[HBR] The long, SLOW HBR project

[Walt Hutchens]

I said:

> The warble turns out to be a real challenge.

Yeah ... and sometimes some or all of the tubes don't light up because the
filament circuit is wired wrong.

After poking around with the scope and finding nothing of interest I did
what I probably should have tried first: Dragged out the signal generator to
localize the problem to a stage.

Coupling a 1700 kcs signal to the plate of the mixer was fine.   Coupling a
3500 kcs signal in the same place and tuning the receiver accordingly was
NOT fine -- same old warble.   That fingered the oscillator, so I replaced
the tube.  Problem gone.

Heater to cathode leakage.  Line operated sets do put more stress on H-K
insulation but the tubes designed for series string operation are supposed
to be able to handle it.   This one, wasn't, and I've seen the problem a
couple of times before.  I don't know why the symptoms were so odd.

The bad tube was branded Alan B. Dumont; I've no idea who actually made it.

I should still move this tube farther down the string (toward the neutral
side of the line, which is where the cathode returns) to minimize the chance
of future problems.  The local oscillator is now probably the most sensitive
stage to this sort of failure.

All of the last few years of vacuum tube TV sets and the great majority of
AM/FM tube radios after 1950 were series filament/power line HV.  A 9-tube
ham band receiver is by no means pushing the state of that art.   But of
course those designers knew what they were doing and the tubes were brand
new in those years.

On this subject as on many others the circuit diagrams in the back of the
RCA Receiving Tube Manual are invaluable.   They show AM/FM series string
sets with the oscillator and mixer stages in mid-string and cathodes that
are 'hot' for RF yet -- something I wouldn't try.   Both the BW and color TV
sets have the front end tubes at the grounded end of the string but they too
have 'hot' cathodes.

Of course all of this equipment is designed for use with MUCH stronger
signals than a ham receiver, so some issues are less difficult.

The receiver sounds fine now, so on to less challenging challenges -- I
hope.  I guess I'll tidy up the local oscillator area first.

Walt
KJ4KV