Help identifying Mystery self contained amplifier and speaker - UK Vintage Radio Repair and Restoration Discussion Forum

AD360 Rob · 18th Oct 2021, 9:47 pm

Evening all,

On the bench tonight is a little amp and speaker combo. I reckon it's a guitar practice amp and it can be mains or battery operated (you have to change a connector over from PSU board to 2 X PP9) It has 2 inputs, a 6.3mm jack socket and 3 4mm banana sockets (I'm guessing it uses some sort of balanced input as both tip and ring of the jack and 2 of the banana sockets go to 2 resistors on the PCB)
The PSU is a simple transformer, bridge and 2 caps plus 27 ohm resistors in series with the caps and output to the main pcb.

It seems to be working as touching the inputs with a screwdriver produces the usual pickup hum/buzz and this is variable by the volume control. it also has a high sensitivity input selectable with a switch, this also works.
The issue I have is that the PSU is designed to supply about +/- 10v but the 27 ohm resistors limit this to about +/-4.5V and get rather hot in the process. (one shows signs of burning ) Basic DC checks on the amp board reveal that the 2 large blue capacitors directly across the psu Input are ok, each measuring about 500uF with an ESR of under 0.2 ohms (according to my Chinese component tester) and there is almost no residual /PSU hum.

Ideally, I'd like to know who made it and maybe get hold of circuit information so I can see what voltages should be appearing at the PSU input to the main board.

It sort of looks like a home brew kit but the internal construction is to a very high standard with cables loomed neatly and the metalwork looks manufactured. The only thing I do know is that someone called Lisa appears to have had a hand in construction.

Any insights much appreciated.

Rob

Nickthedentist · 18th Oct 2021, 10:06 pm

Possibly educational use? Coomber?

There was also a GPO unit which looks similar to this.

1954Fender · 18th Oct 2021, 10:18 pm

I think this may be an earlier version of a GPO Loudspeaker Amplifier 11. The amplifier was used as a speech quality monitor in GPO/BT Repeater stations, the 4mm banana sockets used in balanced 600 ohm configuration. The higher gain setting used to monitor for noise on analogue copper circuits. I don't think it is a guitar amplifier! It was a professionally made amplifier for mains or portable battery use. I can't help with schematics or manufacturers - it's too long ago now!

peter_sol · 19th Oct 2021, 7:40 am

Yes The 11A version is the same only upside down with controls at the bottom. It would have had a transformer on the input which may have bee removed/ bypassed.

AD360 Rob · 19th Oct 2021, 10:51 am

Hi all, thanks to all of you for pointing me in the right direction on this. It is a GPO loudspeaker amplifier type 11A. There is an old post on here with someone who has servicing info so I have messaged him. Hopefully he still has it and can scan and email it.
Peter_sol, it's never had a transformer on the input although I too would have expected to see one.
My plan with this is to use it as a signal tracer, the high gain setting will be very useful in this regard. Obviously I'll have to knock up some probes and possibly modify the input to accept an unbalanced input.

Rob

18th Oct 2021, 9:47 pm	#1
AD360 Rob Hexode Join Date: Jun 2012 Location: Chester, Cheshire, UK. Posts: 421	Help identifying Mystery self contained amplifier and speaker Evening all, On the bench tonight is a little amp and speaker combo. I reckon it's a guitar practice amp and it can be mains or battery operated (you have to change a connector over from PSU board to 2 X PP9) It has 2 inputs, a 6.3mm jack socket and 3 4mm banana sockets (I'm guessing it uses some sort of balanced input as both tip and ring of the jack and 2 of the banana sockets go to 2 resistors on the PCB) The PSU is a simple transformer, bridge and 2 caps plus 27 ohm resistors in series with the caps and output to the main pcb. It seems to be working as touching the inputs with a screwdriver produces the usual pickup hum/buzz and this is variable by the volume control. it also has a high sensitivity input selectable with a switch, this also works. The issue I have is that the PSU is designed to supply about +/- 10v but the 27 ohm resistors limit this to about +/-4.5V and get rather hot in the process. (one shows signs of burning ) Basic DC checks on the amp board reveal that the 2 large blue capacitors directly across the psu Input are ok, each measuring about 500uF with an ESR of under 0.2 ohms (according to my Chinese component tester) and there is almost no residual /PSU hum. Ideally, I'd like to know who made it and maybe get hold of circuit information so I can see what voltages should be appearing at the PSU input to the main board. It sort of looks like a home brew kit but the internal construction is to a very high standard with cables loomed neatly and the metalwork looks manufactured. The only thing I do know is that someone called Lisa appears to have had a hand in construction. Any insights much appreciated. Rob Attached Thumbnails

18th Oct 2021, 10:06 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,841	Re: Help identifying Mystery self contained amplifier and speaker Possibly educational use? Coomber? There was also a GPO unit which looks similar to this.

18th Oct 2021, 10:18 pm	#3
1954Fender Triode Join Date: Aug 2009 Location: Devon, UK. Posts: 23	Re: Help identifying Mystery self contained amplifier and speaker I think this may be an earlier version of a GPO Loudspeaker Amplifier 11. The amplifier was used as a speech quality monitor in GPO/BT Repeater stations, the 4mm banana sockets used in balanced 600 ohm configuration. The higher gain setting used to monitor for noise on analogue copper circuits. I don't think it is a guitar amplifier! It was a professionally made amplifier for mains or portable battery use. I can't help with schematics or manufacturers - it's too long ago now!

19th Oct 2021, 7:40 am	#4
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Help identifying Mystery self contained amplifier and speaker Yes The 11A version is the same only upside down with controls at the bottom. It would have had a transformer on the input which may have bee removed/ bypassed.

19th Oct 2021, 10:51 am	#5
AD360 Rob Hexode Join Date: Jun 2012 Location: Chester, Cheshire, UK. Posts: 421	Re: Help identifying Mystery self contained amplifier and speaker Hi all, thanks to all of you for pointing me in the right direction on this. It is a GPO loudspeaker amplifier type 11A. There is an old post on here with someone who has servicing info so I have messaged him. Hopefully he still has it and can scan and email it. Peter_sol, it's never had a transformer on the input although I too would have expected to see one. My plan with this is to use it as a signal tracer, the high gain setting will be very useful in this regard. Obviously I'll have to knock up some probes and possibly modify the input to accept an unbalanced input. Rob