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Author Topic: SB-200 Grid Current?  (Read 5436 times)
K8AXW
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« on: April 06, 2011, 08:33:19 PM »

I'm having a problem wrapping my head around the various grid current readings with the SB-200 amplifier.   

The "white scale" represents a scale of 0-100ma.  I'd like to know the grid current readings the readers of this forum are getting for 80-40 & 20M.

I guess I am also trying to determine if my grid current readings are valid or if I have a problem with the grid current dropping resistor.

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KA5N
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« Reply #1 on: April 07, 2011, 04:09:55 AM »

First off there is no "grid current dropping resistor".  The grid current is measured across a
1.5 Ohm resistor, which you can measure (power off AC cord unplugged) with a DVM.  The other resistors in the grid circuit should also be checked for correct resistance and replaced if needed. You will have to use resistors that are not carbon composition (as the originals are) as they are not currently available (been replaced with better quality types).  It would have helped if you had given a table of grid currents you are getting. 
The grid current varies with the power input and if you are using SSB the meter will be flicking upscale as you talk.  With CW the grid current should be constant as long as the key is depressed (with a keyer the current will go from zero to upscale depending on the keying speed set). 
Usually you are OK if the grid current goes upscale but doesn't go over the white scale.
So what readings are you getting that have you concerned?
Allen
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K8AXW
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« Reply #2 on: April 07, 2011, 08:23:26 AM »

Allen:  I opened my mouth about the "grid resistor" without checking what I was saying..... I was setting here with the laptop in the "Amplifer Forum" and just ripped off the question.

I have to get a set of 572b tube curves and study them as well as the metering circuitry of the SB-200.

What is bothering me is that on the higher bands my grid current runs around 50ma or so.... but last night I was on 40m CW and my grid current was running 15ma with 550ma plate current and 650W output. 

This is with the LOAD control on 2.  Recall that 0 (zero) is fully meshed.  What has me really spooked on this Allen is that I've smoked two bandswitch wafers so far and just installed the third.  At $30.00 a pop and the BIG hassle of tearing out the bandswitch to replace the wafer..... well, every time I get on 40m with this low grid current reading and LOAD control close to maximum mesh, my pucker factor ramps up close to 10.  Actually, I haven't got the nerve to get on 80m yet after this last wafer replacement. 

So, that's why my tail is twisted.
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W8JX
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« Reply #3 on: April 07, 2011, 08:30:37 AM »

How much drive are you applying? Also want is SWR in input and output. Smoked wafers tend to suggest problems in these areas.
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KA5N
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« Reply #4 on: April 07, 2011, 09:12:07 AM »

I think  you are pushing it too hard.  I have two SB200's and never run more than 60 watts into them.  Depending on the band used 45 to 50 watts input will give 450-500 watts output and that is about as hard as you need to go.  Trying to get the last few watts out won't make any difference to the other operator and may cause problems. 
As pointed out the input swr of the amp is important.  The two lower bands are a problem as the loading capacitor is too small and the extra switched capacitor may not be large enough.
(It's 500 pf).  As has been pointed out on this forum and other places, the input circuits are less than optimum.  I modified my coils on 15 and 10 meters to accomodate 12 and 17 meters better.  I found that changing the cap on the 20 meter circuit from 360pf to 390pf made that band 1.1:1.  I seldom operate on 80 and I peaked up 40 so that it has a better SWR.  All unmodified SB200's seem to be the same.
Remember that this amp was designed when most transmitters had tunable pi net output
circuits and so the ops were able to match to the input more closely.  So don't push it too hard and check in the input SWR of the amp. Report back what you find.

Good Luck Allen
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K8AXW
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« Reply #5 on: April 07, 2011, 10:00:07 PM »

I'm a bit confused now.  My complaint (for the lack of a better word) was LOW grid current on 40m..... which tells me that the drive is also low.  The amount of grid current is 15ma. 

But be that as it may, my drive level is quite low....probably on the order of 60-70W.  I don't have a power meter between the TS-830S.  I'm just estimating this level by the way my Carrier Level control is backed down.  I'll get into it tomorrow and give you more accurate numbers. (After loading the amp, I'll bypass the amp and get a power reading that way.)

The amplifier input SWR is 1.5:1 or lower on all bands. The load SWR is flat.  I'm using a half sloper with an antenna tuner.

