Fuse or sophisticated Grid Bias arrangement?

[thevalveset]

Hi,
I am restoring an early Beethoven 101 "Baby" battery portable with a moving iron L.S. In the absence of any known trader service data, I have been laboriously tracing the schematic out by hand.
As usual I have been re-stuffing the paper condensers and carefully replacing the innards of the ceramic tube resistors with modern components.
So far the component restoration is going well and the set still looks original.

Now I have what I believe is the true schematic, I am bemused by the addition of an MES pilot lamp in the negative HT supply line.
This lamp is NOT visible from outside the set, and is screwed in a Bakelite holder on the chassis.
At first I imagined it would be directly across the LT supply although why such a lamp would then be hidden from view was a mystery.
As I continued to trace the circuit It became apparent the lamp was in the Negative HT supply, so what is it for?
As you can see the HT battery must have a 'tap' a couple of cells up from the negative end to generate negative G.B.
So, as the set draws HT current the lamp will raise the chassis up w.r.t. the G.B. supply cancelling some of the negative bias.

This arrangement seems too sophisticated for such an early wireless and my other thought was that the lamp is just a screw-in fuse, (such that if the owner shorts the HT) the pilot lamp "blows" thus protecting the expensive HT battery.

But which purpose is correct (or is there a. n. other)?
Someone must have seen this 'pilot lamp' arrangement elsewhere on another wireless set and have a correct explanation.
PLEASE tell?
Thanks in anticipation..
Dave

[Cobaltblue]

It's a Fuse bulb commonly used in battery sets to protect the valve filaments from the HT battery if there's a short circuit.

Cossor used them way into the the late 1940's
The snips are from the Cossor 338 of 1937.
The 488B of 1947 still has the same arrangement

Cheers

Mike T

[thevalveset]

Brilliant, thanks Mike,
It made more sense to be the latter of my guesses, it's strange that I had never encountered this arrangement before.
Y'see one is never too old to learn something new.
Mystery solved, you are a star!
Dave

[emeritus]

The 1936 Bulgin catalogue offers a couple of MES lampholders specifically intended for holding fuse bulbs, one for internal mounting, the other for mounting in a visible position, the holder having a removable cover enclosng the bulb and having an opening through which the bulb does not protrude but through which the end of the bulb can be seen. However, the only bulbs they listed were "illumination bulbs". The illustrations of the fuse bulb holders (sold without fuses) show bulbs in phantom that seem to have an obscured region covering the end of the bulb envelope. Presumably in practice it was just a question of using an ordinary bulb of the appropriate current (and voltage) rating?

if there's a short circuit

Or (more likely) and idiot fingers moment. Any bulb with a rating in current a bit bigger than the HT current (a few 10's of mA) and much less than the LT current (half an amp or so) will do. The readily available 60mA rear bike bulbs do nicely.

[Herald1360]

Originally Posted by thevalveset

Hi,


.......



As you can see the HT battery must have a 'tap' a couple of cells up from the negative end to generate negative G.B.
So, as the set draws HT current the lamp will raise the chassis up w.r.t. the G.B. supply cancelling some of the negative bias.

.......



Dave

But not enough to matter..... the hot resistance of the bulb is only about 23R at 150mA, at say 20mA HT current it's probably less than 5R, which will only upset the bias by 100mV or so.

[Cobaltblue]

Originally Posted by merlinmaxwell

Or (more likely) and idiot fingers moment. Any bulb with a rating in current a bit bigger than the HT current (a few 10's of mA) and much less than the LT current (half an amp or so) will do. The readily available 60mA rear bike bulbs do nicely.

They are almost always 150mA 3.5V for Cossor sets.

Cheers

Mike T

[thevalveset]

But not enough to matter..... the hot resistance of the bulb is only about 23R at 150mA, at say 20mA HT current it's probably less than 5R, which will only upset the bias by 100mV or so.

Yes thanks, I too did the calculation and even set the bulb up outside the set in series with a milliammeter and wound the current up to the combined HT current of a similar valve lineup.
As you say the voltage across the lamp was nearly 200mV leading me to almost discard the "sophisticated G.B." theory LOL
So I posted here to ask those more knowledgeable than me....
Thanks everyone
Dave

[Chris55000]

Hi!

Only minor point to add to this is that there were still circuits being published in "Practical Wireless", (after the more modern circuit symbology of BS 530 that came in 1948 with the Uno Sloping diagram lettering) well into the 1950s with these "fuse bulbs" in the h.t. – lead. It was the use of "all dry" circuits with combined h.t./l.t. batteries with a four pin plug and socket that generally eliminated them!

Chris Williams

[G6Tanuki]

Some of the early B4//B5 type valve-sockets [or the early radio-designs where individual turned-brass sockets were fitted to an Ebonite sheet which was also the radio's front panel] had a design deficiency in that the individual sockets protruded above the bakelite base - if you were foolish enough to try to swap a valve without turning the radio off it was possible for you to make inadvertent contact between the HT sockets and the filament-pins, with the risk of a distinctly unhelpful outcome.

Valve-sockets with fully insulated individual sockets were the obvious answer.

Some valves also had shorter heater pins to safeguard against inadvertent mis-connection during insertin/removal.