Military Valve Amplifier Strip - UK Vintage Radio Repair and Restoration Discussion Forum

SteveCG · 16th Mar 2021, 10:43 am

Any information on this unknown please.

I've attached two photos - the ruler scale is in Inches.

1) Unfortunately somebody has removed the identity label on the unit - otherwise it looks to be unaltered. Modification label no.1 has a cross on it.

2) The unit has six B9G valves, three Brimar 8D3 / 6AM6, three Mullard EF91. Note they are civilian valves. The input is via the Flying lead, the output via a chassis socket.

3) Trying to follow the circuit makes it a six in line amplifier chain. The valve heaters are grouped as two sets of three in series, ie the heater voltage is approx 18 Volts.

SteveCG · 21st Mar 2021, 8:33 am

Perhaps some of the Forum's experts on vintage military equipments can answer this thought of mine in my attempt to reduce the 'search space' for this amplifier?

Does the use of an 18 volt heater configuration indicate that the unit was used in an airborne environment?

Previously I had assumed that most airborne equipment was fed off A/C supplies (albeit at differing voltages and frequencies). However reading about the TR1985 Transceiver I found that it was run off the aircraft's 24 DC supply and used a 'carbon pile' set-up to reduce and regulate the heater voltage down to 18 volts. I guess this was because this Transceiver - and others like it - might be needed when the engines were not running.

So is this thinking leading me anywhere please?

AlanC · 30th Mar 2021, 9:17 am

Hi Steve,

I would have said it was the IF strip from an airborne radar- and whilst you might automatically think military, actually such things have been commonplace on civil airliners as well, in the form of the weather radar which became available in the 1950's and is now a mandatory requirement on aircraft above a certain size.

Assuming this is the case, it will likely work in the 30-50MHz region and have a flat-top bandwidth of 1-2MHz. Normally the IF strip input would be connected to the output of a passive crystal diode mixer in the RF assembly (via that flying lead?), so all of the receiver gain is provided by the IF unit which is why it needs a lot of stages.

A lot of speculation going on here but I have a strong hunch you are right!

Alan

Radio Wrangler · 30th Mar 2021, 10:14 am

Such amplifiers are often arranged for logarithmic compression, which also benefits from a large number of stages, otherwise the range of amplitudes of returns is greater than can be displayed.

David

SteveCG · 30th Mar 2021, 6:29 pm

Thanks for the thoughts. I'll try setting up a test rig and report back in due course.

SteveCG · 1st Apr 2021, 12:27 pm

A brief update:

It seems to work at about 45Mc/s - but its bandwidth is not great, possibly a couple of 100 kc/s.

I'm still working on it ...

AlanC · 4th Apr 2021, 6:20 pm

Interesting Steve, you could stagger tune it I guess if you needed a wider bandwidth. What's the sensitivity like? Do you have an application in mind for it? I like your use of 'proper' units for frequency by the way!

Alan

SteveCG · 5th Apr 2021, 11:35 am

AlanC,

Firstly, thanks for the units compliment.

Secondly, I've just put the unit away for the moment as I've other things to do on the bench.

The unit comprised a five stage band-pass coupled amp. The 6th stage is a bit of a puzzle for me as by tracing the circuit as much as possible (some components overlap in a tight space), I found that it is not complete, nay curious. The valve (EF91) almost certainly has a 10 kilohm cathode resistor for starters. Also the anode load is not present. I assume it was provided by the unit to which this IF unit was connected. However the connection between the anode pin and the output socket is via an RF choke and so the stage could not be just another IF amplifier. I guess this valve was being used in some form of detector mode ('anode' bend, 'grid' bend, or something like that??) I tried a couple of different resistor values as load but did get anything discernible on an attached 'scope.

Perhaps if this brief description gives a clue to somebody as to the unit's Government identifier it might make more sense!

Electronpusher0 · 5th Apr 2021, 2:36 pm

Quote:

Originally Posted by SteveCG

AlanC,

The valve (EF91) almost certainly has a 10 kilohm cathode resistor for starters. Also the anode load is not present.

Cathode follower?

Peter

Radio Wrangler · 5th Apr 2021, 4:08 pm

No, it sounds like the anode circuit is completed by the subsequent equipment as the anode pin goes to the output socket.

If there is lowpass filtering (capacitor) as well on the later equipment it elegantly also filters any noise pick-up, and also the constant current nature of the output from the anode of this equipment neatly avoids any voltage ingress from hum/noise loops. Current mode signal connection!

david

SteveCG · 5th Apr 2021, 6:02 pm

Electronpusher0,

Just to confirm: It is not a cathode follower. Radio_Wrangler has re-stated the correct configuration.

I also tried a 400 c/s AM modulation RF input (at approx 45 Mc/s) to the unit - but did not get any demodulated output across an external resistive load (connected to HT) , which puzzled me. The valve was drawing current.

