[AMRadio] 6146A's Arrived (was DX-100B on Phone - bad audio distortion / oscillation)

[Rick Poole]

At 09:44 AM 12/7/2011, Gerald Stockinger wrote:

 >>>>>
>Rick, I have been following your tale of woe for a bit and this morning I
>looked up a copy of the scheme of your rig. I could not find one for the
>DX100B only the DX100. The first thing I see is the DX100 did not have
>nuetralizing of the 6146's. That may have been added for the B version.
<<<<<

Good morning, Jerry.

As best I can determine, neutralization was never a part of the 
DX-100 or -B design.  There was an aftermarket modification that 
involved some #12 wire run from the driver variable cap, through a 
hole in the shield to the PA compartment, and then it ends hanging in 
the breeze, and you bend it closer to or farther away from the plate 
choke or tubes to vary the capacitance.  Mine has (had) that modification.

There are those who say that modification was only needed to make it 
play nice with 6146B's or -W's and that with straight 6146's or 
6146A's it is not needed.  There are others who say it's not needed 
at all.  I tried adjusting it through its entire range with the -B's 
and again with the -A's and then ended up taking it out altogether, no help.

 >>>>>
>The
>thing I would try in any event is to replace the 1300pf screen bypass caps
>for the finals. Use a good quality low inductance cap like a centralab door
>knob no.850 type if you can get them.
<<<<<

I will try that.  The previous owner (or maybe the owner previous to 
him) had already done some of the recapping.  All of the 
electrolytics have been replaced with modern equivalents, sometimes 
in an interesting manner (the filter caps in the power supply were 
replaced with radial lead electrolytics that were glued to the 
underside of the chassis).  Some of the signal-level caps look 
relatively new but I haven't gotten to things like searching for date 
codes yet.

The original equipment screen bypass caps were disc ceramics.  Those 
aren't good enough?

I guess the first thing I need to do is clean and retighten all the 
grounds, then take it from there.

Rick WA1RKT