Message 20732 in TECH.WW
From: G8MNY
Date: Mon, 13 Apr 26 06:48:00 Z
Newsgroups: TECH.WW
Subject: Old Valve Radios
Message-ID: <57131_GB7CIP>
Path: N2MH4|GB7BED|EI2GYB|GB7CIP
R:260413/0652Z 30160@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260413/0652Z 9723@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260413/0651Z 52922@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
R:260413/0648Z @:GB7CIP.#32.GBR.EURO #:57131 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Nov 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I have collected a few over years, 4 of the bigger ones I regularly display at
an RAF memorial flight do, where WW11 vintage kit is appreciated by oldtimers.
My oldest & biggest one is a Marconiphone 279, it was made in 1934/5 and sold
Last heavily used in the 1960s in the "prefects room" at my school, listening
to all the MW boat pirates, so its wooden case condition is poor!
_________________
A low frequency IF of 125kHz (kilo cycles back then) is used and gives very
good selectivity, and with the 2 RF tunned circuits the nearby RF image is not
a problem. The IF is double tuned both sides of the variable mu IF amp (VMS4B).
The DC in this IF stage also drives the "fluid light" dial tuning indicator (no
magic eyes back then, so this is a magnetised metal strip in a coil in the dial
light path). N.B. These 3 tetrodes have the ANODE as the top cap!
The detector is conventional double diode triode valve (MHD4) 7 pin, with high
level signals from the IF anode used to generate the negative AGC line for the
RF & IF amps. The AF detector feeds the volume control together with the Gram
Pickup input and back to the triode preamp stage. This is coupled to the AF
output. A leaky capacitor here, is a major cause of valve radios not working
with the output stage taking all the HT power!
The output stage is a single power pentode (MPT4) with side screw connection
for the screen grid, it produces about 5W RMS onto the LS transformer.
The LS is a large mains energised magnet (HT choke) in the earth side of the
PSU, the winding also has a tapping for the negative bias voltage for the AF
output valve.
In series with the LS voice coil is the "Hum bucker" coil on the side limb of
the choke magnet, this picks up the same hum voltage as the voice coil and
cancels the hum.
Across the LS is a shorting mute switch operates when the band change changes
over.
Due to the negative side HT choke, the smoothing caps in the seperate PSU have
separate -ve rails. The rectifier valve (MU12/14) has 2 anodes fed from the
350-0-350V transformer. No fuse or earth is used in the set, (I recommend a 1A
mains plug one and an earth tag screwed to the PSU case!)
The 4V heater line has a hum balancing pot ("Hum Dinger"), slider earthed, this
is used to minimise heater emitted hum in the 5 indirectly heated valves.
===============================================================================
My next largest set is the famous EKCO A22 with the fragile iconic round black
bakerlite case, mine is still 100% OK and works on LW MW & SW.
/ ___ \ First made in 1945 there were earlier
\()( )()/
_> <_
/________\
It has no frame aerial and needs a good wire aerial, then it does very well.
outer SW scale quite well. Despite the round case it is fairly conventional
valve radio design.
Again a leaky signal capacitor to the AF output valve, is a major cause of
valve radios not working with the output stage taking all the HT power!
===============================================================================
This one has a strange history, as it is still band new and looks it with its
shiny varnished wooden case! Removed unsold from my Grandfather's shop when it
closed down, it is still in original box, with guarantee, operating
instructions and sales information etc. It had an intermittent fault "no RF
osc" which I guess is why it was never sold. (tuning cap)
_______________ Valve line up is ECH35 mixer osc, EF39 IF, EBC33
voltage as the voice coil and cancels the hum.
===============================================================================
Roberts Valve Portable LW MW & SW Radio
______
____\.....\_____
See my Tech buls on "Valves (tubes)" & "Early AVO Valve Tester".
Why Don't U send an interesting bul?
73 De John G8MNY @ GB7CIP
← Return to Bulletin List