I think that tube load lines involve finding the center of the Eg-Ip curve to set the
class A or B or C quiesent operating point based on plate load (Eg-Ip = grid voltage vs plate current).
Know what... I like tubes but it has been years since I last touched one, and I do mean YEARS
I was a high school guy in 1966 or so when I began learning serious electronics and I kind of grew
up with both tubes and transistors. Old-school tube techs who couldn't adapt were dropping out of
the field like flies. I never knew there was a problem. Parts were plentiful and there was still WW2
surplus to be had for pocket change. It was IC's that for a while screwed me up. But... but... but
what's inside. Who cares. I do. It doesn't matter. Stuff inside. Look... apply power, signal in, the
innards do this and that and signal out. It doesn't matter, you can't fix an IC... you replace it!
I graduated high school with a Second Class Radiotelephone License and an Advanced Class
Amateur Radio License in 1968. Joined the Navy, spent 6 years as a steam turbine engineroom
mechanic, then switched to aviation electronics. Later got First Class Radiotelephone with Radar and
made Extra Class Amateur Radio. Saw the world and retired. Then the FCC told me I don't need NO
license anyway but here's a GRC good for life and never bother us again. But getting WAY off subject!
I had to work for those licenses and the high school diploma was "free". I valued my FCC tickets more
than that silly diploma. There was no money for college and I wasn't smart enough at the time. Those
FCC tickets were my "degree". And I learned most of what I know on my own from books. Now, I have
taught EE engineers things and THEY have taught me a LOT. Engineers and techs learn differently. Now,
about a receiver...
I said "wow" when I read 200 MHz in your post. But 20 MHz is still quite adaquate for what you are
doing. I once had a great big Tektronix 'scope that was designed for color television work. Don't
remember the model number but it was full of tubes! It was made for working on the old NTSC
analog color TV system of late demise. I had that puppy displaying all 8 cycles of the 3.54xxx
reference color burst looking for fleas at 270 degrees of the 5th cycle. There weren't
any but it would have displayed them. Darn thing would stop and lock a single cycle of RF from
my 2M rig.
I have two 'scopes. A Tektronix 454 dual trace 100 Mhz classical CRT instrument and a Tektronix
THS720 LCD display 'scope. The 454 was old when I got it and now its older. Years ago when they
were a great big deal I had a dead 5.25 inch floppy drive that I traded for it. I'm serious, I really did.
I found the LCD 'scope at a yard sale. Just sitting there. I was lovingly touching and fondling it. The
owner asked me if I know what it was (well it DOES look like a TV set). I assured him that I very well
DID know. I'd been wanting a digital LCD o'scope for a long time. I knew that I couldn't afford it but I
asked the price. $160.00 said the owner. O-M-G as I whipped out my weekly allowance. Owner knew
very well what he had, no longer used or wanted it and just wanted a bit of cash. Had the original receipt
for something like $2500 bucks. Saw the same 'scope at a hamfest asking price $1600.
Anyway, back to your receiver. Without that audio bypass cap you would actually have better fidelity
but much lower gain in a common emitter/common cathode circuit. Now, you spoke of signals sounding
too "bassy". The IF filter is too tight (too narrow bandwidth). The high frequency component of voice or
music are outside the IF filter passband. Well, this is primarily a CW/SSB receiver anyway. But are
you aware that the frequency of xtals is NOT exactly what is printed on them? Depends on the
capacitance of the crystal socket and other components connected to the crystals. You can experiment
with variable series/parallel small value trimmer caps and small series inductances to "pull" the
crystals slightly +/- the IF frequency. To shift the IF passband. That may improve the bassy quality. I doubt that the quality has much to do with a cathode bypass capacitor.
Keep homebrewing because that is an excellent way to learn a lot! And let us know what you're up to.
Post more questions and we'll enjoy trying to answer them.