1960 green magic eye

[Trevor Leese]

Hi everyone, I'm restoring a 1963 valve tape recorder, the green magic lamp is working fine but my question is this:

The lamp has some red tape in the centre of the valve which must donate maximum input levels, this tape has come off, can you buy this tape or what type of replacement tape should i use as i reckon it must be heat resistant.
Thanks
Trev

[DMcMahon]

I have only seen tape on a Magic Eye valve once before, not sure if it was an original fit or something that someone had added as a level indicator.

Could you get by with not replacing the tape ?

David

[vidjoman]

There were quite a few with a strip of red tape around the magic eye to show if the recording level was too high. Any red transparent tape, sweet wrapping, or similar will do. Not critical if you remember not to let the two beams get too close to joining.

[unitelex]

Kapton tape would withstand the heat and be transparent and durable, but I have never seen it in red colour
Could you consider a thin coat of red varnish carefully applied using masking tape (removed before varnish sets) to get a clean edge

Best Regards
Chris

[vidjoman]

You can get transparent red gels for theatre lighting that will withstand the heat from the magic eye.

[Trevor Leese]

Originally Posted by DMcMahon

I have only seen tape on a Magic Eye valve once before, not sure if it was an original fit or something that someone had added as a level indicator.

Could you get by with not replacing the tape ?

David

Hi David, this tape was put on when the machine was new, i can managed without but for originality i would like to have it as it should.
Thanks, Trev

[Trevor Leese]

Originally Posted by vidjoman

There were quite a few with a strip of red tape around the magic eye to show if the recording level was too high. Any red transparent tape, sweet wrapping, or similar will do. Not critical if you remember not to let the two beams get too close to joining.

Hi, ive got red tape but what about the heat generated by the valve? Could it be a fire risk.

[Trevor Leese]

Originally Posted by unitelex

Kapton tape would withstand the heat and be transparent and durable, but I have never seen it in red colour
Could you consider a thin coat of red varnish carefully applied using masking tape (removed before varnish sets) to get a clean edge

Best Regards
Chris

Yes i could Chris and its a reasonable idea matey, thank you.
Trev

[Bill]

There is very little heat in a magic eye (EM84?) . Risk of fire with any kind of tape must be miniscule. Different matter if it was round a thrashed EL84! Yes, I've seen tape on a M.E. long time ago, no idea what sort of tape though.

[Ed_Dinning]

Hi, try a band of red nail varnish

Ed

[Chris55000]

Hi!

This any good?

https://www.ebay.co.uk/itm/273601434333

I'm not sure how heat resistant it is, but indicator valves only take very low cathode–currents with a typical anode dissipation of typically 0.25–0.5W, and don't run more than luke–warm in my experience!

Chris Williams

[paulsherwin]

Yes, this valve only gets warm rather than hot.

[ricard]

If I recall correctly, the EM84 valve when connected as suggested by the manufacturer has no overlap between the green segments at maximum input level, whereas the slightly later EM87 does. However, by juggling the resistor values - I think by placing a resistor in the connection between the target and B+, it's possible to get overlap also on the EM84, and some manufacturers such as Grundig did just that, whereas others, notably Philips, tended to opt for a red band.

The problem I feel with the red band is that it tends to cut down the light quite a bit, so rather than glowing bright red to indicate overload, one just sees the green segments disappear into the red area, so I the visual impact of just having two overlapping green segments is better.

[Trevor Leese]

Originally Posted by Bill

There is very little heat in a magic eye (EM84?) . Risk of fire with any kind of tape must be miniscule. Different matter if it was round a thrashed EL84! Yes, I've seen tape on a M.E. long time ago, no idea what sort of tape though.

Thanks for the info Bill.
Much appreciated.
Trev

[Trevor Leese]

Originally Posted by Ed_Dinning

Hi, try a band of red nail varnish

Ed

Hi Ed, thanks for your thoughts, my wife said the same.
Trev

[Trevor Leese]

Originally Posted by Chris55000

Hi!

This any good?

https://www.ebay.co.uk/itm/273601434333

I'm not sure how heat resistant it is, but indicator valves only take very low cathode–currents with a typical anode dissipation of typically 0.25–0.5W, and don't run more than luke–warm in my experience!

Chris Williams

Thanks Chris, I'll check it out as soon as possible.
Trev

[Trevor Leese]

Originally Posted by ricard

If I recall correctly, the EM84 valve when connected as suggested by the manufacturer has no overlap between the green segments at maximum input level, whereas the slightly later EM87 does. However, by juggling the resistor values - I think by placing a resistor in the connection between the target and B+, it's possible to get overlap also on the EM84, and some manufacturers such as Grundig did just that, whereas others, notably Philips, tended to opt for a red band.

The problem I feel with the red band is that it tends to cut down the light quite a bit, so rather than glowing bright red to indicate overload, one just sees the green segments disappear into the red area, so I the visual impact of just having two overlapping green segments is better.

Yes you're right, the red band does tend to cut down the light and having the two overlapping green segments might be a better option.
Thanks for your input.
Trev

[Paulus.d]

I used a Piece of red leader from one of my tapes.... formed it in a loop round the valve and stuck it with a bit of glue!!
It's still quite see through enough to see the beams //
Regards Paul

[DMcMahon]

Originally Posted by ricard

If I recall correctly, the EM84 valve when connected as suggested by the manufacturer has no overlap between the green segments at maximum input level, whereas the slightly later EM87 does. However, by juggling the resistor values - I think by placing a resistor in the connection between the target and B+, it's possible to get overlap also on the EM84, and some manufacturers such as Grundig did just that, whereas others, notably Philips, tended to opt for a red band.

The problem I feel with the red band is that it tends to cut down the light quite a bit, so rather than glowing bright red to indicate overload, one just sees the green segments disappear into the red area, so I the visual impact of just having two overlapping green segments is better.

Have never seen a red band on a EM84, would be interesting to see a photo, am so used to seeing the overlap on the Grundigs.

Checking a typical Grundig that uses the EM84 they use the same anode load resistor of 470k on pin 9 as the Elizabethan but whereas on the Elizabethan where the Target (pin 6) is connected to the top end of the anode load resistor/HT supply, the Grundig fit a 100k resistor between pin 6 and the top end of the anode resistor.

Note on the Elizabethan the anode via the resistor is connected to a lower HT of around 160 volts (i.e. not the full/main HT) whereas on a typical Grundig the anode via its resistor is taken to the main HT of around 260 volts.

Sometime out of interest I will link out the 100k resistor to see the operational effect.

David

[Chris55000]

Hi!

The Robuk RK3/RK4 range of machines fit a 100k resistor between the target–anode pin 6 and the h t. supply lead – the Robuks tended to use a higher value of h.t. than was customary in most such machines, the Robuks I came across had a smoothed h.t. of 275–280V.

(None of the Robuk Service Data I came across gave any component–values, valve types, operating–voltages, etc., on it)

Chris Williams