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1-9 of 9 messages
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Mods for SB-200, 50Mhz
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by SM6TMR on December 8, 2003
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Hi
Has anyone ever converted on of these to 50mhz operation ?? and how it´s work?
Manuel
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RE: Mods for SB-200, 50Mhz
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by WB2WIK on December 8, 2003
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The SB-200 is a lousy candidate for 6m modification because it uses tubes only rated to 30 MHz (572Bs).
The SB-220 is a good candidate, as are any amps using 3-400Z, 3-500Z, almost any kind of metal-glass or metal-ceramic tube (3CX/4CX), etc. The old NCL2000 works great on 6m once converted, as do old Thunderbolts (4-400s), AL-80As, etc.
I wouldn't bother with the 572Bs on six meters...too hard to drive and they don't run much power at 50 MHz.
WB2WIK/6
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RE: Mods for SB-200, 50Mhz
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by PHINEAS on December 13, 2003
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I have done this modification(Actually my Own Mod). The only problem with my mod is once you do it, it will only be usable on 6m. The ouput is quite resonable. I have seen up to 600watts output once you get it tuned in right(400w on average). My modification is a little simplified in comarision to the ones you see on the net.
1. I bypass the input band selection network, and run an 8X coax straignt from the relay to the begining of the tube input circuitry(See the schematic, you will see what I am talking about.
2. I replaced the wire going from the output side of the tubes to the load coil, and the antenna side Cap(The big variable cap on the left side when you are facing the front of the amp). You cant miss this wire cause it runs to the band coil selector switch and a grounded choke on the bottom side of the amp. I replaced this wire with a copper #10 Solid. Its over kill, but during this conversion process I learned how importatnt connections are.
3. On the 10/15 meter coil, I tap the coil at about a turn. You will have to play around with this to get the best results. I ran that #10 tapped from the main conductor straight to the tapping point bypassing the band selector switch.
4. Took 2 blades out of the Load Cap(The one on the right). It will work without doing this, but the tuning is very tight if you dont.
Voila
Phineas
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RE: Mods for SB-200, 50Mhz
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by AB5Q on December 28, 2003
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I agree with Steve on this, the SB-200 or even more specifically the 572B is not the best choice for 6 meter. Yes it can be done, however they are not “cut and dry” conversions like some amplifiers. The conversion process is pretty simple however the issues as a result of modification can be a real bear.
1) The only 572B that will provide you with satisfactory results are manufactured by Cetron. The rest of the 572Bs have an issue relating to the length and gauge of the conductor between the pin and grid. This small issue limits the tube at about 30 MHz for full output.
2) The RF compartment is very tight, not much room to incorporate a properly designed tank circuit. You will experience significant losses and instability as a result.
A few years ago I converted a SB-200 for 6m monoband. It was capable of 300w out with RF Parts 572Bs, and 500w with Cetrons (100w drive). The amplifier was not exactly stable and was prone to self-oscillation. I could not tame the beast so to speak regardless of what I did. I scrapped the project and proceeded to build up an amplifier that uses a single 3-500Z. The 3-500Z is a much better option for 6-meter operation. I have had zero problems with the 3-500Z on 6 meters.
My advice to you is to sell the SB-200 slated for 6-meter conversion and purchase a SB-220, SB-1000, AL-80 or 80B. Convert the 3-500Z amplifier to 6 meters and enjoy the fruits of your labor.
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RE: Mods for SB-200, 50Mhz
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by WB7QXU on February 21, 2004
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A few years a Amp pro N6TYF and I did this mod for 6m and it worked but not well and only for 6m, The time and effort was a total loss, You can buy a solid state unit for less, and the 572B must be a Centron, the taylors might work but the 572's are a poor choice and I DO NOT RECOMMEND THIS AMP FOR 6M. It would be better to convert it for 160m instead.
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RE: Mods for SB-200, 50Mhz
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by M0BCG on February 22, 2004
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ive so far converted 12 of these amps to run on hf and 6m , they are all still working and will make between 700 and 800w out on 6m when tested into dummy load.
the sb200s do become hot under heavy use but for short duty cycles they are the cheapest way to a sub 1kw amp youll get on 6m .
the usual problem is that even though people follow the QST featured conversion , which is fine in itself and works ok , they dont beef up the anode parasitic chokes which is allways where the amps fail on 6m useage if you stick to standard chokes , they burn out rather quickly usually down to ops tuning up in keydown mode .
there is no need to remove 40m , you can remove 10m coverage and even make it so that you can change back to 10m in about 15 minutes of work on the amp .
to convert by removing 10m .
just swap the 10m input capacitor [68pf] for a 20pf one and thats the input sorted , 6 & 10m use same input coil dimensions ..
move the plate coil tap point for 10m back along the plate coil to a point which is one turn round from the start of plate coil for 6m , ie from plate tune capacitor end .
