SB-220 Troubleshooting

[Kenneth G. Gordon]

On 1 Aug 2010 at 18:31, Paul wrote:

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

Agreed. In my experience, amps such as the SB-220 do not have 
that problem. Most of them are properly designed.
 
> 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! 

Thanks for the pointer, Paul. I'll visit that asap.

> The next major problem 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 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 will cause the tube socket to eventually fail. 
> 
> 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 500
> watts output. Again, its proper operation of the amplifier.

Very interesting and thank you for this information.
 
> 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. 

I agree that this would certainly be the next most likely culprit 
following a long storage period. I have seen many rigs that have 
had an incredible amount of dirt built up over many years of 
storage, leading to arcs when fired up.

> 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 grid is a recipe for oscillation!

Indeed!

> 
> I'd suggest a visual inspection as suggested by Ken.

Which SHOULD find blown grid chokes, if any exist! :-)

> It's good practice to
> let any power grid tube run with B+ applied for several hours before doing
> anything. 

Yes.

Thank you, Paul. You given me some good ideas. I hope to have 
an SB-220 here soon, myself.

Ken W7EKB

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