BUSH MB60 |
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NOTES |
The first incarnation of this classic cabinet design.
The set uses the standard range of battery valves. However it is unusual in having an extra I.F. stage, hence the presence of two DF96's. This should improve both sensitivity and selectivity and presumably the extra gain this gave also allowed the designers to include -ve feedback in the audio stages to improve sound quality. The extra valve meant that the the valve heaters could
not be series-connected since standard batteries for the job were 7.5V
or 1.5V yet 9V would have been necessary. They therefore had to be connected
in parallel. The total filament current of 300mA meant that, unlike many
other mains/battery sets, it was not practical to run the heaters via
a dropper resitor from the rectified-and-smoothed H.T. (the generated
heat would be considerable and the H.T. smoothing cap would need to be
large). Thus the mains transfromer has an additional winding to generate
the low "1.5" volt filament supply. It is however difficult
to accurately obtain such low voltages and so a forward-biased metal rectifier
MR2 (which presumably has a forward voltage of around 1.4V) is used to
regulate the supply. Note the huge (by 1950's standards) smoothing capacitors
that the supply requires.
The mains on-off switch is part of the tone control potentiometer and standard controls of this type would include one or two switches. However we appear to need three (battery H.T. and L.T., plus one mains). This problem was resolved by adding two extra switches so as to feed the battery's H.T.+ via the mains on/off switch. This is not as risky as it first sounds since the mains/battery switching is operated by the insertion/removal of the mains input connector, the arrangement illustrated opposite. |
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SERVICE DATA | There is a copy of the Trader service sheet #1403 on CD#1. |
CURRENT STATE |
Physically sound, just needing a light clean. However
half of one of one of the mains connector pins is missing and I dont have
a suitable mains lead either. From the quality of the circuit design I have high expections for the performance of this set. |
WHERE FOUND | Bought locally via an advert in the B.V.W.S. magazine for £15. |
SEE ALSO | TR82 - Later all-transistor version VTR103- Updated TR82 including V.H.F. waveband. |
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Last updated 5th May 2003 |