I've had the impression for a long time that the padder caps for 80 & 40m are too small because as the frequency goes down, the closer to full mesh the LOAD cap becomes.  In order to get the grid current up to around 50ma, it will be necessary to almost close (full mesh) the LOAD cap and this is when I smoke the damn wafers!




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W1QJ
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« Reply #6 on: April 08, 2011, 04:13:37 AM »

You are smoking those wafers because you have thew wrong idea about what your job is to load the amp up.  Why do you insist on having higher grid current?  The grid current is limited to a maximum not a minimum, so if you load up and the amp makes loads of power at low grid current, well that's real good, yet, you feel something is wrong because it seems too low.    So for some reason you change the loading to increase the grid current because you feel it should be higher.  By you adjusting the load for higher grid current you are making the exact opposite adjustment than you should be making.  What you are doing by adjusting the grid current higher with the load is actually loading the amp more lightly.  When you load the amp more lightly you create stress on other parts like the band switch contacts.  Now I know why you are burning them up.  Did you know that by lessening the load (turning it to the higher numbers) that you are actually loading more heavily?  By reducing the amount of loading you increase the loading and by adding C you are making it more lightly loaded.  In fact, you actually want to overload the amp to the point that the power actually falls a tad.  At that point the grid current falls nor rises.  Now you know why you are burning conracts?
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KA5N
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« Reply #7 on: April 08, 2011, 05:23:38 AM »

Ditto W1QJ.

I don't know where the idea that the load control should be jockeyed around to get a certain grid current came from but it is incorrect.  As I said in my first post if the grid current is less than 100 ma you are ok. 
One of the things that cause problems with inexperienced operators is the notion of "loading".  Loading increases as you turn the load control clockwise and reduce loading as you turn the control counterclockwise.Don't worry about whether the capacity increases or decreases.  Don't worry about what the grid current is.  Basically just tune for maximum power output using your plate and load controls. 
Since you are using a TS-830, how do you tune it?  You should be doing the same thing with it as with the SB200!! 
Allen
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W1QJ
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« Reply #8 on: April 08, 2011, 07:36:01 AM »

Correct Allen.....I have been building and repairing amplifiers for over 20 years on a regular basis.  On one occassion I had AN Ameritron AL-1500 in for repair.  Customer said it was not making much power.  I set it up on the test bench, bird wattmeter and bird dummy load and loaded it up.  I easily obtained almost 2500 watts output.  No problem with this amp.  Contacted the customer and we went over the loading procedure.  Low and behold he had no clue as to the proper loading procedure.  I delivered the amp back to him and gave him a lesson on how to load it up.  He could not believe it.  He thought I must have made some type of repair.  When I told him "no charge" I guess he believed me.  This is a common occurance. 
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K8AXW
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« Reply #9 on: April 08, 2011, 09:22:53 AM »

OK guys!  Now if you go back to the original post, I said I was having a hard time getting my head around the various grid current readings. 

Ever since I've had this SB-200 I've had a vapor lock (some call it mind lock) on the correct tuning procedure. 

I downloaded and actually studied the tuning procedure for days that W8JI has on his website.  I can understand what he is saying and when it comes time to use the amp, I am back to this "mental" problem.

I think some of this confusion (other than stupidity, mental block or any other description you might have) is that I am associating grid current with RF drive.....

I'm having an equally difficult time making myself adjust the amplifier by watching the power output meter.  Maybe it's 'old school' or once again simple stupidity, but I have a very difficult time getting away from the load and "dip" routine.  I've gone so far as to load and "dip" and then compare it to adjusting the load/tune controls for maximum output and find the settings are the same.
 
But then the next time it's back to load and "dip."  I suppose at this point it's either learn to do it or blow it up!

As for the 830S, I tune it up to the rated output, back the Carrier Level control to a lower value, switch on the amplifier and preset the controls.  I then inject RF into the amp by raising the Carrier Control until I get plate current which I then dip. I also watch the grid current, keep adjusting the Carrier control and tune/load controls on the amp until I get around 500ma plate current (dipped) and keeping the grid current in the white, which is well below 100ma.

Thanks guys.  I'll be studying these posts for a long time to come.  Maybe print it out and post it next to the amplifier.

