SB-200 Modifications to make it better.

[@joesitter20101]

I like your Thermistor in the filament circuit. I've added a small 12V, 1A DC supply, replaced the T/R relay with one having a 12V coil and a small solid state relay to drive that relay. The internal 12V is used as pullup on the SSR internal LED so the modern radio only sources 7.5mA with its' open collector transistor. I kept the RF grid grounding with SM caps but DC grounded them with chokes. For DC bias applied +50 DC during standby, or a 10W, 10V Zener diode bias through a 3rd T/R relay to the filament transformer's center tap. This make the Q current much lower (about 40mA) so the tubes don't glow as much and stay much cooler. The RF output doesn't contain out of band emission.

[@hankosaurus]

Great video! Very instructive and good ideas for improving an SB200. Tnx es 73 de kc4i.

[@geekthesteve6215]

Can you make your PDF available that shows the mark-ups for the modifications? That would be very helpful. If not, could you show full screen views on your next post of each of the mods? Thanks in advance. Steve, WA0A

[@CIRCUITDESIGNER]

I will make a blowup of the markup in the update.

[@geekthesteve6215]

@@CIRCUITDESIGNER I just bought another SB-200 (my other one is actually a SB-201). I really like these amps and the price was such that I could not walk away. This one has the Harbaugh caps mod. Best 73'a, Steve, WA0A

[@louw1qjparascondola821]

The transformer is designed to limit start up current, you don’t need a step start. The best thing I ever did to an sb200 was to directly ground the grids! Using a stock set of Cetron tubes and tweaking the inputs a bit the amp was perfectly stable. Power output shot up drastically. I loaded the amp up for performance purposes to maximum grid current without going out of the white zone. Power out figures were 80m-950watts pep 40m-900pep 20-900 pep15m-850watts pep and 10m -750-800pep. If you throttled the drive back to be conservative 750-850pep output on all bands it will just load along.

[@CIRCUITDESIGNER]

Sounds like something to try for more power. My only concern would be the 2nd and 3rd harmonics. I have a Rigol DSA815 Spectrum Analyzer that could check that. My big concern was getting a 1:1 match of 50 ohms between the radio and the amplifier on all bands. I finally accomplished that and will post soon.

[@geekthesteve6215]

Looking forward to that post about the input circuit matching CIRCUITDESIGNER! I am in the process of buying an SB-201 from a buddy and I also have a IC-745 I will be driving the amp with. Steve, WA0A

[@williamogilvie6909]

Interesting discussion. The high SWR on the input side of the SB200 might be due to reduced capacitance of some of the capacitors in the filters. I like to use a return loss bridge or a Boonton Rx bridge in cases like this. A nanoVNA might work ok, but I have no experience with one.

[@CIRCUITDESIGNER]

Did you check out the 2nd video on the SB200? It explains a lot more. Thanks

[@itsonlyme9938]

I have reversed engineered mine and know using GI6B ceramic tubes the grids are connected to earth and using cathode bias and using a BD 250 transistor and a zener diode. If you look up GI7B amp same tube as GI6B and under QSL net it shows the diagram. You could with the 572b ground the grids and use cathode bias which I would improve the output of 572B's and might improve the stablity. The modified SB200 can be seen on my QRZ page G4LMZ with the GI6B tube fitted.

[@CIRCUITDESIGNER]

Thanks for the tip, I will look into it.

[@dezertraider]

EXCELLENT! I HAVE A DENTRON 160-10L,I USE ON 75 METERS AND WOULD LIE TO ELIMINATE THE TUNER FROM THE RADIO,.ANY SUGGESTIONS? THAN YOU 73

[@CIRCUITDESIGNER]

I looked at the schematic of your Dentron. There are no selectable tuned circuits on the input like the SB-200 has. The signal from your transceiver goes directly into the cathodes of the 572's. Your stuck with a tuner between.

[@dezertraider]

@@CIRCUITDESIGNER THANK YOU

[@fredguitarman3536]

I had a problem with self oscillation until I removed the parasitic suppressors replaced the 47 ohm resistors rewound the coil like the original no more problem. I had problems with those type of suppressors

[@CIRCUITDESIGNER]

Nether type of parasitic suppressors seemed to work well for me. I stayed with the teflon loop type because the results were about the same as using the resistor/inductor type. Using the toroid to induce some negative feedback worked great. Thanks

[@dk4hi]

Hello RCD66, You did a good job indeed! I have a simple question regarding the neutralizing circuit. Did you remove the 1000pf cap between the toroid input connecting to 1 leg of the 572B's filament?

