[ARC5] BC-610 questions

[Kenneth G. Gordon]

On 19 Nov 2012 at 7:53, Richard Knoppow wrote:

> > 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 fed through 
the "cathode". It 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 stages plate coil (which was 
also the final amp's grid coil) and hooked a piece of RG-58 to it. It worked 
very well.

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. 
Thankfully, 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 
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 successful one) was a 66 
foot long piece of #36 black enameled wire which ran from one dorm to 
another on the second or third floor (I can't remember which 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 
home to 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 
instead of in his room since it stunk so badly after that.

Kenneth G. Gordon W7EKB

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