take out the plate tuning capacitor and remove from it 2 inner [from shaft] and 2 outer vanes [fixed side] from the capacitor [ the outermost vane at each end of capacitor ], use long nosed pliers to do this and free the vanes from the crimped on points .
put capacitor back in place and thats the output tuning sorted .
make new anode parasitic chokes using 3x 130 or 150ohm 2watt carbon compound resistors in each choke [47ohm +- approx total value ] and a u shaped inductor made from copper/brass strip 85mmx8mmx1mm thick with a 2.5mm hole in each end to solder resistor legs to .
fitting the chokes is a little fiddly but they will go in there .
next step is to sort out the missmatch at 50mhz across the tx/rx changeover relay contacts .
shunt feed a compression trimmer capacitor [postage stamp type ] about 1-15pf 375vdc at the rf input side of the relay , ie at the solder tag where the short piece of braid comes in from input so239 connector .
solder 2 1/2" of insulated multistrand wire to the tag on the relay [facing forward ] and solder one side of comp trimmer to this .
then solder 1" of insulated solid core wire to the earth tag next to relay , solder other end of comp trimmer to this .
with 50 ohm load in output socket and 4 feet patch lead on input side check thru swr with mfj or similar analyser and adjust the compression trimmer cap so you get lowest thru swr reading @ 50mhz , usually 1-1 @ 50ohms .
thats it .
put cover back on and tune up carefully , low drive at first then gradually increase drive.
pse use pulsing to tune amp, either mfj pulser or fast cw dits , never use keydown .
tuning is very peaky so you need to be carefull or youll miss it .
fitting 6-1 vernier dials will improve 6m tuning ability/range
i have tested these amps installed with cetron, svetlana ceramic base and general electric [chinese ]572b tubes and all run fine on 6m .
it is adviseable to fit a harbach electronics fan upgrade kit and a soft start kit .
some original sb200s will have phono rca jacks on the input socket but you can change these to so239 easily .
do not remove the swr circuitry , keep the sb200 as standard as possible except for the 6m mods ive mentioned .
this is the easiest and most effective conversion for the sb200 amp .
for later or increased duty cycle use it would be adviseable to make a solid plate outer case for the sb200 and fit 2x 120mm papst fans directly above the rf deck/tube area , also with a 5" x 3" hole made in the outer case under the original fan area filled in with perforated plate , to allow air to be drawn up through the "breathing" area below original fan and then drawn out of rf deck by the papst fans .
you still keep original fan in place and the inner aluminium case as it was, no changes made to it .
hope you find this usefull .
73 ian
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RE: Mods for SB-200, 50Mhz
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by N3JBH on December 4, 2005
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i have done M0BCG's mod and works wonderfully. i wouldnt recomend any but this one it is not hard to do. and got great result's in the process. my output? well around 650 if i drive it with 100 watt's tuning is kindy touchy i thought but hey it worked very well indeed.
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RE: Mods for SB-200, 50Mhz
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by W5JON on July 20, 2006
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Well, I only lasted one year after selling the 6 & 2 Thunderbolt and getting off 6 Meters "forever". After a week on 6 Meters with only 100 Watts, I decided I had to have a little more power.
I decided to convert an SB-200 to 6 Meters, as I really did not want another amp with a big amplifier "footprint". So I searched the Web, and found a EHAM topic on the subject. In many comments on the topic many said it was a bad idea because of the cut-off frequency of the 572B's, etc, etc, etc…...
Further down that topic someone said they has converted an SB-200 using the M0BCG mods and they worked great. So I thought I would give it a try, and contacted Ian (m0bcg), and got a copy of the mods and pictures from him, as he has modified about a dozen with great success on all.
I have been on 6 Meters almost every night, (just like the old days) and with 60 Watts drive, it has 600 Watts output. Clean with no TVI on my own TV 20 feet away, with great on the air signal and audio reports. I am driving with an IC-7000, to a 4 Element SteppIR @ 85". With 90 Watts of drive it outputs about 750 Watts but the finals run HOT, so 600 Watts is a nice compromise.
It has worked perfect since the first time I turned it on after performing the mods.
A couple of notes:
1. I added the Harbach Fan, and moved the original Heath fan over a bit . At 600 Watts the Finals run very warm, but NOT hot. His mods suggest that you only change it to the Harbach fan, but I figured why remove a perfectly good fan, and it really paid off. I also have all the Harbach, Power Supply Mod, Soft Start Mod, and Relay.
2. Parasitic chokes that Ian suggested work great and do not run hot, and really do the job.
3. The tuning is very sharp, so I added 6 to 1 vernier drives (recessed) to the TUNE and LOAD.
4. Only 10 Meters was modified to 6 Meters on the SB-200, 15 thru 80 Meters remain and operate normal. However, as I have the QRO-2500DX and the 30L1, the SB-200 will only be used on 6 Meters.
If you are looking for easy 600 Watts on 6 Meters, the SB-200 is a nice package and is going great.
Thank to Ian for all his clear instructions and pictures.
73,
John W5JON
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