HeathKit SB-220 Parasitic Oscillation Supression

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Loy Beckett:
I installed 3 seperate 470 pico farad caps and a 30 ohm 1/2 watt resistor in the grid of each output tube to protect the grid from excessive current. I also installed 10 ohm 3 watt resistors in the cathode circuits and a 10 ohm 10 watt glitch resistor ini the HV supply line.
I replaced the zener diode with 6 diodes so that I could set my grid bias at 3.67 volts for a grid current of 120 ma @2500 volts and 150 ma @ 3000 volts. My amplifier works good but my output seems to be less than it was. My max output is 900 watts with  120 ma grid current and 400 ma plate current. I tuned-up with CW dits (50%).
My question is this: Did my changes lower my output or are my tubes getting weak?

Loy   W4LCB

Steve Katz:
Beats me, but I can't see how you'd ever get more than 780W output with only 400mA plate current!  That's only 1200W DC *input* power; at even 65% efficiency, which is really pushing it for a grounded-grid linear amplifier, that would result in at most 780W RF output.

Normally, an SB-220 would run about 600mA key-down Ip at ~2700Vdc to provide 1620W DC input and 1050W RF out.  Under low duty-cycle conditions (SSB), peak Ip can reach 800mA and peak output ~1300W PEP.  At least, that's the case with every SB-220 I've owned or used, including the one I built back in 1969...


Lee Buller:
I agree with the above statement.  My question is,
why all the stuff on the grids of the tubes?  WHen I had my SB220, I ripped all that out and grounded the tubes with thick wire to the chasis.  That is what a grounded grid amp is all about.  I never had any trouble with the amp. Grid current can and is monitored and should be a function of drive, resonance and loading.  The SB220 is a good amp, but you have to load it heavy or it will go goofy on you.

Mark S. Graalman:

 The best thing to do with that amp is to fround the grids directly to the chassis with several pieces
of SHORT wire, or better yet SHORT wide strap.
 I would install about 10 ohms at about 5 watts of NON-inductive resistance in the cathode RF return, install
about 15-20 ohms at about 25-50 watts in the B+ lead,
paint the chassis around the tubes flat black, sequence
the antenna relay so the output contact "makes" just before the input contact, and basically leave the rest of the amp alone.
 Resistors in the grid don't protect anything, all they do is further unstablize the bias supply voltage
under grid current conditions, and in addtion to the
grid bypass caps, lift the grid above RF ground
which is NEVER a good thing.
 I would question your wattmeter,also you have to realize also that you raised the drive requirements of that amp slightly as well.

73,  Mark  WB8JKR

Loy Beckett:
The main reason that I used the 30 ohm grid resistors was to protect my filament transformer from damage in case of a filament to grid short. I was told that sometimes filaments sag and short to grids and that this would be a good thing to do just to be safe. However, my amplifier now has greater drive requirements. What would be a better way if I still
want something to act as a grid fuse? I no longer have the original RF grid chokes. I didn't save them when I replaced them with the 30 ohm 1/2 watt resistors.


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