1928 Osram Music Magnet No3 - UK Vintage Radio Repair and Restoration Discussion Forum

retroaudio · 22nd Mar 2019, 5:06 pm

Hi I was looking for advice on restoring this set and I wanted to ensure I had understood the theoretical diagram correctly.

1) There are only 2 HT inputs one at 120 volts and 1 at 80 volts. I was going to try and achieve these voltages by use the using a load of PP3s in series.

2) Grid bias is about 4 volts I was going to achieve that by using a flat 4.5 volt battery of the type they used to use in cycle lamps. Cant remember the actual number of that battery.

3) Not sure what the low tension is . Am I right in assuming that's about 1.5 volts. What's the best way of achieving that without the supply being dissipated very quickly. when I tried to do this some time ago with post war battery sets I struggled to maintain the low tension voltage.

thanks Alex

AC/HL · 22nd Mar 2019, 6:41 pm

Grid bias was normally adjustable in 1.5 volt steps to allow for HT battery ageing, but you must turn the set off to adjust it. LT is usually 2 volts (Lead acid or Cyclon) but 4 or 6 volt versions were made.

Silicon · 22nd Mar 2019, 9:09 pm

If you forget to connect the grid bias battery before connecting the HT, the valves and transformers may be damaged.

It may be safer to wire a resistor network and zener diode on the HT negative side to make sure the grid bias is always present when the HT is connected.

Mike. Watterson · 22nd Mar 2019, 9:27 pm

Indeed the six cells in a GB pack are exactly the same cells in a flat 3 cell pack, used to be 1289. I've bought Panasonic ones and made replica packs, soldering tubes made of cut coffee tin as the wander plug sockets. The +9V goes to 0V. Start with GB wander lead at -9V and work towards 0 ether for distortion to end or measure anode current. Often the valves were graded with a colour or letter to indicate "ideal" GB tap until 1930s when valve characteristics were more uniform per model.

I've also used alkaline AA cells. The GB pack should normally last practically the shelf life.

Ed_Dinning · 22nd Mar 2019, 10:27 pm

Hi Alex, I have 2 of these sets and they work well, main problems can be poor connections and the audio coupling transformer can be open circuit (easily rewound}.
Some of these sets could also use a 1.5 v supply to the det valve as well as the 4.5v to the AF valve.
A high impedance speaker is needed for the output, but a small output transformer with a 3R speaker will also work.
A long wire aerial, say 10m is needed. An earth is not always necessary, but do not use the mains earth as it may increase noise levels.
I may have the construction sheet for this set

Feel free to contact me if you have any problems

Cheers, Ed

retroaudio · 22nd Mar 2019, 11:18 pm

Hi at the moment its a pig in a poke until I can power it up. Its all there and very clean but that doesn't mean all the components are working
! I will need to buy some wire tomorrow for the terminals as I haven't done anything this for some time. I will get a Panasonic battery tomorrow for the grid bias and connect it first in the hope that in the past someone has not tried to power it up without one, I have a set of high impedance earphones a horn speaker and a high impedance paper cone speaker so if there is a kick out of it I should be able to hear something. I will keep you all posted.

Alex

Mike. Watterson · 23rd Mar 2019, 12:35 am

I've worked on one of these. You'd want to check coil continuity. A pair of NiMH with a series rectifier is also close enough. But neither Cyclone, Lead Acid nor NiMH with dropper should ever be connected while on charge, as the terminal voltage rises a lot.
A cheap clock radio power transformer will work as an anode load to 8 Ohm speaker.

22nd Mar 2019, 5:06 pm	#1
retroaudio Pentode Join Date: Mar 2005 Location: Cairneyhill, Fife, UK. Posts: 135	1928 Osram Music Magnet No3 Hi I was looking for advice on restoring this set and I wanted to ensure I had understood the theoretical diagram correctly. 1) There are only 2 HT inputs one at 120 volts and 1 at 80 volts. I was going to try and achieve these voltages by use the using a load of PP3s in series. 2) Grid bias is about 4 volts I was going to achieve that by using a flat 4.5 volt battery of the type they used to use in cycle lamps. Cant remember the actual number of that battery. 3) Not sure what the low tension is . Am I right in assuming that's about 1.5 volts. What's the best way of achieving that without the supply being dissipated very quickly. when I tried to do this some time ago with post war battery sets I struggled to maintain the low tension voltage. thanks Alex

22nd Mar 2019, 6:41 pm	#2
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: 1928 Osram Music Magnet No3 Grid bias was normally adjustable in 1.5 volt steps to allow for HT battery ageing, but you must turn the set off to adjust it. LT is usually 2 volts (Lead acid or Cyclon) but 4 or 6 volt versions were made.

22nd Mar 2019, 9:09 pm	#3
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Re: 1928 Osram Music Magnet No3 If you forget to connect the grid bias battery before connecting the HT, the valves and transformers may be damaged. It may be safer to wire a resistor network and zener diode on the HT negative side to make sure the grid bias is always present when the HT is connected.

22nd Mar 2019, 9:27 pm	#4
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: 1928 Osram Music Magnet No3 Indeed the six cells in a GB pack are exactly the same cells in a flat 3 cell pack, used to be 1289. I've bought Panasonic ones and made replica packs, soldering tubes made of cut coffee tin as the wander plug sockets. The +9V goes to 0V. Start with GB wander lead at -9V and work towards 0 ether for distortion to end or measure anode current. Often the valves were graded with a colour or letter to indicate "ideal" GB tap until 1930s when valve characteristics were more uniform per model. I've also used alkaline AA cells. The GB pack should normally last practically the shelf life.

22nd Mar 2019, 10:27 pm	#5
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: 1928 Osram Music Magnet No3 Hi Alex, I have 2 of these sets and they work well, main problems can be poor connections and the audio coupling transformer can be open circuit (easily rewound}. Some of these sets could also use a 1.5 v supply to the det valve as well as the 4.5v to the AF valve. A high impedance speaker is needed for the output, but a small output transformer with a 3R speaker will also work. A long wire aerial, say 10m is needed. An earth is not always necessary, but do not use the mains earth as it may increase noise levels. I may have the construction sheet for this set Feel free to contact me if you have any problems Cheers, Ed

22nd Mar 2019, 11:18 pm	#6
retroaudio Pentode Join Date: Mar 2005 Location: Cairneyhill, Fife, UK. Posts: 135	Re: 1928 Osram Music Magnet No3 Hi at the moment its a pig in a poke until I can power it up. Its all there and very clean but that doesn't mean all the components are working ! I will need to buy some wire tomorrow for the terminals as I haven't done anything this for some time. I will get a Panasonic battery tomorrow for the grid bias and connect it first in the hope that in the past someone has not tried to power it up without one, I have a set of high impedance earphones a horn speaker and a high impedance paper cone speaker so if there is a kick out of it I should be able to hear something. I will keep you all posted. Alex

23rd Mar 2019, 12:35 am	#7
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: 1928 Osram Music Magnet No3 I've worked on one of these. You'd want to check coil continuity. A pair of NiMH with a series rectifier is also close enough. But neither Cyclone, Lead Acid nor NiMH with dropper should ever be connected while on charge, as the terminal voltage rises a lot. A cheap clock radio power transformer will work as an anode load to 8 Ohm speaker.