Carl wrote:
>>
>> You can make a good shunt screen stabiliser using an op-amp and a
>>tetrode, this was done many times for 4CX250B, 4CX1000A etc. before
>>the Tetrode Boards from Ian G3SEK...
>>
>> See here (second half of article):
>>
>> http://www.agder.net/la8ak/d23.htm
>>
>> and here:
>>
>> http://www.qsl.net/g3uvr/4cx250/G4AJW-psu.pdf
>>
>> for the 4CX10000D you would need a bigger tube ;-) The BBC
>>long-wave transmitter (198KHz) used to use a 4CX1500A as the shunt
>>screen stabilizer for the 250KW tubes made by Thompson ESF.
>>
>> I'm just not sure which tube you should select for the job...
>>perhaps someone on the list can make recommendations...?
>>
>>
>> Mike G8TIC
>>
>Id be tempted to hear Ians view of the 2nd one particularly since it is
>so dear to his heart....2M IMD in his back yard.
>
There is much less 2m SSB activity than there used to be, but the G4AJW
screen stabilizer is still "dear to my heart" as the ancestor of the
solid-state circuit that I use.
As far as I know, G4AJW invented the idea of using 30V of zener diodes
in series with the cathode of the shunt stabilizer tube, and using this
point to provide the 30V supply rail for the op-amp. The output voltage
swing from the op-amp is about 5-25V negative of the cathode, which is
just right for connection to the control grid of the tube. As far as I
can see, the only major design weaknesses are that the reference voltage
for the control loop is derived from the 'soft' 30V rail (rather than a
dedicated IC regulator as we would do today); and the use of an op-amp
with fixed internal compensation (which actually makes the control loop
less stable).
G4JZQ made the transition to a MOSFET stabilizer and greatly improved
the loop compensation using a 748 op-amp [1]. Like Dave, I have
experimented a lot to optimize the balance between static voltage
regulation, response to sudden steps in current demand and avoiding
higher-frequency noise in the feedback loop.
Regarding the best type of tube for a shunt stabilizer, modern circuits
have plenty of loop gain available in the op-amp, so I don't think it is
essential to use a low-mu triode any more. Simply identify the range of
currents that the shunt stabilizer will need to pass, under all possible
operating conditions, and then choose a tube with the right
characteristics to be controlled by the available voltage swing from the
op-amp... and of course has the necessary anode voltage and dissipation
ratings. For ruggedness and low cost, the 813 would be well worth a
look.
[1] Taking up Dave's point about using such an ancient device as the
748, it has survived in the market because there is no other mainstream
op-amp with quite the same combination of features. There was a glitch
in supply during the changeover to 'lead-free' manufacturing, but the
748 came back in a new SMD package as well as the original DIL, which
looks like a vote of confidence in future demand. (The same can be said
of the 723, which has survived in the market because it was extremely
well designed in the first place, and still does some things better than
newer devices.)
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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