[HBR] Fw: Building a HB-65.... (sort of)

[hbr@mailman.qth.net]

Hugo W. Catta said:

> ... since the IF is 1700, the front end has two tuned circuits (band
> image) calling for Miller adjustable coils (4406). How about T-68-2
> toroids? What is the issue (other than non-adjustable cores) with
> using iron powder toroids with tubes? 

The 'issue' with torroids is a non-issue -- torroids became cheap and 
common just after vacuum tubes began seriously phasing out, and 
since bipolar transistors are crummy receiver devices (very limited 
dynamic range), high-Q coils were absolutely essential.  In addition, 
torroids are smaller for a given inductance and Q and they are self-
shielding.  With all that going for them they swept the field.

Torroids work very well indeed in vacuum tube circuits -- the HBR2K 
project is a vacuum tube receiver following the conversion scheme 
(and using the chassis and key parts from) the FT-101 and torroids 
have been very helpful.

Solenoid coils are 'more vintage' and they may be easier to make.   
By using a tuning slug you can easily make the inductance 
adjustable.   An air core solenoid on the right form is more easily 
made temperature stable, so a better choice for a critical VFO like 
that in your HB-65, which, since it will tune about 5200-5800 Kcs, 
needs careful attention if you are to be able to receive SSB without 
your hand on the tuning knob.   Other than those considerations, 
solenoids are not in general as good as torroids in receiver circuits.

The only thing that might be an issue is the change of mixers.   A 
6U8 will certainly work as a mixer but has much lower gain than a 
7360 or other beam deflection tube.  Looking at the HB-65 diagram, 
you have two mixers, one IF stage of modest gain (6BA6), a no-gain 
2nd detector (solid state diodes) and a moderate gain AF amp.   
That's probably not really enough gain if using a pentode instead of a 
beam deflection 1st mixer -- you may not have enough volume or 
decent AGC action.  I'd consider using a 6BA6 RF stage between 
the two front end coils -- the circuit can be found in almost any of the 
common receiver designs.   If genuinely tight on plate milliamperes, 
go to a 6BJ6 in both RF and IF sockets.   

A second issue is that a beam deflection tube provides better 
isolation between the VFO and the signal path than does a pentode.  
You might want to consider using a 6C4 as the VFO and making the 
triode of the 6U8 1st mixer into a buffer.   Otherwise, voice signal 
peaks will tend to pull the oscillator off frequency and cause 
distortion.   

Finally, if moving IF transformer coils apart is needed and not easy to 
do, you can get the same effect by adding one or more shorted turns 
between them.   Just put a barely long enough loop of hookup wire in 
the space between the coils and solder the ends together.  If that 
doesn't lessen the coupling enough (it usually will) you can add 
another *separate* shorted turn.

The HBR2K project has been shelved the last few weeks due to 
outside work, a family wedding, and other such trivial non-homebrew 
activities;  I'll be back to it shortly.

Walt Hutchens
KJ4KV