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[AMPS] 572's

To: <amps@contesting.com>
Subject: [AMPS] 572's
From: W8JI@contesting.com (Tom Rauch)
Date: Mon, 28 Aug 2000 15:31:47 -0400
>a while back someone said that there are problems with svetlana 
572b
> tubes??
> 
>       hank,  n6buu

The Svetlana 572 tubes have two problems. They require more bias 
to cut the tube off. In some amplifiers with marginal cutoff bias (the 
FL2100) the tubes do not cut off.

This causes a problem because the anode and input of the tube 
are unloaded when the relay transfers to the receive mode. The 
poor anode to filament shielding in the 572 creates a feedback 
path, and the amplifier will oscillate on higher bands while on 
standby.

The "band-aid" fix is to increase cutoff bias in the original circuit by 
changing to a voltage doubler in the bias. A better fix is to open the 
cathode return path and let the tubes self-bias into cutoff. The 
ultimate fix would be to neutralize the tubes, and open the cathode 
path.

The second problem relates to anode voltage breakdown. In a 
traditional class AB or B PA the peak anode voltage when the PA 
is properly loaded and driven with correct drive for that loading is 
just under two times the anode voltage. 

In abnormal operation....like when the tube is being momentarily 
overdriven by a transient from the exciter, the antenna presents a 
mismatch of the right angle at the antenna port due to an arc or 
other fault, or the antenna is not transferred to the amplifier the 
peak anode voltage can be several times the normal operating 
voltage. Generally a switch or capacitor will arc and limit the 
voltage, but if the tube has inadequate hold-off it can flash over 
internally.

About 2/3 of the new Svetlana 572B's failed a dc breakdown test of 
5000-6000 volts. Some cleaned up after running, but then a large 
percentage of them failed in the field. Almost none of the tubes 
handled 8 kV. 5000 volts would be typical in a properly loaded 
amplifier running the 572 within ratings. IMO, the tube should have 
headroom of about 50% over the maximum expected peak voltage 
in normal operation. For example, good 3-500Z's will take about 3 
times the rated dc operating voltage in a dc test, which is about 
50% more than the typical peak voltage in normal operation.

The 572 will always be a good sub for an 811A because they have 
enough headroom for 1500 volts or so dc anode voltage. Any 
vendor should work.

Some Chinese tubes are good, some aren't, depending on the 
manufacturer in China and how the tubes are tested. You have to 
trust the retailer to know the difference, and select the reliable 
sources, since they can be sent out in whatever name the 
customer wants printed on the tube.




   
    
73, Tom W8JI
w8ji@contesting.com

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