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G8MNY > TECH 22.08.09 10:11l 125 Lines 5882 Bytes #999 (0) @ WW
BID : 35165_GB7CIP
Read: GUEST
Subj: Old Valve Radios
Path: IZ3LSV<IQ3GO<SR1BSZ<F4BWT<GB7COW<GB7CIP
Sent: 090821/2236Z @:GB7CIP.#32.GBR.EU #:35165 [Caterham] $:35165_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Aug 09)
(8 Bit ASCII Graphics use code page 437 or 850)
I have collected a few over years, 4 of the bigger ones I recently displayed at
an RAF memorial flight do, where WW11 vintage kit is appreciated by oldtimers.
My oldest one is a Marconiphone 279, it was made in 1934/5 & sold for 16+pt,
It uses six 5 pin valves, it is still working fine. Last heavily used in the
1960s in the prefects room at my school, listening to all the MW pirates, so
its condition is poor!
_________________
.' /³
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ The design is quiet a complex
³ ....ÚÄÄÄ¿.... ³ ³ superhet for its day. It gave good
³ : ÚÄÙ[=]ÀÄ¿ : ³ ³ sensitivity with just its MW/LW
³ .ú ³@ @³ ú. ³ ³ frame aerial which had a manual
³ : ÀÄÄÄÄÄÄÄÙ : ³ ³ peaking capacitor. The RF &
³ : _____ : ³ ³ external aerial is fed to a
³ : '³³³_³³³' : ³ ³ variable mu (gain) stage (VMS4B)
³ : ÙÀÄÅÄÙÀ : ³ ³ then RF tuned again & on to the
³ : ³ ³ ³ : ³ ³ oscillator mixer stage (VMS4B).
³ : ³ ³ ³ : ³ ³
³ : .¿__³__Ú. : ³ ³
³ :.............: ³/
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
A low frequency IF of 125kHz (kilo cycles back then) is used & gives good
selectivity, it is double tuned both sides of the variable mu IF amp (VMS4B),
the DC in this 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 pentodes have the ANODE as top cap!
The detector is conventional double diode triode valve (MHD4), 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 & back to the triode.
The AF output stage is a single power pentode (MPT4) with side screw connection
for the screen, it produces about 5W RMS. The LS is a 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 (a complex way to get -ve tapping
rather than 2 Rs). In series with the LS voice coil is the "Hum bucker" coil on
the limb of the choke magnet, this picks up the same hum voltage as the voice
coil & cancels the hum.
Across the LS is a shorting mute switch briefly operates as the band change
changes over.
Due to the negative side HT choke, the smoothing caps have separate -ve rails.
The rectifier valve 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 & an earth!)
The 4V heater line is balanced & a "Hum Dinger" pot, slider earthed, is used to
minimise heater emitted hum in the 5 indirectly heated valves.
===============================================================================
The next largest set is the famous EKCO A22 with the fragile iconic round black
bakerlite case, mine is still 100% OK & works on LW MW & SW.
_.-ÄÄ-._
/ ___ \ First made in 1945 there were earlier
| ÜÛÛÛÛÜ | round types with live chassis & no
| Ûß ßÛ | chrome L.S. trim, but the A22 is a
| ÞÛ ÛÝ | safer mains transformer model with
| ÛÜ ÜÛ | four IO based valves ECH35, EF39, EBL31,
| ßÛÛÛÛß | AZ31 ! Sold for 14-14s (14 Gns) +pt.
\()( )()/
_> <_
/________\
It has no frame aerial & needs a good wire aerial & then it does very well.
There is a semi slow motion tuning drive, so SW tuning is good & the huge 270ø
dial lets you see the scale quite well. Despite the round case it is fairly
conventional valve design.
Looking on the Web, I see these sets now go for nearly 800 ($1400)!
===============================================================================
Ferguson 450, SW & MW, first made in 1946 sold for 14-14s (14gns) +pt.
This one has a strange history, as it is still band new & looks it with its
shiny varnished wooden case ! Removed unsold from my Grandfather's shop when it
closed down, still in original box Guarantee operating instructions & sales
information. It had an intermittent fault "no RF osc" which I guess is why it
was never sold.
_______________ Line up is ECH35 mixer osc, EF39 IF, EBC3
/ _______ /³ Detector AF amp, EL33 AF output. AZ31 rectifier.
/.../ / /.../ ³
³'''ÀÄÄÄÄÄÄÄÙ'''³ ³ Again it needs a good external wire aerial.
³ ============= ³ ³ I does use a mains energised magnet
³ ============= ³ ³ (HT choke) on the L.S. in the +HT side of
³ ============= ³ / the PSU. In series with the L.S. voice coil
³ (/) (/) ³/ is the "Hum bucker" coil on the limb of the
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ choke magnet, this picks up the same hum
voltage as the voice coil & cancels the hum.
===============================================================================
Roberts Valve Portable LW MW & SW Radio
______
____\.....\_____
/ =³_____³= /³ It has a blue covering over wooden box.
/ /_\___/ / ³ LW, MW with built in frame aerial & SW
ÚÄÄÄÄ¿0 0 0ÚÄÄÄÄ¿ ³ on added wire aerial. It was quite
³ ÀÄÄÄÄÄÙ ³ ³ popular as I have seen many examples.
³ ³ ³ It used a large 1.5V+90V battery & has
³ /~~~\ ³ ³ a battery eliminator to work on mains.
³ ³ ³ ³ ³
³ ³ ³ ³ ³ Care must be taken not to exceed 1.5V
³ \___/ ³ / on the valves heaters, as the heaters
³ ³/ will easily lose their cathode coating
'ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ' (emision) or burn out.
See my Tech buls on "Valves (tubes)" & "Early AVO Valve Tester".
Why Don't U send an interesting bul?
73 De John G8MNY @ GB7CIP
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