The Collins 30S-1 also used grounded screen and was neutralized.
73
Jim W7RY
-----Original Message-----
From: sm0aom@telia.com
Sent: Monday, July 27, 2015 1:49 PM
To: mike@tubby.org
Cc: amps@contesting.com
Subject: Re: [Amps] Follow-up...re: Time to ask for help.
"Grounded screen" will work as it did in the Collins 204F-1 and the
208U-3/10 amplifiers,which were neutralized
despite using liberal grid swamping.
The electron transit time input losses at 50 MHz for the 4(C)X150/250 can be
approximated with a series resistance of about 5 ohms in series with the
grid input capacitance of 20 pF.
This translates roughly to a parallel equivalent of 4000 ohms and 19 pF.
4 tubes will correspond to 1000 ohms and 76 pF,and the grid voltage for AB1
will be about 55 V peak.
With reasonable coupling and circuit losses, the required peak driving power
is around 2 W.
Output power should be about 1000W,so the power gain is 37 dB, which is way
above the 20 dB
that are considered "safe" even with neutralization, and assuming no other
feedback paths.
A more reasonable safe gain without neutralization would be in the order of
15 dB, which requires
about 100 ohms extra swamping.
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från : mike@tubby.org
Datum : 2015-07-27 - 21:01 (V)
Till : amps@contesting.com
Ämne : Re: [Amps] Follow-up...re: Time to ask for help.
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@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@gmail.com
Datum : 2015-07-27 - 20:17 (V)
Till : donroden@hiwaay.net, amps@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@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@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@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@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@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@hiwaay.net> wrote:
Quoting Ron Youvan <ka4inm@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
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