, 2010, 10:02:01 AM »

Tom,

Thanks for the comments.
I tried both my Icom -730 and a Drake TR-4C and got the same results with both.

Unkeyed the bias is -135V. Keyed it is -2V. I only measured at the pin of one of the tubes. Plate voltage (on the front panel meter) is about 2.3KV. No signal, keyed plate current is about 60mA. Keyed goes up to about 300mA. Plate voltage drops down to about 1.9KV.

73, Bill
Logged
W0BTU
Member

Posts: 1686


WWW

Ignore
« Reply #6 on: September 08, 2010, 10:48:55 AM »

... Weak tubes can cause high SWR. ...

Just when I think that I understand most things that are really important, I read this.

I'm sure there's a simple explanation for somebody simple-minded like me. Please excuse my ignorance, but how can weak tubes in my SB-200 cause high SWR?

TIA, Tom.

Logged

KA5N
Member

Posts: 4380




Ignore
« Reply #7 on: September 08, 2010, 04:22:28 PM »

Tom is great on advice but often short on explaination.  The reason bad 572B's can cause high SWR is that the input impedance of an amplifier is a dynamic quanity.  If the tubes are dead the input impedance is going to be nil.  The input impedance depends on the condition of the tubes and the operating conditions of the amplifier.
Nominally the input impedance of a 572B is 215 Ohms, so a paralleled pair would have an impedance of about 107 Ohms.  So the input circuit has to transform 50 Ohms into 107 Ohms (assuming all is OK otherwise).
To tune the input circuit with the amp off one must substitute a fixed load of 107 Ohms.  Four 430 Ohm resistors in parallel (i.e. each tied from a grid connecton to ground) will work.  Tom calls out for four
390 Ohm resistors which would be 97 Ohms for the paralleled connection.  Why he calls for those values will have to come from him.  Probably won't make much difference anyway.  A problem with the SB200 is that most of the input circuits are LC circuits instead of the more predictable PI section usually used at the input. 
Of course as has been said:  "SB200's have worked for 40 years, why is there suddenly such a problem."
(or close to that)Try to use them on 17 and 12 meters and see.

Allen
Logged
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #8 on: September 08, 2010, 06:29:08 PM »

... Weak tubes can cause high SWR. ...

Just when I think that I understand most things that are really important, I read this.

I'm sure there's a simple explanation for somebody simple-minded like me. Please excuse my ignorance, but how can weak tubes in my SB-200 cause high SWR?

TIA, Tom.



Weak tubes cause higher SWR because the input impedance of the tube increases as emission goes away. Current decreases for a given voltage, and driving impedance is the ratio of driving voltage to cathode current.

I've found 390 ohm resistors to work better on the SB200's I've looked at. I'm not a big fan of the SB200 input circuit. It is a minimum Q L network on most bands.

http://www.w8ji.com/tuned_input_circuit.htm
Logged
AD4U
Member

Posts: 2167




Ignore
« Reply #9 on: September 09, 2010, 06:08:33 AM »

I am the proud owner of a SB-200 I built in 1970.  It still runs the original Cetron 572 tubes and it still puts out 500 - 600 watts on all bands.  I only use my SB-200 in conjunction with my SB-102 and the combo works great, so I really never checked the input SWR of my amp.

Another way to look at it is the SB-200 was designed in an era when every transmitter or transceiver had tube finals and a pi-net that would match the rig to most any SWR condition up to around 3:1.  In those days there was no functional reason to have an amp with an input network that yielded a perfect 1:1 SWR or a 52 ohm input impedance.

I think Heath advertised an input SWR on their amps to be less than 1.5:1, but I am not sure all of their amps met those specs.  In those days with tube finals and a pi-net  "close was good enough".

Granted that over 40 years component values can change and band switch contacts can get cruddy and today's 572 tubes (IMO) cannot compare to Cetrons, and people who built Heathkits often did not do a very good job, etc, etc. 

All these variables can often add to the "problem" you are having.  With care you can adjust or re-work the SB-200 input network to get the input SWR to acceptable levels.

Dick  AD4U
Logged
NO2A
Member

Posts: 786




Ignore
« Reply #10 on: September 09, 2010, 02:38:31 PM »

Bill,I`d like to know what the normal plate voltage should be for the SB-200. 1900 volts key down sounds to low to produce 500-600 watts out,but maybe someone who owns one or Tom can comment on that. The plate current sounds low also. Even if the tubes are soft I would want to know the proper voltage level.  -Mike.
Logged
W1QJ
Member

Posts: 1447




Ignore
« Reply #11 on: September 09, 2010, 03:50:50 PM »

Normal plate voltage on an SB-200 with today's line voltage (it will vary a bit depending on your line voltage) but normal is about 2400 volts.  The plate voltage will sag as you drive the amp and the sag is dependant on how much drive and more importantly how stiff your AC mains are.  This is why I like a dedicate 120v or 240v line for the amp.  I prefer 240v always but an SB-200 is OK on a good 120v line. Often times the plate metering series resistors change value and give a bogus reading but ones that are close to the right value should yield about 2400v on the plate.
Logged
AD4U
Member

Posts: 2167




Ignore
« Reply #12 on: September 10, 2010, 06:39:42 AM »

To take W1QJ's comments a step further......The HV metering resistors in a SB-200 almost always change value (increase) over 40 years of operation.  This is due to heat and age.  When this happens the HV on the meter reads lower than it really is.

To correct this problem and to minimize the chance of it happening again, most SB-200 owners and restorers replace the original 3 HV metering resistors with 6-8-10 resistors whose total OHM value is the same as the original 3 resistors.  When I do this I use 2 watt resistors so they are not dissipating heat at their rated limit.

Dick  AD4U
Logged
W5LZ
Member

Posts: 477




Ignore
« Reply #13 on: September 12, 2010, 07:18:33 AM »

Your original test measured the SWR/impedance of the by-pass circuit in that SB-200.  The 1.1:1 you found is nice for that by-pass circuit, but doesn't tell you anything about the amplifier's active circuit.
Substituting two tubes is certainly one way of checking things, but using dissimilar tubes doesn't tell you much.  Replacing those tubes with similar, or the same 'size' tubes might.  Replacing those 572's with 811's is a good way of destroying the 811's too.  Difference in the plate voltages, but the biggy is the filament voltage differences.
Hopefully, you have the Operating/Construction manual(s).  I think I'd take a very good look at those and the amplifier.
I don't have much 'faith' in 'eBay' descriptions!  So, your first 'guess' may have been closer to the truth than is pleasant to think about...
Good luck.
Paul
Logged
W0BTU
Member

Posts: 1686


WWW

Ignore
« Reply #14 on: September 12, 2010, 04:40:03 PM »


Weak tubes cause higher SWR because the input impedance of the tube increases as emission goes away. Current decreases for a given voltage, and driving impedance is the ratio of driving voltage to cathode current.

I've found 390 ohm resistors to work better on the SB200's I've looked at. I'm not a big fan of the SB200 input circuit. It is a minimum Q L network on most bands.

http://www.w8ji.com/tuned_input_circuit.htm

Okay, this makes sense now! I should have read the entire thread. Thanks, Tom!
Logged

Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!