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Old 17th Feb 2018, 8:42 am   #1
petervk2mlg
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Default Australian Philips 124 6X5GT cathode heater connection

Hi all,
I'm repairing an Australian Philips model 124. One thing puzzled me on the schematic. The cathode of the 6X5GT rectifier valve is connected to one side of the heater supply.
Relevant part of the schematic attached.

Peter
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Old 17th Feb 2018, 9:11 am   #2
barrymagrec
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Default Re: Australian Philips 124 6X5GT cathode heater connection

Since the 6X5 heater is supplied from a separate winding this simply eliminates the possibility of a heater cathode problem with the valve.
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Old 17th Feb 2018, 11:10 am   #3
boxdoctor
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Default Re: Australian Philips 124 6X5GT cathode heater connection

This seems to be a fairly common practice. It prevents the heaters floating at an undefined potential, otherwise the heaters may drift to an undesirable voltage with reference to the cathode due to leakakges in the transformer insulation etc., and as barrymagrec has stated, they are on a separate winding on the transformer. Tony
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Old 17th Feb 2018, 11:26 am   #4
G8HQP Dave
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Default Re: Australian Philips 124 6X5GT cathode heater connection

Almost all octal rectifiers either have an internal connection from the cathode to one side of the heater, or are directly heated. The 6X5 is unusual in allowing a separation, but I guess designers stick with familiarity - and there are good reasons for doing this, as outlined above.

Note that all indirect heater circuits, without exception, should have a DC reference voltage - which is often zero. This is because no valve electrode should be left floating.
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Old 17th Feb 2018, 11:53 am   #5
petervk2mlg
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Default Re: Australian Philips 124 6X5GT cathode heater connection

Many thanks, guys.

Peter
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Old 17th Feb 2018, 1:40 pm   #6
Sideband
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Default Re: Australian Philips 124 6X5GT cathode heater connection

It does mean that the transformer heater winding (the separate one) will have extra insulation since it will have to withstand the HT voltage as well.
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Old 17th Feb 2018, 6:27 pm   #7
G6Tanuki
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Default Re: Australian Philips 124 6X5GT cathode heater connection

Quote:
Originally Posted by G8HQP Dave View Post
Note that all indirect heater circuits, without exception, should have a DC reference voltage - which is often zero. This is because no valve electrode should be left floating.
True: if you look at the circuits of quite a few 1940s-1950s Eddystone radios you'll find the heater-chain is raised to something in the region of 50-100V by way of a potential-divider network connected between HT+ and ground.

The reason being that it stops the bits of the heaters that are exposed outside the [indirectly-heated] cathodes of the valves acting as 'secondary' cathodes, and so introducing hum-modulation to VFOs, BFOs and product-detectors.

This caught me out once: an Eddystone radio with a burbly local-oscillator which made SSB reception horrible-to-impossible was restored to normality once I fixed the inadvertent heater-to-chassis ground which shorted-out the heater-line-lift bias.

Of course the downside of not being able to DC-ground one side of the heaters is that you need to fit heater-decoupling chokes/capacitors around local-oscillators, RF-amps and suchlike to stop inadvertent cathode-to-cathode coupling.
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