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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 23rd Sep 2018, 10:03 am   #1
NickG0HIK
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Default A1134a-BOAC

I recently obtained a A1134A, that has been modified for/by BOAC.

Initially I found it very confusing, but since working on it for the last couple of days, things are much clearer.

The four pin power connector only had two wires connected, the valves fitted were 28D7 and 6SL7, the Grid bias battery had been replaced with a capacitor and a choke.

Also the original markings have been covered over with plastic labels, for the two switch positions and a metal label for the power connector.

My initial thoughts were that is had been modified by an amateur at some point, but the mods were far better than the usual kitchen table and scissors modifications that you usually come across.

The two valve heaters are in series and the types were the same as the Packard intercom Amplifier.
There was no shunt resistor across the 6SL7.

Right, so the supply to the unit was 28v and supplied the L.T. and H.T.

The 6 volt valve had the heater over run and the 28 volt valve having the heater under run. I have now selected a resistor on test of 100 ohms, (the Packard uses a 60 ohm) to give a much better balance.

Looking at the wiring of the Octal base, it was discovered that a twin triode would never work in this position and a single triode would work with the wiring scheme used.

I've selected a 6J5, I was confused that there was no provision for any bias for this valve. But looking at the datasheet, the 6J5 with an H.T. of 30 volts it will only pull around 2 mA with no G.B.
I went ahead and fitted this.

Skipping forward, the amplifier appears to work quite well on a 28/30v supply. But I wonder if I have selected the best valve. It needs to be I.O. base, single triode, work with no bias and low H.T.

I wonder if a drawing or any info on the unit exists.

Mine is S/N 0083, I am lead to believe it was used in an AVRO York, and I presume maybe also the Lancastrian, what other aircraft did BOAC use at this time?

It must be more rare than the A1134/A as only 259 Yorks were built and even less flying for BOAC.

Any further information would be fantastic.

Nick
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Old 23rd Sep 2018, 3:15 pm   #2
Keith
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Default Re: A1134a-BOAC

Hi Nick,


With the valve line-up as described, this is no longer an A1134A! It sounds to me as if it has been converted electrically to a Packard Bell K intercom amplifier, although I can't imagine BOAC performing such a modification.

The A1134 (A1134A has only minor component differences) uses 2V battery valves VR21 (triode) and VR35 (twin pentode). I use one regularly with a T1154 transmitter (see http://www.tibblestone.com/oldradios...adio/T1154.htm). In the aircraft (Lancaster, etc) it required a 2V accumulator and a 120V dry battery, plus the internal GB battery.
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Old 23rd Sep 2018, 3:32 pm   #3
M0FYA Andy
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Default Re: A1134a-BOAC

Keith, I've been following Nick's detective work on this amplifier with interest, and whilst you are correct that it isn't an A1134A, it obviously started life as one, and has been converted to, according to its ID plate, an A1134A-BOAC.

I don't think it has been converted to a Packard-Bell circuit, but is a half-way house, I would assume with the desire to eliminate the HT and LT batteries and allow it to be run off aircraft 28v alone (as per the P-B). Maybe not done by BOAC, but professionally done to their requirements?

It would be educational if Nick could draw out the whole circuit and confirm just how much has changed.

It's a fascinating item!

Andy
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