51 vs 47 bulb with 35z5

11/7/2009 1:52:03 AMPeter G. Balazsy

In radios of the 40s I'm used to seeing a 150ma #47 pilot bulb used on the lamp tap of a 35z5 tube.

So I was puzzled to se a 200ma #51 bulb called for on this RCA print.

Why was a 51 chosen?

The 51 has a smaller physical bulb size... however physical size is not a restriction here.

And there is also no shunt resistor across the bulb in the schematic like the tube manual suggests for DC current in excess of 60ma.

So it piqued my interest.
Any ideas why it was chosen and why I shouldn't use a 47 instead?

http://www.nostalgiaair.org/PagesByModel/039/M0015039.pdf

11/7/2009 7:13:57 AMEdd

Sir Peter. . . . .

If it's involved with being in a shunt function with that 35Z5, it should last longer than the 1847 /47.

Possibly mechanically, the 47/1847 might be placing the units hot filament housing tip end up close to a plastic dial scale that might end up getting "heat scorched" , as compared to the shorter round end profile of a 51.

That extra spacing then affording a degree of heat relief to the dial scale.

A remote, last possibility . . . you would know . . . of that rare chance of the lamp being involved in the projection of an index line or pattern /numbers to another surface. Where the light source / filament spacing . . . to that area would be critical, it causing an out of focus condition if that spacing was not met.

73's de Edd

In radios of the 40s I'm used to seeing a 150ma #47 pilot bulb used on the lamp tap of a 35z5 tube.

So I was puzzled to se a 200ma #51 bulb called for on this RCA print.

Why was a 51 chosen?

The 51 has a smaller physical bulb size... however physical size is not a restriction here.

And there is also no shunt resistor across the bulb in the schematic like the tube manual suggests for DC current in excess of 60ma.

So it piqued my interest.

11/7/2009 7:59:56 AMPeter G. Balazsy

Thanks EDD...
..all very interesting possibilities...

on this Radiola 61-1 set the dial scale is glass though.

also the dial lamp bracket is on a sliding clip that affords a great deal of play/adjustment towards or away from the entire dial area.

As far as shunt? goes the lamp is connected electrically that same as all other #47 lamps ... right across pin 2 and 3 of the 35z5.

did you have a peek at the schematic?

Thanks for all the thoughts.

11/7/2009 11:09:22 AMEdd

Sir Peter. . . . .

And there is also no shunt resistor across the bulb in the schematic like the tube manual suggests for DC current in excess of 60ma.

And assuredly your current drain imposed by that circuit design is not to be in excess of 60 ma, nor there being a potential other situation of there not being a pilot lamp used at all , but with the 'Z5 plate receiving its raw AC from the filamant tap off point at 3.

Otherwise, I still think that the lamp longevity is probably the situations answer, with its 7.5 Vac filament spec.

73's de Edd

:Thanks EDD...
:..all very interesting possibilities...
:
:on this Radiola 61-1 set the dial scale is glass though.
:
:also the dial lamp bracket is on a sliding clip that affords a great deal of play/adjustment towards or away from the entire dial area.
:
:As far as shunt? goes the lamp is connected electrically that same as all other #47 lamps ... right across pin 2 and 3 of the 35z5.
:
:did you have a peek at the schematic?
:
:Thanks for all the thoughts.
:
:
:

11/8/2009 11:08:01 PMThomas Dermody

Interesting radio. I have the 55X (or something similar). In mine a #47 is called for. Otherwise it is pretty much the same.

In your radio a 120 ohm cathode bias resistor is used on the 35L6, as opposed to the typical 150 ohm. That would increase tube current draw a bit, both because of the smaller resistor and because of the lower bias voltage on the grid. Another interesting design calls for a 30 MFD on the rectifier, and a 50 MFD after the hum bucking coil in the output transformer, which is a reversal of the typical layout. Perhaps this eases up any surges on the rectifier and pilot lamp.

11/9/2009 5:26:58 AMPeter G. Balazsy

:Interesting radio. I have the 55X (or something similar). In mine a #47 is called for. Otherwise it is pretty much the same.
:
:In your radio a 120 ohm cathode bias resistor is used on the 35L6, as opposed to the typical 150 ohm. That would increase tube current draw a bit, both because of the smaller resistor and because of the lower bias voltage on the grid. Another interesting design calls for a 30 MFD on the rectifier, and a 50 MFD after the hum bucking coil in the output transformer, which is a reversal of the typical layout. Perhaps this eases up any surges on the rectifier and pilot lamp.
:
:T.
That 120 ohm cathode resistor is reading 137 ohms actually with @ 5.5 volts.

the print show 5.6v there... so with a 120 ohm resistor that would be 46ma
...but I have 5.5v and 137 ohms = 40ma

The total B+ current measured 62.5ma right at the rectifier cathode.