SB-220 Troubleshooting

[Paul]

While I do agree that just about any amplifier can oscillate under the right
circumstances, it is a rare exception when this occurs in a properly
designed amplifier.

 

Much has been made of the VHF parasitic oscillation problems centered around
grounded grid triodes. I refer people to W8JI's web site for a thorough
discussion on the subject. Tom gets it! 

 

IF (and that's a big if) an oscillation occurs, you will see Ip and Ig
increase very quickly. If the amp is connected to a load (and watt meter)
you will see SOME power output. EVEN IF THE AMP IS NOT KEYED. The T/R relay
only provides just so much isolation (about 30db). If the amp is keyed when
the oscillation occurs you will see even more output power. It's impossible
to say how much power would be observed, but runaway Ip and Ig can not be
refuted.

 

The main problem with SB-220's is improper use. I've owned at least 6
SB-220's over my 37 years as an amateur and never once experienced a failure
in operation, much less due to parasitic oscillations. And that includes a
LOT of contest operation. 

 

The next major problem with the SB-220 is the way Heath cools the 3-500z's.
The fan blows across the chassis (above and below). Eimac tubes have a
phenolic spacer that secures the tube pins AND provides about a 3/16" air
gap with the socket and base of the tube. If you use brand "XYZ" tubes, they
won't have this gap, so the tube basically sits directly on the socket and
air flow around the tube base seal is reduced. Putting a fan on top of the
amplifier does little to improve this situation. The extreme heat of the
filaments without adequate cooling could cause the tube socket to eventually
fail. 

(Remember, SB-220's have been around a LONG time and who knows how the
SB-220 was treated before you got it)

 

The failure mode that is realized is the receptacles on the socket will not
make a good connection to the filament pins and will eventually lead to tube
failure. Don't believe me? I can provide pictures of SB-220 sockets that
I've replaced. I will agree that the majority of this heat problem can be
avoided by not holding the SB-220 at full power key down for extended
periods of time. RTTY with the SB-220 should be limited to no more than
300-400 watts output. Again, it points back to proper operation of the
amplifier.

 

Back to the amplifier at hand, I'm more suspicious of a tube flash over due
to the amp sitting unused for so long and one of the tubes being a little
bit gassy. This would most likely take out the grid chokes. If the grid
chokes are blown, replace them. Trying to run the SB-220 without a DC Path
to ground for the grids is a recipe for oscillation!

 

I agree with Ken; a visual inspection is in order. Finally, it's good
practice to let any power grid tube run with B+ applied for several hours
before doing anything. 

 

 

Paul

 


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