[ARC5] BC375 and Dave's Excellent Tube Adventure, Part Two

[Mike Feher]

Hey - just remember who recommended the 1625 first, HI. Regardless,
congratulations on the great progress so far.

BTW, did you see the bidding on that TN-6? Good thing you got mine trough
the generosity of Mike H. I think my GF/RU was a lot better deal than this
TN-6 is going to be - 73 - Mike 

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

-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of David Stinson
Sent: Saturday, October 26, 2013 12:44 PM
To: ARC-5 List
Subject: [ARC5] BC375 and Dave's Excellent Tube Adventure, Part Two

Since I've satisfied the restless spirits of my Celtic and Confederate
ancestors by a screaming, headlong-charge  into this project (with the
predictable heavy casualties), I decided that less bravo and more brain
might be in order.
(Ya think??)

Note: All this work with the 1625s is "proof of concept."
I'm not building these assemblies to be permanent or I'd make them neater
and more rigid.
So don't laugh when you see the photos ;-)

Also-  Yes; this is going to run at less than the specified
70 or so Watts out.  The point is not to get exactly the same output or the
same performance as a set of 211s and a 10Y.
The point is to get the transmitter working on-the-air with a set of tubes
that cost $20 instead of *$1000*.
------------

I decided to back the B+ on the 1625s down to about 500 Volts and the
screens down to something like "normal" for that B+.
Since the BC-375 needs a "floating B-" and I don't want to build-up a new
supply for a temporary experiment, I put a 5000 Ohm, 50 Watt dropping
resistor in the B+ lead from the dynamotor and that worked.
Installed 10 Kohm 5W resistor with a .004uFd 2KV cap across it from the
plate pin to the 1625 screen (pin 3).
This means the assembly is slightly taller than a regular 211,
and that means looong leads.   So besides the parasitic suppressor in
the 1625 plate lead, I installed a .25x.25 in. ferrite bead in the grid lead
to help prevent parasitics.  Here's a photo of the assembly before
installing it in the 211 base  (I did trim the leads shorter. Red is grid,
green fils):

http://home.netcom.com/~arc5/BC375/375-1625B.jpg

When tuned-up with 211s in CW mode and the normal 1 KV B+, the set rans
about 200 mils Plate current for 70 watts out.
Running 500 B+ with these 1625s, I got just under 100 mils for about 23
Watts out.
Neutralization was "touchy," but not very far at all from the proper setting
for the 211s.

One thing that shows up quickly- any gas in the 1625 oscillator tube.
Once a little of this ionized gas causes the tube to heat up, the gas starts
swirling-around and the oscillator wanders all over the place.  A good tube
gave a solid note and as much stablility as could be expected with the
oscillator tube on stilts ;-).

http://home.netcom.com/~arc5/BC375/375-1625A.jpg

The 211s in the modulator at 500 volts don't like these and don't modulate
them well, but that's another problem I'll get to later.
For now and for the primary mission- a working CW 375 rig, I think we're off
to a good start.

73 OM DE Dave AB5S

______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html