[Milsurplus] [3890_AM] TCS Filter Adaptor- Corrected Schematic

[Bill Carns]

I would worry about decoupling the grid on the filter.

Here is the way that Collins did it in the 32S-3 transmitter. While this is
a transmitter application and not a receiver, it is representative and takes
all of the switching complications out of the picture.

The only thing that might get the filter is a plate to grid short on the
output amp side and that is pretty rare in a receiving tube.  Notice the
grid biasing.

I say keep it simple.  :-)



Bill

-----Original Message-----
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of
Mike Feher
Sent: Monday, February 15, 2016 10:55 AM
To: 'AKLDGUY .' <neilb0627 at gmail.com>
Cc: boatanchors at mailman.qth.net; 'David Stinson' <arc5 at ix.netcom.com>;
milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] [3890_AM] TCS Filter Adaptor- Corrected Schematic

Dave -

I apologize, Neil is absolutely correct on the output side of the filter.
The grid needs to be at ground, for proper bias, and as he points out that
is sufficiently done through the filter. Since no resistor is needed no
decoupling is needed on the output side either. Do not know what I was
thinking. In either case,, realistically, since there is minimal grid
current, the resistor would have no effect either, but then you would need
decoupling. Save two parts. Next time I'll think before posting. 73 - Mike 

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 


-----Original Message-----
From: AKLDGUY . [mailto:neilb0627 at gmail.com]
Sent: Monday, February 15, 2016 11:45 AM
To: Mike Feher
Cc: David Stinson; milsurplus at mailman.qth.net; boatanchors at mailman.qth.net
Subject: Re: [Milsurplus] [3890_AM] TCS Filter Adaptor- Corrected Schematic

> On the output side, I assume the 1K in the cathode is to provide some 
> bias to the tube. Unfortunately the coil in the filter essentially 
> shorts the grid to ground, so, you may encounter distortion with high 
> level signals. Here I would suggest a 100K from the cold end of the 
> filter to ground with a 0.01 across it for RF purposes.

I'm really puzzled by that recommendation.
With bias provided by the 1K cathode resistor, the grid absolutely SHOULD be
returned to ground, DC-wise.
The filter winding does that and drives V2 with signal.
It's correct as shown.

The 1K is not decoupled for RF, but there may be a reason that I missed in
an earlier post.

73 de Neil ZL1ANM


On 2/16/16, Mike Feher <n4fs at eozinc.com> wrote:
> As it is you do not have 22K anyway as the coil resistance is much lower.
> Once you isolate the filter, 22K may still be OK. As I recall the 
> filters themselves like to see the 120 pf and about 1K or so of R. So, 
> you could possibly try playing with getting the matching correct also 
> on both input and output. The 22K needs to be in the sold end and not 
> across the filter anyway. Should have mentioned that before. Keep up 
> the good work. 73 - Mike
>
>
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960
>
>
> -----Original Message-----
> From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf 
> Of David Stinson
> Sent: Monday, February 15, 2016 8:56 AM
> To: milsurplus at mailman.qth.net; boatanchors at mailman.qth.net
> Subject: Re: [Milsurplus] [3890_AM] TCS Filter Adaptor- Corrected 
> Schematic
>
> To my happy surprise, several of our great "smart people" have written 
> me and offered good suggestions on improving the filter adaptor.  I 
> will give them a try and report back.  I especially like those that 
> get the DC off the filter input.  Will probably need to change the 22K 
> plate resistor.  73 Dave S.
>
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