>
>I mentioned that simply to show the progression over the decades Ian,
from
>basics to modern circuits that werent possible in the 50's or took up
>considerable real estate and high cost.
>
>Carl
>KM1H
>
That was exactly how I took your remark, Carl, but it seemed worthwhile
to explain why the resistor was still needed.
73 from Ian GM3SEK
>-----Original Message-----
>From: Carl [mailto:km1h@jeremy.mv.com]
>Sent: 06 November 2013 16:01
>To: Ian White; amps@contesting.com
>Subject: Re: [Amps] Initial RF testing of 4CX1000A amplifier
>
>
>
>> >
>>>> My meter reads -10 - 0 - 40. Actually it is a normal meter with a
new
>>>> scale. There is a 10 mA bleed which brings the pointer to zero when
>> there
>>>> is no screen current. Very clever idea by GM3SEK.
>>>>
>>>> I'll be testing with full drive soon.
>>>>
>>
>>>
>>>That screen bleed goes back to the 50's or so when tetrodes were more
>>>popular than triodes.
>>>
>>>Carl
>>>KM1H
>>>
>>
>> Back in the 1950s, the screen bleed resistor was a rather poor effort
to
>> provide some voltage stabilization. It never really worked, which is
why
>> modern screen supplies are electronically stabilized.
>>
>> The Tetrode Boards use an active shunt stabilizer. At first sight
this
>> shouldn't need a bleeder resistor at all, and the bleed current
(about
>> 10mA) contributes nothing to the voltage stability... so why is it
there
>> at all?
>>
>> The bleed resistor is not part of the power supply. It is installed
>> close to the tube itself, connected directly between the screen grid
and
>> ground (cathode). Its purpose is to prevent the screen grid from
>> floating unconnected during the few milliseconds while the screen
relay
>> contacts are in motion between the RX and TX positions. This is
>> particularly important for "problem" tubes like the 4XC1000A and the
>> 4CX250/350 family whose screen grids are very prone to secondary
>> emission (negative screen current). Without that resistor acting as a
>> pull-down, the screen voltage can easily float upward causing a
runaway
>> spike of anode current.
>>
>> The bleed current of about 10mA will move the meter needle up-scale,
but
>> probably not enough to display the full range of negative current for
a
>> 4CX1000A. The best solution is to mechanically adjust the meter
needle
>> further up-scale toward the center. As well as the front-panel
>> screwdriver adjustment, many meter movements have a coarse
>adjustment on
>> the rear pivot.
>>
>> (That was the short version. Many more details are provided on my
>> website, both in the User Manual for the boards and in a QEX article
>> which explains what was wrong with the large majority of older screen
>> supply circuits.)
>>
>> 73 from Ian GM3SEK
>
>
>I mentioned that simply to show the progression over the decades Ian,
from
>basics to modern circuits that werent possible in the 50's or took up
>considerable real estate and high cost.
>
>Carl
>KM1H
>
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|