[HBR] My approach to the HBR... Suggestions?

[Peter Bertini]

On Sat, Sep 13, 2008 at 3:39 PM, Walt Hutchens <waltah at earthlink.net> wrote:
> I
> My favorite AGC circuit uses a plate detector with almost any of the
> common triodes with the cathode returned to a negative bias of 50
> volts or more and the plate through a resistor of 3 meg or so to a
> negative bias that is panel adjustable from 0 to whatever is required
> to cut off your AGC'd stages. This panel control serves as an RF/IF
> gain control for most purposes, although you may also want a
> conventional variable cathode resistor in the RF stage for extremely
> strong signal/crowded band conditions -- 75M on winter nights.
>

Hi Walt..

I redrew the schematic so I could get a better handle of what Henry was
doing.  It looks too simple :)  Is the cathode voltage critical,or will any
voltage over -50 or 60 volts work?  Also, is the original design, with VR102,
10K trimmer, for AGC Adj. needed in the HBR?  I don't see what purpose
it serves, and it doesn't have much authority over the cathode voltage??
>
> You do need a tapped final IF transformer but the tap on the command
> set output units works fine or you can use a capacitive tap.

I assume at this point you're talking about the IFT tap for the triode product
detector? I also further assume on the Command IFT I would be grounding
the tap, and taking the signal from the lowest Z side of the winding?  That
circuit also seems to use the CT to reverse the IF signal phase to the
PD, so the neutralizing cap is able to cancel the BFO leakage?


> The signal detector must be comparably high gain.  For SSB the usual
> triode, gated or switched beam tube detectors work fine.  What I've
> done, however, is use another plate detector, adding a BFO signal for
> SSB.

Again, we're talking about the triode PD used in the Henry Tempo One?
Are you happy with the PD?  The circuit looks deceivingly simple!
And sure would make the project easier, I was going to use a 6JH8
for the PD.
>

>
> One added complication is the need to neutralize the grid-plate
> capacitance of the detector tube in order to prevent the BFO from
> affecting the AGC. This isn't difficult: You build the circuit without
> the cap, see what you get for an S-meter reading when the BFO is
> switched on, and fiddle with cap values until the reading goes away.
> The T-1 uses a 'gimmick,' I've used up to about 5 mmf mica.

I just want to make sure that the Tempo grounds the IFT tap to
reverse the IF signal phasing to the PD, and that the PD IF tap is
at a very low impedance point on that transformer...
>
> With a 6EH7 RF stage, any of the better detector circuits (Pullen is
> low noise -- not 'better' for an HF receiver having an RF stage) and
> two stages of 6EH7 IF this will give you about the best that's
> possible in an HF multiband receiver.

I may try the 6EH7, nothing is cast in stone, I see there are several
semi remote pentodes that were made for TV IFs that look promising.
>
> If you use an RF stage, I don't think it's a good idea to use any gain
> at the 1st IF. The 2nd mixer overload problems are considerably
> increased by doing so.

That would be ONLY to overcome additional losses if I cascade IFs at 1600kc,
and that IF stage would be throttled back to the minimum needed gain. It's
easier to remove a stage than to add one... :)

Three 6BJ6 IF stages at 85 kcs will give AMPLE
> gain. I personally would go for two 6EH7 stages because of the amazing
> overload tolerance (with high gain) of this tube, but using what you
> have is a ham tradition and the three 6BJ6's will work fine.

And Peter has two sleeves of 6BJ6s, and nary a 6EH7...
>
> With the AGC circuit described above you can even use sharp cutoff
> tubes in the IF stages because the signal levels at the output are
> never greater than the fraction of a volt range.
>
>
> Even matching the 85 kcs IFTs can make you fairly crazy if you don't
> use something close to the original IF circuit because the tuning
> range of is next to nothing.
>
I have two HFO rocks, one 1515kc, and the other is 1710.080kc. One or
the other will hopefully work in this area.
>

Pete