[Amps] Follow-up...re: Time to ask for help.

Mon Jul 27 16:57:25 EDT 2015

The 640 Socket does not have the built-in screen bypass.
It has only one screen lead unlike the 630 that has several screen  
bypass connections .

E F Johnson sold a screen shield ring  that worked Well with the sk-640

Quoting Mike Tubby <mike at tubby.org>:

> I've looked at the schematic.
>
> As you have SK640 sockets why not modify the design to run "grounded  
> screen" ... like the 144MHz 4CX1000A design in the 1990 ARRL handbook?
>
> You would connect B- to the cathode, and the +ve side of the screen  
> supply to chassis and the -ve side of the screen supply to cathode.  
> You would need some good quality (large) silver mica caps to return  
> the cathode to deck at RF.
>
> In this mode you have a 'perfect' RF ground on the screen and the  
> isolation between grid and anonde circuits should be at least 10dB  
> more than the stage gain and as a result you shouldn't need any  
> neutralizing at all and you can maintain your pi-tuned input and  
> pi-tuned output.
>
> To execute this approach successfully you would probably need to use  
> some fingetstock to fabricate a low inductance ground for the screen  
> contact with multiple earth points.
>
> Just a thought ...
>
>
> Mike G8TIC
>
>
>
>
> On 27/07/2015 19:45, sm0aom at telia.com wrote:
>> If you are using the SK-640 sockets, which lack integral screen  
>> bypass capacitors,
>> what kind of other screen bypass do you use?
>>
>> Be advised that attempting linear operation of the 4X150 on 50 MHz  
>> using conventional screen bypassing
>> is a disaster waiting to happen. There is very little control over  
>> the self-resonant frequency of the screen circuit under these  
>> circumstances, and a "cold screen grid" is essential for stable  
>> operation.
>>
>> The previous suggestion of operating the amplifier in "grounded  
>> grid" will only work if the proper voltages and bypassing are  
>> present on both grids.
>>
>> Just tying both grids together and to ground will easily damage the  
>> control grid, as the 4X150 family is not suitable for operation as  
>> an "high-mu" triode.
>>
>> 73/
>> Karl-Arne
>> SM0AOM
>> SM0AOM
>>
>>
>>
>>
>>
>> ----Ursprungligt meddelande----
>> Från : ai7rogerroger at gmail.com
>> Datum : 2015-07-27 - 20:17 (V)
>> Till : donroden at hiwaay.net, amps at contesting.com
>> Ämne : Re: [Amps] Follow-up...re: Time to ask for help.
>>
>> Sockets in use are Eimac SK-640.
>>
>>
>> 73, Roger
>> AI7RR
>>
>> On Mon, Jul 27, 2015 at 10:49 AM, <donroden at hiwaay.net> wrote:
>>
>>> I woke up and the thing on my mind was .....
>>>
>>> Are you using Eimac SK-630 sockets ?
>>>
>>> I think those sockets may have been the reason my amp was so stable on six.
>>>
>>>
>>> Don R.
>>>
>>>
>>>
>>> Quoting Roger <ai7rogerroger at gmail.com>:
>>>
>>>  Don,
>>>> Thank you for the information. I've been studying other circuits and
>>>> combined with the info you just mentioned regarding screen loading, I have
>>>> a change to make. Both my grid and screen supplies are very stiff with
>>>> capability of supporting reverse current. My screens however are shunted
>>>> with very high resistance at the terminals just to hold them at ground
>>>> during rx.
>>>>
>>>> I've gone through many different wiring configurations with this amp and
>>>> have even had output once or twice, but every time I try to improve it, I
>>>> screw something up. This time finding the error hasn't been so easy.
>>>>
>>>>
>>>> 73, Roger
>>>> AI7RR
>>>>
>>>> On Sun, Jul 26, 2015 at 8:17 PM, <donroden at hiwaay.net> wrote:
>>>>
>>>>  No problem... Just think of the 250Bs as a big variable DC load on the
>>>>> plate power supply.  You should be able to change the grid voltage and
>>>>> see
>>>>> a smooth
>>>>> increase in plate current from zero to near 1000ma ( very briefly !!! )
>>>>>
>>>>> I would suggest zero to 500ma as a minimum range to check.
>>>>> No more than that until absolutely sure everything is stable
>>>>>
>>>>>
>>>>> THEN hook up the grid circuit and see if you can do the same thing..
>>>>> something may be breaking down in the grid circuit and killing the bias
>>>>> voltage and
>>>>> then ...... BOOM ... Plate current runs away.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Quoting Roger <ai7rogerroger at gmail.com>:
>>>>>
>>>>>  "I'm confident my DC is stable."
>>>>>
>>>>>> Scratch that. I misunderstood the statement:
>>>>>>
>>>>>> "He has to make the amp DC stable before hooking up the input and output
>>>>>> RF
>>>>>> networks."
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> 73, Roger
>>>>>> AI7RR
>>>>>>
>>>>>> On Sun, Jul 26, 2015 at 5:50 PM, Roger <ai7rogerroger at gmail.com> wrote:
>>>>>>
>>>>>>  Sorry, I thought it was clear what I was trying to do. I have four
>>>>>> 4x150a
>>>>>>
>>>>>>> tetrodes in parallel, attempting to make them operate on 50mhz. I'm
>>>>>>> confident my DC is stable. Additional information includes, a Pi tuned
>>>>>>> input and output. Input and output terminations are quality 50 ohm
>>>>>>> devices.
>>>>>>> Power supply and rf deck are in separate boxes. My problem appears to
>>>>>>> be
>>>>>>> related to instability possibly caused by insufficient or incorrect
>>>>>>> neutralization.
>>>>>>>
>>>>>>> It was mentioned that the neutralizing cap needed to be by-passed and
>>>>>>> that's something I have not even considered. By-passing anything in the
>>>>>>> grid circuit seems contradictory to me.
>>>>>>>
>>>>>>>
>>>>>>> 73, Roger
>>>>>>> AI7RR
>>>>>>>
>>>>>>> On Sun, Jul 26, 2015 at 3:56 PM, <donroden at hiwaay.net> wrote:
>>>>>>>
>>>>>>>  Quoting Ron Youvan <ka4inm at gmail.com>:
>>>>>>>
>>>>>>>>
>>>>>>>>   I am trying to find out what you are trying to do.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>   Me Too Ron,
>>>>>>>> He has to make the amp DC stable before hooking up the input and
>>>>>>>> output
>>>>>>>> RF networks.  We don't know if that's the case yet.
>>>>>>>>
>>>>>>>> Don W4DNR
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> Amps mailing list
>>>>>>>> Amps at contesting.com
>>>>>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>
>>>
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