DX-100 question

Garey Barrell k4oah at MINDSPRING.COM
Thu Aug 16 09:55:08 EDT 2007

John -

There are several reasons for changing from 5R4s to 866(A)s.  Some more 
important than others!

1.  The 5R4s lose about 50-60 Volts in EACH diode.  So that's a little 
over 20W lost in each tube, and a reduction of 50VDC in the high voltage 
applied to the final tubes in the DX-100.  The 866(A) only drops about 
15 Volts in each diode.  Less heat lost in the rectifiers / more HV to 
the final.

2.  That pretty blue glow, that pulses with keying or modulation.  
Unfortunately, it's relatively unseen inside the DX-100 cabinet, 
although you can lift the top of the B cabinet!  NO other rectifier 
offers THIS "feature".  3B28s offer the efficiency, but are essentially 
dark in operation.  SS rectifiers are more efficient, losing less than 
one volt per diode, but ...  YUK!  NO GLOW!

The downside of the 866(A) is that it ONLY operates in the vertical, 
base down orientation, and MUST be warmed up for at least a minute or 
two at each operation session.  This relatively fast warm up only 
applies if the tube orientation has NOT been disturbed since the last 
time!  IF the tube(s) have been disturbed, a MINIMUM of 30 minutes warm 
up is necessary.  This can make it (even more) difficult to work on the 
DX-100 with it turned on it's side.  Can't do it!!  I kept a pair of 
3B28s to use in my Valiant for just this purpose.  :-)

You hear all sorts of horror stories about mercury vapor rectifiers, but 
99% of them are the result of improper use, most often insufficient warm 
up before application of high voltage.  They run for decades in 
commercial transmitters and ham transmitters, as long as they are 
treated properly, i.e., warmed up sufficiently before application of HV.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 4-B & C-Line Service Supplement CDs

John N. Ritter wrote:
> Hi everybody,
> I've had a DX-100  for a number of years and hope to get it 
> reactivated soon.
> A previous owner replaced the high vacuum rectifiers with 866's.  Can 
> anyone
> tell me if this modification is commonly done and what its purpose 
> might be?
> Thank you,
> John

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