[ARC5] BC-610 adventures

[Kenneth G. Gordon]

On 18 Nov 2012 at 23:13, Richard Knoppow wrote:
 
>It appears that there were many versions of 
> the BC-610 and evidently they were built into the 
> 1950s.
>
> Thanks Ken, that's about what I wanted to know.
> Perhaps when I win the lottery I will find one. I have 
> just been curious about them since I was a kid and they were
> fairly common then. I can't remember ever seeing one in 
> the flesh. Its interesting to compare it to a Viking 500 
> which puts out about the same power.

One other thing I might add: I never had one speck of TVI 
with mine when I was using it as a linear amp. Neither did 
my friend, Bob Preston W7DPG.

When we first got my unit, I had thought that to use it as 
a linear amp I might need to drive the pair of 807s that 
were the driver stage.

We very quickly discovered that, at least in mine, all the 
TVI came from the 807 stage.

The tube line-up is a 6L6 oscillator, a 6L6 doubler/driver,
and a pair of 807s as the driver to the 250TH.

I could never figure out why Hallicrafters used a 6L6 as 
an oscillator (both VFO and crystal). I modified mine to 
a 6AG7 and got rid of some of the TVI. I tried to clean 
up the 807 stage, but never really succeeded.

Even with the stock 6L6, the VFO was unusually stable, 
although the tuning rate was very quick so one had to 
have a very light touch on the VFO tuning knob to get 
it on frequency. When used on CW, there wasn't any chirp to
speak of either.

In any case, we quickly found that we didn't need the 807 
stage as long as we used a bias-shunt-regulator to hold 
the bias on the final to the correct point. Without the 
bias-shunt-regulator, excitation-bias would drive the amp
way into Class C and drive requirements would jump way up.

I "stole" the circuit for the bias-shunt-regulator from the Wilcox 
96-A. It was an 811 with its plate grounded, the grid-bias voltage to 
the final was fed through the "cathode". There was a 100K resistor between 
the 811's grid and cathode. The circuit worked extremely well, 
holding the final amp's grid bias to the Class B setting within a 
1/2 volt. It is a very simple circuit.

We drove the grid of the 304TL directly through the 807 
stage's tuned circuit.  We simply wound a few turns around 
the 807 stage´s plate coil (which was also the final amp's 
grid coil) and hooked a piece of RG-58 to it. It worked
very well. We removed all the tubes which were ahead 
of the final amp tube

The 304TL when used in push-pull Class B, requires just
about 100 watts of drive for 1500 watts output.

I will repeat: the 304TL is not a very good linear amp 
tube: it exhibits much too high levels of distortion for 
SSB, even when operated within ratings.
Luckily, we were using them on frequencies that were 
not anywhere near a ham band, so no one ever complained.

Besides, we were severely over-driving the single-304TLs 
in our amps. We didn't know any better at the time. We 
were being heard reliably in Vietnam and SEA when we 
were needed and those on the other end seemed to 
appreciate our efforts. That was all that counted with us then.

Prior to using the BC-610s for AFMARS, and while I was 
going to College at Montana State University in 1960, 
I and a couple of other hams sneaked a BC-610 into 
a dorm room there with the help of a friendly janitor who 
let us use the freight elevator. 

The first time we fired it up, 
there was a dead short in the HV circuit, and it blew 
the main breakers for the entire dorm floor. We
eventually got it working. 

Our 2nd antenna (and first successful 
one) was a 66 foot long piece of #36 black enameled 
wire which ran from one dorm room window to another
in an adjacent dorm on the second or third floor of both
(I can't remember which floor at this point).
We used it to work DX on 20 meters CW. I can't remember 
what we used for a receiver. 

When the semester ended, the fellow who owned it took it back
to his home in Billings, Montana.

Our first attempt at an antenna was an old military whip of 
some sort, about 12 feet long. We stuck it in the thumb-hole 
of a bowling ball to use as an insulator and leaned it out 
the window. When we keyed the rig, there was a huge ball 
of corona that appeared at the tip of the antenna. It scared the
heck out of a fellow on the floor just above ours and partially 
melted the thumb-hole of the bowling ball. The owner had 
to leave the ball out in the hall for the rest of the semester
instead of in his room since it stunk so badly after that.

Great fun.

Kenneth G. Gordon W7EKB

"Courage is being scared to death but saddling up anyway."--- John   Wayne