[HBR] Question about homebrew receiver

[hbr@mailman.qth.net]

Don Kelly describes a very sophisticated receiver design, then says:

> My only real problem is that when I tune the front end I get an
> occasional "squiggling" type of noise. Usually when I tune off the
> resonant freq and it is not bothering operation. I realize this is
> probably some HF or VHF parasitic. I placed resistors in parallel
> with the front end coils and it reduced it. I think this problem
> occurred in part because I used toroids in the front end and 
> I am a victim of Q.

Kees asked for 'experts'; if having felt your pain (Don) is a 
qualification, I guess I'm one ... there was some of this stuff in the 
HBR2K design and in most rigs before that, too.

This is a VHF parasitic.

First, the reason you don't see much about this kind of problem in 
the old tube gear handbooks (like the Radio Amateur's in the 60's) is 
that when hams were doing a lot of building tube gear, the tubes they 
used mostly weren't high enough gain at VHF to have the problem if 
the design was at all reasonable.   Look at the 'command' recievers -- 
grid leads all a couple inches long, some longer than that, but who 
ever saw a VHF parasitic in one of them?   

With higher VHF-gain tubes (and transistors), however, you have to 
be a lot more careful.  And you have to know what to be careful of.   

*The problem isn't the toroids.*   Take the shunt resistors off.   
They're hurting the Q at the intended working frequency and 
(evidently) not hurting it enough at VHF.   You are indeed a victim of 
Q, but not the Q of the toroids.

The 'squiggle' you hear -- I assume you mean a sort of warbling, 
unstable beat note -- is the beat between a high harmonic of some 
oscillator in the set (like the first LO) and the unwanted VHF 
oscillation in some stage.   First thing to do is pin down which stage 
is oscillating.   Usually you can do that by touching various points 
with the tip of a screwdriver 6" long or so; all the points which make 
the warble disappear (even when you retune the front end) are part of 
the oscillating stage and circuit.   Since you notice the problem when 
you tune the front end, the chances are that it's the 6BA6 that's 
oscillating but it could be the 6BE6.    Poke around (with due care for 
high voltage) -- you'll figure it out pretty quickly.

Now you need to figure out what is the oscillator circuit.   Usually the 
wiring in the grid circuit is operating as parallel lines and the plate 
circuit looks the same and is tuned close enough to the same 
frequency to produce a TPTG oscillator at something like 100 Mcs.  

Usually the only parts that are really in the VHF circuit are the wiring 
and shunt capacitances.   The toroids probably are part of the shunt 
capacity and otherwise just chokes.   The 'cure' will be to put a small 
resistor *in series* with one or both VHF tuned circuits at a high 
current point, i.e., close to the end of the wire that has the most 
capacitance.   

What I'd try first is 100 ohms, 1/4 watt, carbon film (anything but 
wirewound will work) at the point where the grid of each stage 
connects to the tuning cap or shunt padder cap, whichever comes 
first.   Do the 6BA6 grid first as that's probably where the trouble is.

If you can shorten the grid and/or plate leads of this stage, do it.   
That moves the resonant frequency higher, where the stage gain is 
less and you might fix the problem that way.   However, there is no 
significant loss from the 100 ohm resistor as there is no grid current 
flowing in it at the HF signal frequency -- Class 'A' amp, right?   Even 
if you put one in the plate circuit it doesn't hurt 'cause these tubes 
have plate resistances like 200k -- current is very low there, too.

BTW it's not true that these parasitics only hurt performance if you 
can hear them.   They usually occur over a substantial range of the 
tuning and they increase stage noise levels and decrease gain.   You 
can't have a top performing receiver without killing this thing.

I'm impressed that you got good performance with a toroid for the LO 
coil ... I go air core only for that job.   You do need AGC on the RF 
stage; the big problem on the lower bands isn't getting max 
sensitivity, it is coping with all the garbage coming in from your 
antenna, and the 6BE6 is far from a great strong-signal mixer -- it 
needs all the help it can get.   Sounds like a great piece of work -- 
you really ought to join this list.   Quiet right now, but we go through 
periods of serious activity and you'd fit right in!

Walt Hutchens
KJ4KV