[Drake] Re: [drakelist] reduced bias on T-4XB

[Garey Barrell]

Jason Buchanan wrote:

>
>> Probably NOT a good idea.  The final stage, regardless of "mode", is
>> operated as a  _linear_  amplifier in Class AB1 (no grid current). 
>> Increasing the bias voltage to cut the tube off between elements 
>> essentially takes the stage more towards Class C (normally biased 
>> WELL past cutoff) and  _non-linear_  operation.   The result is 
>> "distortion" of the signal, which translates as spurious output 
>> signals.  I've never tried this, but would expect clicks, "extra" 
>> output signals and just general crud.   The degree would vary 
>> depending upon just where the bias actually ended up, just barely 
>> "on", just barely "off", etc.
>
>
> I did some experimentation and found that the original 70mA spec was 
> right on for the majority of the worst spurs and somewhere around 
> 100mA it tapers off a bit more.  Over 120mA it seemed to have 
> essentially no benefit...  so I left it at around 100mA.  As it just 
> so happens, 100mA was where it was pretty darned clean, however I did 
> notice that for 100W output i'm around 360-380mA.  Anything 
> approaching 400mA on those tubes scares me - wouldn't 400mA be pretty 
> much the red zone for those tubes?
>
I forgot to mention, the errata sheet also recommended that the plate
(cathode) current not be run over 340 mA.  Again for distortion, not
for
tube protection.   Resonant plate currents over 350 mA are quite often
the result of the meter shunt resistor going high in value.  It is
marginal at 1/2 watt, I always replace them with1W units.  Changing
this
resistor also requires changing the "select at test" resistor up on the
meter switch.  I select that resistor for maximum accuracy at 100 mA,
since that current is far more critical than the maximum plate current
at full output.  The shunt resistor is also usually the cause of "I'm
only getting 35% efficiency out of my T-4XB" statements.

Bottom line is the difference between 100W output and 150W output isn't
discernable at the other end anyway, and is only of importance if you
have a drive hungry linear amplifier.

>
> A while ago you mentioned that there was a procedure to align the 
> radios when a new cable was used either for the injection or antenna 
> cable (can't remember which).  I mostly use the RCVR as the master and 
> never touch the VFO on the T-4XB...  but the downside is max power 
> output is obtained when I rotate the R-4B preselector CW a bit more 
> than it should for best reception. I'm wondering if I need to perform 
> this alignment procedure that i've forgotten about.
>
The key to alignment is to align the INJ trimmers in both units with
them cabled together.  Obviously you need low capacity cable for the
INJ
cable.  Some insist that it MUST be RG-62 coax, but that is internet
myth.  I prefer Belden 8421 microphone cable, which is very similar to
what Drake used originally.  It is MUCH more flexible and more rugged
than RG-62, and a lot less susceptible to shorting from overheating
and/or tight bends.

73, Garey - K4OAH
Chicago