16th Mar 2021, 10:43 am	#1
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Military Valve Amplifier Strip Any information on this unknown please. I've attached two photos - the ruler scale is in Inches. 1) Unfortunately somebody has removed the identity label on the unit - otherwise it looks to be unaltered. Modification label no.1 has a cross on it. 2) The unit has six B9G valves, three Brimar 8D3 / 6AM6, three Mullard EF91. Note they are civilian valves. The input is via the Flying lead, the output via a chassis socket. 3) Trying to follow the circuit makes it a six in line amplifier chain. The valve heaters are grouped as two sets of three in series, ie the heater voltage is approx 18 Volts. Attached Thumbnails

21st Mar 2021, 8:33 am	#2
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Military Valve Amplifier Strip Perhaps some of the Forum's experts on vintage military equipments can answer this thought of mine in my attempt to reduce the 'search space' for this amplifier? Does the use of an 18 volt heater configuration indicate that the unit was used in an airborne environment? Previously I had assumed that most airborne equipment was fed off A/C supplies (albeit at differing voltages and frequencies). However reading about the TR1985 Transceiver I found that it was run off the aircraft's 24 DC supply and used a 'carbon pile' set-up to reduce and regulate the heater voltage down to 18 volts. I guess this was because this Transceiver - and others like it - might be needed when the engines were not running. So is this thinking leading me anywhere please? Last edited by SteveCG; 21st Mar 2021 at 8:34 am. Reason: spelling

30th Mar 2021, 9:17 am	#3
AlanC Hexode Join Date: Jul 2006 Location: Sheffield, South Yorkshire, UK. Posts: 312	Re: Military Valve Amplifier Strip Hi Steve, I would have said it was the IF strip from an airborne radar- and whilst you might automatically think military, actually such things have been commonplace on civil airliners as well, in the form of the weather radar which became available in the 1950's and is now a mandatory requirement on aircraft above a certain size. Assuming this is the case, it will likely work in the 30-50MHz region and have a flat-top bandwidth of 1-2MHz. Normally the IF strip input would be connected to the output of a passive crystal diode mixer in the RF assembly (via that flying lead?), so all of the receiver gain is provided by the IF unit which is why it needs a lot of stages. A lot of speculation going on here but I have a strong hunch you are right! Alan __________________ Always expect the unexpected...

30th Mar 2021, 10:14 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Military Valve Amplifier Strip Such amplifiers are often arranged for logarithmic compression, which also benefits from a large number of stages, otherwise the range of amplitudes of returns is greater than can be displayed. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

30th Mar 2021, 6:29 pm	#5
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Military Valve Amplifier Strip Thanks for the thoughts. I'll try setting up a test rig and report back in due course.

1st Apr 2021, 12:27 pm	#6
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Military Valve Amplifier Strip A brief update: It seems to work at about 45Mc/s - but its bandwidth is not great, possibly a couple of 100 kc/s. I'm still working on it ...

4th Apr 2021, 6:20 pm	#7
AlanC Hexode Join Date: Jul 2006 Location: Sheffield, South Yorkshire, UK. Posts: 312	Re: Military Valve Amplifier Strip Interesting Steve, you could stagger tune it I guess if you needed a wider bandwidth. What's the sensitivity like? Do you have an application in mind for it? I like your use of 'proper' units for frequency by the way! Alan

5th Apr 2021, 11:35 am	#8
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Military Valve Amplifier Strip AlanC, Firstly, thanks for the units compliment. Secondly, I've just put the unit away for the moment as I've other things to do on the bench. The unit comprised a five stage band-pass coupled amp. The 6th stage is a bit of a puzzle for me as by tracing the circuit as much as possible (some components overlap in a tight space), I found that it is not complete, nay curious. The valve (EF91) almost certainly has a 10 kilohm cathode resistor for starters. Also the anode load is not present. I assume it was provided by the unit to which this IF unit was connected. However the connection between the anode pin and the output socket is via an RF choke and so the stage could not be just another IF amplifier. I guess this valve was being used in some form of detector mode ('anode' bend, 'grid' bend, or something like that??) I tried a couple of different resistor values as load but did get anything discernible on an attached 'scope. Perhaps if this brief description gives a clue to somebody as to the unit's Government identifier it might make more sense!

5th Apr 2021, 4:08 pm	#10
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Military Valve Amplifier Strip No, it sounds like the anode circuit is completed by the subsequent equipment as the anode pin goes to the output socket. If there is lowpass filtering (capacitor) as well on the later equipment it elegantly also filters any noise pick-up, and also the constant current nature of the output from the anode of this equipment neatly avoids any voltage ingress from hum/noise loops. Current mode signal connection! david __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Apr 2021, 6:02 pm	#11
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Military Valve Amplifier Strip Electronpusher0, Just to confirm: It is not a cathode follower. Radio_Wrangler has re-stated the correct configuration. I also tried a 400 c/s AM modulation RF input (at approx 45 Mc/s) to the unit - but did not get any demodulated output across an external resistive load (connected to HT) , which puzzled me. The valve was drawing current.