[@CIRCUITDESIGNER]

1000pf is placed in circuit to couple the toroid secondary to the RF input of the tubes 180deg out of phase from the plate RF signal. Thanks

[@dk4hi]

@@CIRCUITDESIGNER Thank you very much!! My case being is that I have modified my SB200 to pa0FRI mods that replaces the existing grid design grounding the grids to ground. The biasing of the 572B; in short, by using the ct of the filament transformer and creating an independent biasing power source to the filament for biasing, thus keeping the grids firmly grounded. So knowing that, using the 1000pf serves 2 important functions in my case. However, since its purpose is neutralizing VHF, perhaps paralleling a 100pf and maybe a 1 pf to the 1000pf might not be a bad idea? After seeing your response, I will proceed to install your design as my suppressors, if not perfectly tuned exhibits a potential problem with heating on 15 and 10 meters. I modified a Drake TR4C replacing the PA sweep tubes with 6146W...etc and when using the SB200, carefully tuned, its output that now reaches 700-800 watts for short duration with the grid current at the top of the meter's white bar, before the presence of the smell of overheating occurs. Hey... this is fun stuff for me as I love peeling onions *LOL@self* ANYHooo... Thanks Again and 73's... AH6BS (blue sky)

[@dk4hi]

Indeed your valued comment makes sense as it would make the design more sensitive to detect VHF oscillations of sort! BTW... I reviewed PA0fri circuit using the filament CT voltage for biasing the 572B... it worked fine but, for reason, I had to change my 572B to Penta's version as they seem to be still obtain-able at this time. Again stability on 15 and 10 mtrs is problematic and need to be troubleshot for a solution... *here I go again!*

[@ASmith-vu2wo]

Do you have a download link for your modified schematic. The screen display is just to small for me to discern many of the modification values. Thank-You.

[@CIRCUITDESIGNER]

I can only Email images. I will work on that.

[@doughendry115]

Why remove the input tune network and replace with a 50 ohm 100 watt non inductive resistor

[@dk4hi]

Hello RCD66, I like your commentary as it is simple but very much understand-able! I have a question regarding W8Ji's neutralizing. W8JI called out 15T #28 wire however, your schematic calls for twisted pair 30T bifilar wound? Can you help me understand the process bifilar wound, to me, it imply 2 wires side-by-side wound Vs. twisting a pair of 30T of 28 gauge wire looping around the toroid's loop core. Also looping a bifilar wound15T Vs. 30T of 28 gauge wire is a substantial change, perhaps 30T was required? Thank You for your time! :) 73's AH6BS (blue-sky *LOL*)

[@CIRCUITDESIGNER]

The toroid core I used is very small so I needed more turns. The 2 wires don't need to be twisted, just wound together.

[@norbertstepien9185]

I've recently purchased an SB200 and mods are interesting. Could you give me value of "back to back" diodes you installed to protect the meter?

[@CIRCUITDESIGNER]

Hi, The diodes are 1N914.

[@dominicryan5227]

I just picked up an SB-200, this is a perfect set of mods! Could you clarify a detail on the key circuit? The Vishay opto-coupler part seems to want a max input current of 50mA, or the PVT part about 25mA. You have used a 68ohm resistor after the 5.1v zener. That would be more like 75mA. Is that still ok? (I have minimal circuit construction experience).

[@CIRCUITDESIGNER]

Don't forget to subtract the forward voltage drop across the LED. I used 1.2V. The drive is actually around 6ma.

[@AdamosDad]

73's🎙KD9OAM🎧

[@captlarry-3525]

Change the tubes, I bet it works fine on ten. Use stock parasitic chokes. The input coils are tuneable you don't need a tuner, check your input coax length. Yes you need a keying interface. Learn more before you post nonsense. Look at those bullshit parasitic chokes. Make stock ones !

[@CIRCUITDESIGNER]

Please watch the update video and be a little less aggressive.