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W1QJ
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« Reply #10 on: April 08, 2011, 10:31:58 AM »

Everybody has there way of loading amps.  W8JI has his way etc.  I must have had 25 SB-200 amps in my time (or more).  "If" adjustments are made quickly which they should be you can load an SB-200 in 30 seconds.  Here's my recommendation.  Forget the grid meter, forget the plate meter and use an output indicator like the internal forward power meter or external.  Inject about 20 watts of drive from the radio then peak the load and tune quickly for maximum output, then go to 50 watts and again quickly peak load and tune for maximum output, then go 100 watts and again peak for maximum output.  At this point go back and forth between load and tune QUICKLY and keep peaking until no more output is seen on the indicator.  Now, switch to the grid mode and key down, look at the grid reading.  If you want to run balls to the wall than back the drive down to the maximum part of the white area on the meter.  if you want to  be conservative then back it to 75% of the maximum white area.  Thats it DONE  If for some reason you have the grid current in the white area with 100 watts drive, then leave it that way.  if it is beyond the white area, drop the drive down to whatever level (power output) you want.  For SSB you should be able to run it at the max level, for CW you would want it less and for AM or digital modes like RTTY you would want it down below 50% of the white area.  Too many people are paranoid about grid current and plate current.  As far as "dipping", well, unless the amp is perfectly nuetralized the amp won't like the dip if it is not exactly at the maximum power output point.  You can't go wrong with maximum output setting because the tube is transferring the most power it can without dissipating whatever power it is not.  And that is the name of the game.
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AD4U
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« Reply #11 on: April 08, 2011, 10:32:56 AM »

Seriously, put an external wattmeter or a relative power meter in line AFTER the SB-200 - or use the relative power position on the SB-200 meter.  

Set the bandswitch on the amp to the desired band.  

Preset the TUNE control on the amp to the center of the white arc for the desired band.

Preset the LOAD control on the amp to around the 10 o'clock position.  

KEY your transceiver on the desired band with around 50 watts drive.  

Quickly adjust the SB-200 TUNE knob for maximum power output as indicated on the external meter.  

Quickly adjust the LOAD control on the amp for maximum power output as indicated on the external meter.  

Quickly readjust the TUNE control (a VERY small adjustment) on the amp for maximum power output as indicated on the external meter.

UNKEY your transceiver.

With a bit of practice this entire process should take less than 5 seconds.

If you are interested in the current the amp is drawing, put the meter switch in the plate current position and REKEY the transceiver.  The plate current should be around 500mA as indicated on the meter.  Put the meter switch in the grid current position and the grid current should be within the white area of the meter scale.

That really is all there is to it.

Dick  AD4U



« Last Edit: April 09, 2011, 04:44:27 AM by AD4U » Logged
K8AXW
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« Reply #12 on: April 08, 2011, 08:49:20 PM »

Everybody has there way of loading amps. 

You got that right brother!! 

I am printing out these last two posts and putting them next to the amp.  I'll be trying them out ASAP and will be back.

Again, thanks guys.  You know what is amazing?  I can remember stuff about internal and external cartridge ballistics that I read one time 50 years ago but I can't remember the tune up procedure for an amplifier for more than 15 minutes!  Ssschhhheeech!
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AD4U
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« Reply #13 on: April 09, 2011, 04:47:23 AM »

Everybody has there way of loading amps. 

You got that right brother!! 

I am printing out these last two posts and putting them next to the amp.  I'll be trying them out ASAP and will be back.

Again, thanks guys.  You know what is amazing?  I can remember stuff about internal and external cartridge ballistics that I read one time 50 years ago but I can't remember the tune up procedure for an amplifier for more than 15 minutes!  Ssschhhheeech!


"My" way of tuning a SB-200 must work.  I am still using (occasionally) the SB-200 I built in 1970.  I have never had one seconds trouble with it and it is still running the original Cetron 572B tubes.

Dick  AD4U
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KA5N
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« Reply #14 on: April 09, 2011, 07:26:08 AM »

Dick I think that Cetron 572b's are the key to long tube life.  Some of the new stuff might last a couple of years.  Using a tuning procedure that doesn't stress the tubes helps a bunch.
My two SB200's are Cetron equipped and I have two sets of spares, but NOS Cetrons are getting hard to find.  The ones I have should outlast me! hihi.
Allen
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