[Boatanchors] 35W4/35Z5 filament tap --

Mike Langner mlangner at swcp.com
Sat Feb 5 12:36:50 EST 2011

Hello everyone !


>From the RCA Receiving Tube manual RC-28 -



A 35Z5 or 35W4  wants the following resistance in shunt with the filament
tap when a #47 lamp is used:


DC output current of 70 ma, 800 ohms

DC output current of 80 ma, 400 ohms

DC output current of 90 ma, 250 ohms


A #47 lamp, 6.3 volts at 150 ma would be 42 ohms.


So, RCA appears to recommend 42 ohms paralleled by the resistance(s) shown
above at the various current levels.


Which means somewhere between 39 and 47 ohms should work OK in lieu of a
lamp. It shouldn't be too "fussy!"


Using the LED system - suggestion:  hook up the LED across the pins (2 & 3).
If the LED doesn't have an integral full-wave bridge rectifier, or isn't two
LED's in parallel providing full wave conduction, put a bridge in series
with the LED.  Measure the current.  Shunt the pins with additional
resistance to bring the current to its correct value.

And now your rectifier filament should be as happy as a clam - however happy
clams are!!


Note:  the open circuit voltage across pins 2 & 3 may reach 15 volts with no
lamp or resistor load.


Good luck !!










Mike Langner

929 Alameda Road NW

Albuquerque, NM 87114-1901


(505) 898-3212 home/home office

(505) 238-8810 cell

mlangner at swcp.com


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