Many of us have noticed the attractive price on the surplus Russian radar tubes and have wondered about using them as linear amps. The cost per watt of dissipation makes this attractive but you should remember you are getting what you paid for. They have been sitting on the shelf for decades and most are not useable without being conditioned first. Of all the ceramic tubes, these may have the worst vacuum seals or poorest getter materials that fail to preserve the vacuum.
If you apply HV and RF before burning in the filament, you are almost guaranteed an internal tube arc in every case. Slowly running the filament voltage up over many hours will burn off contaminants trapped inside the vacuum and prevent these arcs. The topic and how to do this is well covered online and should not be ignored.
Others have suggested the GS35B is like an 8877 on steroids. I say hogwash! Both tubes have a fragile grid at between 25 and 26 watts. If you think you can run a GS35B at twice the output of the 8877 get ready for a failure! The GS35B does NOT have a 2500 plate dissipation as advertised to the ham community and extrapolated from the radar specs.
Don't let that big anode cooler fool you. It actually sucks for two reasons. First it's not part of the tube and has a mechanical bond between it and the anode, causing a significant loss in heat transfer between the two. Second, the tube is a planar triode for microwave and not a conical design. That means the plate surface area is MUCH smaller than other conical tubes of the same physical size.
Many are familiar with the high emissions available from oxide coated cathodes and their lower current consumption. Tubes like the 8877 can be grossly overdriven for short periods of time and produce much higher peak output since the cathode can support these emissions. Not the case with the Russian tubes.
While the Russian tubes are rated for pulse service they can't handle the extra current for any length of time useful in communications duty. Lots of testing on everything from the GI7B to the GS35B were conducted. Drive them hard and the power only comes up for a few seconds. Then get ready for the "fade". Output will begin to drop rapidly. I've watched it fall all the way down to where there was virtually no gain and it then kicks out the over current protection on the test amp.
This condition is very prevalent at higher plate voltages and seems to be dependant on the bias voltage used at higher plate voltages. This presents a situation where bias voltage cannot stabilize the tube at higher than normal plate voltage. You can set the idling current properly but once you apply RF a very interesting thing happens. Grid current will spike in a negative direction and prevent you from using more plate voltage almost regardless of the bias voltage. Only way around this is excessive forward bias current.
I've never seen Western tubes fade in output or spike in negative grid current like this and have tried to figure out what may be the cause. Only two possibilities come to mind. The fragile grids inside these tubes may be turning red hot. Dissection of several failed tubes revealed significant grid damage inside. The second thought is the oxide coating on the cathode may not be deep enough to support these emissions. One clue here is that short 500 hour life span.
Knowing the limitations of these tubes can help you make the right choice. They are a good value but only when used to their RATED military specs. That's 1500 watts on a GS35B, not 2500! Whoever came up with this 2500 watt rating on HF put very little thought into how much more than the rated air flow would actually be required at this power level. You'll need a blower than can nearly push the tube out of the socket to sustain this power level otherwise the copper will turn blue.
If you apply HV and RF before burning in the filament, you are almost guaranteed an internal tube arc in every case. Slowly running the filament voltage up over many hours will burn off contaminants trapped inside the vacuum and prevent these arcs. The topic and how to do this is well covered online and should not be ignored.
Others have suggested the GS35B is like an 8877 on steroids. I say hogwash! Both tubes have a fragile grid at between 25 and 26 watts. If you think you can run a GS35B at twice the output of the 8877 get ready for a failure! The GS35B does NOT have a 2500 plate dissipation as advertised to the ham community and extrapolated from the radar specs.
Don't let that big anode cooler fool you. It actually sucks for two reasons. First it's not part of the tube and has a mechanical bond between it and the anode, causing a significant loss in heat transfer between the two. Second, the tube is a planar triode for microwave and not a conical design. That means the plate surface area is MUCH smaller than other conical tubes of the same physical size.
Many are familiar with the high emissions available from oxide coated cathodes and their lower current consumption. Tubes like the 8877 can be grossly overdriven for short periods of time and produce much higher peak output since the cathode can support these emissions. Not the case with the Russian tubes.
While the Russian tubes are rated for pulse service they can't handle the extra current for any length of time useful in communications duty. Lots of testing on everything from the GI7B to the GS35B were conducted. Drive them hard and the power only comes up for a few seconds. Then get ready for the "fade". Output will begin to drop rapidly. I've watched it fall all the way down to where there was virtually no gain and it then kicks out the over current protection on the test amp.
This condition is very prevalent at higher plate voltages and seems to be dependant on the bias voltage used at higher plate voltages. This presents a situation where bias voltage cannot stabilize the tube at higher than normal plate voltage. You can set the idling current properly but once you apply RF a very interesting thing happens. Grid current will spike in a negative direction and prevent you from using more plate voltage almost regardless of the bias voltage. Only way around this is excessive forward bias current.
I've never seen Western tubes fade in output or spike in negative grid current like this and have tried to figure out what may be the cause. Only two possibilities come to mind. The fragile grids inside these tubes may be turning red hot. Dissection of several failed tubes revealed significant grid damage inside. The second thought is the oxide coating on the cathode may not be deep enough to support these emissions. One clue here is that short 500 hour life span.
Knowing the limitations of these tubes can help you make the right choice. They are a good value but only when used to their RATED military specs. That's 1500 watts on a GS35B, not 2500! Whoever came up with this 2500 watt rating on HF put very little thought into how much more than the rated air flow would actually be required at this power level. You'll need a blower than can nearly push the tube out of the socket to sustain this power level otherwise the copper will turn blue.
