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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 23rd May 2021, 1:46 pm   #81
Heatercathodeshort
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Default Re: Marconi VC76DA

If you are only recording 4v across the Mullard heater it has a partially shorted heater, a very common fault with Mullard tubes.
Fitting a 6.3v transformer will certainly give a brighter picture with 6.3v force applied to what has actually become a 4v heater due to the short. The heater will also appear to be much brighter than normal.
The heater voltage of the 3/16 and 3/18 tubes is a bit of a dark area. It can be 8.5v 10.5v or 13.5v and they are not marked but it makes no difference because the heater current is .3amp and it will only drop what it requires in a series chain.
J.
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Old 23rd May 2021, 5:47 pm   #82
EF80TVVALVE
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Default Re: Marconi VC76DA

Ah thanks for explaining John, it certainly gives a much better picture now so I'm happy enough with it. Was there any methods to try and break the short in the heaters or is this something that will just have to remain.
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Old 23rd May 2021, 7:04 pm   #83
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Default Re: Marconi VC76DA

I don't know of any reliable method of dealing with the partial heater short in the Mullard tetrode tubes.
Some guys have been lucky by simply turning the tube in the mask hoping that the short may clear due to the 'sag' factor being reversed.

The MW31-16 can be replaced with the later MW31-74. The MW31-18 can be used but this tube does not have an ion trap so may display an ion burn in the center but this might be only a minor stain. John.
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Old 23rd May 2021, 10:08 pm   #84
EF80TVVALVE
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Default Re: Marconi VC76DA

Hi John
Yes rotating may be the only thing to try, with the age of the tube I don't think it would be worth risking anything else. The tube fitted is the later MW21-74, I don't know if this was fitted from factory or a later replacement but it does look to have had very little use.
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