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@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
>>>
>>
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