[Amps] {Collins} Understanding Screen Current in a Tetrode

[Mike Waters]

FYI, here was my post to the Collins reflector that prompted that reply.
What do you think?

"There is a lot of misinformation out there about screen current. But it's
the most important indication of proper loading!
"When I used to DX on the low end of 144 MHz with my homebrew 4X150A
amplifier (30 years ago), I knew that the amp was tuned just right when the
screen current was either zero or very slightly negative.
"According to www.w8ji.com/loading_amplifier.htm :  'In grid-driven
tetrodes or pentodes the most important parameter is screen current, with
control grid current a close second, and output power a close third.'
Believe it. :-) "

73, Mike
www.w0btu.com

On Tue, Jan 27, 2015 at 5:51 PM, Mike Waters <mikewate at gmail.com> wrote:

> Are you saying that we should always tune a grid-driven tetrode linear amp
> for the maximum screen current allowed by the manufacturer?
>
> Well, that would have been 30 or 40 mA on my homebrew AB1 grid-driven
> 4X150A amp. I think that would have been 30 or 40 mA too much! :-)
>
> It would be interesting to see what the people on the AMPS reflector have
> to say about this.
>
> 73, Mike
> www.w0btu.com
>
> On Tue, Jan 27, 2015 at 5:15 PM, David C. Hallam <david.hallam at knology.net
> > wrote:
>
>> For a GRID Driven tetrode amplifier, the correct way to tune and load is;
>> apply dome drive and dip and load in the normal so that you have some
>> indication of resonance.  At that point this is what you do.  Keep
>> increasing the drive and adjusting the loading for maximum screen current,
>> check the tuning occasionally.  You keep doing this until you reach one or
>> the other of two conditions.  One, You reach the maximum screen current
>> allowed by the manufacturers spec or Two, you have applied the maximum
>> drive you have available.  Once you reach either of the two conditions,
>> reduce the loading just slightly, and you will be properly loaded for the
>> best IMD your setup will produce.  I've been doing this for years on my HB
>> linears with never a complaint about the quality of my signal.
>>
>> This is the point of the QST article.
>>
>> David
>> KW4DH
>>
>>
>> On 1/27/2015 4:45 PM, Tony Sokol wrote:
>>
>>> Hi Kurt,
>>>     Good information because it backs up what I have been reading.
>>> EVERYONE tells you that monitoring the screen current is important but they
>>> really do not say what to look for or what to do about it. From reading the
>>> spec. sheet on the 4CX-1000A a comment is made that some tubes may exhibit
>>> a negative screen current as much as -25Ma. I did read that having the
>>> loading capacitor meshed too far will raise the voltages (and heat) in the
>>> tank circuit.  I also have read that when the screen goes positive the IMD
>>> goes to pot and you start splattering.  I guess the way to go is to tune up
>>> for max output with minimum plate current and then back off the loading to
>>> keep the meter at or under "0" screen current.  As far as the screen
>>> current goes...positive is not good and negative (within reasonable limits
>>> ie. >20Ma) is nothing to worry about as long as the screen voltage remains
>>> stable.  There was one other comment about dipping the screen current
>>> around "0" like you would with the plate meter I assume.  I will try using
>>> that theory tonight on the net and see what happens.  So far it seems to be
>>> running cooler and I had several unsolicited reports of "great sounding
>>> signal".  With all of the 30S-1's out there I am surprised that we haven't
>>> seen more on proper tuning procedures for it.
>>>
>>> 73
>>> Tony - W9JXN
>>>
>>>
>>> On 1/27/2015 2:51 PM, W6ph--- via Collins wrote:
>>>
>>>> FYI, there is an excellent article about screen current on page 26 or
>>>> July
>>>> 1961 QST.  In the article it says that negative screen current is not
>>>> unusual.  But it also says that maximizing screen current (within its
>>>> limit) is
>>>> the best indication of resonance.  It also says that a tetrode amplifier
>>>> should never be tuned for maximum output as is common for cathode
>>>> driven
>>>> (grounded control grid) amplifiers.
>>>>   Kurt W6PH
>>>>
>>>
>