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

Wed Jan 28 01:29:45 EST 2015

Carl is correct. In the 4CX1000a the spec sheet calls for 35 ma screen current in typical AB1 service. The screen dissipation is 12w, and at the rated 325v, that would be 36 ma! Of course the duty cycle in SSB/CW helps you out.
When tuning, the screen current is a very sensitive indicator of resonance and loading. On CW I would try to keep it between 25-30 ma so that I could QSY without causing it to exceed 40 ma in the positive or negative direction, the point at which it was set to trip out.

Vic K2VCO /4X6GP 

> On Jan 28, 2015, at 4:26 AM, Carl <km1h at jeremy.qozzy.com> wrote:
> 
> I don't care for that JI answer since grid driven tetrodes are almost always run in AB1 where there is no grid current and in AB2 there is only a flicker of current on peaks but a lot more IMD. Tuning is via the screen and power meters and let the mike gain control take care of the grid. Just remember that AB1 is less efficient than AB2 or B...or C for that matter
> 
> Some tubes exhibit negative screen current, the 4CX1000A is known for that and it is normal under some conditions and right in the spec sheet.
> Many of the small 4X150 class exhibit negative screen current on VHF and if output doesn't increase when going positive it is OK.
> 
> I get a small amount of negative with a 6M NCL-2000 while tuning but output always peaks in the positive 10-15ma region for 1200W. There is no negative with the HF NCL's unless it is a sudden downward drift signaling an impending BANG; this is a fairly common fault with 8122's that have been beat hard and misused. They often work OK in the CW position at the lower plate and screen voltages and are still linear.
> 
> The AM-6155 I use on 432 and several of the AM-6154 and 6155's Ive converted or repaired for 144, 222, and 432 all show positive screens when loaded properly. The 8930 that is used is a 4CX250B with a bigger anode cooler and the Russian GS-36B/4CX400A is a direct replacement except for sometimes having to bend the plate cavity finger stock a hair; it performs the same.
> 
> Carl
> KM1H
> 
> 
> 
> --------------------------------------------------
> From: "Mike Waters" <mikewate at gmail.com>
> Sent: Tuesday, January 27, 2015 6:53 PM
> To: <amps at contesting.com>
> Subject: Re: [Amps] {Collins} Understanding Screen Current in a Tetrode
> 
>> 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
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