[Milsurplus] BC-9: Blessings and VT-1 Report

[David Stinson]

Why The Almighty has blessed me, the least worthy
of men, to know people on these lists so kind and
so generous of spirit, I cannot guess.
I can only be humbly grateful and do my best 
to be worthy of such men.  One of our members
sent me the matching BE-48 Equipment-Battery
Box-Mount and another member sent me three
working VT-1 tubes.  I am overwhelmed.
Thank you, thank you.

To refresh where we were before the VT-1s:
The manual is at:
http://tinyurl.com/n2ljjo3

Many photos at:
https://goo.gl/photos/1gM2VvHFYUxrWL316

The radio was refurbished to the original-design 
circuit, save with these substitute tubes:
Triode-connected 6AQ5 Osc/Detector 
for external tank coil and link.
VT-25A Osc/Det for Loop.
Two 01As for Audio Amp.
Original-Design power requirements are:  
120V RF B+
40V Amplifier B+
-20V Bias
4V A+
When using substitute tubes, which have 
different filament requirements, the A+
voltage is set at 6.3V.

Power is currently provided by four DC-DC
converters, allowing the set to be run from 
a 12V storage battery.  120V is provided by
two isolated dual-output converters in series
(+/-48V, +/-15V).  40V is provided by one
end of another +/-48V converter.  Bias voltage
is provided by two 9-Volt batteries.  A+ is 
from a non-isolated adjustable Buck converter.
Ground currents from the HV side are causing 
some heat issues with the A+ converter, even
though the total Filament current is withing spec,
so I will be replacing this one with a linear
regulator.  Birdies only become an issue when
the storage battery nears discharge.  
No idea why.

We found that, when using the set "as designed,"
the transmit frequency was offset many KCs 
from the receive frequency and agreed that 
this could not have been the original intent.  
After much "chewing" on the problem and 
discussion by our group, and some experiments, 
we found that inserting one of the common 
"Radio Shack" 100 uH choke coils at the input 
of the Plate filtering circuit:
https://goo.gl/photos/riJSGYi71HTuYboR6
netted the set and it is successfully making contacts.
The offsets between transmit and receive frequencies 
are somewhat dependent on the Oscillator tube used
and the antenna configuration, all of them providing
a usable offset.

The chance to test the set in "full original design"
configuration came with the generous gift of 
a working set of  three VT-1s.
I bypassed the inserted choke, reset the A+ to
the design-level of 4V and tested the set.

To my great surprise, the VT-1 tubes made 
no difference in the frequency offset issue- 
they exhibited the same wide TX/RX frequency 
displacement as any of the other tubes.
Re-inserting the 100uH choke again netted
the rig with a pleasing 900 Hz offset from
TX to RX frequencies when using the loop.
As with the substitute tubes, with the 100uH 
choke in-circuit, changing the value of C6 had 
little to no effect on offset
(there was one operational difference- later).

Applying logic- I think it safe to propose that,
since there is no substatial difference in the offset
from the original VT-1s to a substitute and that 
the same filter modification produces essentially
the same results, and that this could not be the
designer's intent, there is likely an as-yet
undiscovered fault in the filter circuit.

I do not intend to chase that issue further.
I have learned through unpleasent experience
that continuing to "fiddle" with a working system
invites disaster.  
I define the rig as "working-"
making successful, comfortable contacts 
and easily field portable as was intended.
If I keep twiddling I'm going to break some
irreplaceable part or blow open a transformer
primary, etc.  "Don't let "the perfect" 
be the enemy of "the good," lest you 
end-up with neither," a wise man once told me.
I will concentrate on refurbishing this lovely
Battery/Equipment box and mounting a power
supply in it.

The operational issue when using the VT-1s:
While the VT-1s function, I found these tubes
incredibly "noisy."  Using the 01As at reduced 
filament in the amplifier stages, all three VT-1s
in the Osc./Detector stage exhibit very high 
"hash" noise levels.  Signals which were easy copy 
with the VT-25A or 6AQ5 were buried in 
hash noise using the VT-1s.
I'm sure that would not be a problem when working
another BC-9 a couple of miles away, considering
that the loop on my kitchen table has QSOed 250
miles, but the noise buries all but the stronger signals.

The "best working' tube so far when using the loop
is the VT-25A at the low 6.3V filament voltage
( S3 filament switch closed).
Would like to find a less expensive tube that works
as well on the loop (there I go fiddling again).
The triode-connected 6AQ5 is excellent when 
using the external tank coil and link- so much so,
I see no reason to search for anything else.

That's where we stand today.  Again, my deep 
gratitude to all those who have contributed 
discussion, ideas and such valuable items 
to this project.  Our community is awesome.

GL OM ES 73 DE Dave AB5S