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Old 8th May 2021, 9:14 pm   #1
unitelex
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Default Philips 624A/544A/644A heaters biased to +30V ?

I was wondering if anyone could shed light on why Philips have biased the 6.3V heater winding to +30V.

This radiogram has B8A valves with conventional independent 6.3v transformer winding providing the heater supplies. One side of the 6.3V winding is connected to the tapping of a potential divider across the HT; 2M7 to HT+ and 390K to ground with a decoupler of 10n across the 390K to ground. (R37, R36. C62)

I assume it is some kind of noise cancellation. I tried disconnecting it and there was a slight increase in mains buzz (not hum) at low volume. But why this DC level? is it to reduce stress on the heater-cathode insulation?

Chris
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Old 8th May 2021, 10:41 pm   #2
PJL
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Default Re: Philips 624A/544A/644A heaters biased to +30V ?

The rectifier is indirectly heated and does not have it's own LT winding so your 2nd guess of heater-cathode insulation is most likely correct.
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Old 9th May 2021, 8:13 am   #3
Robert Gribnau
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Default Re: Philips 624A/544A/644A heaters biased to +30V ?

I do not think that the biasing of the heater voltage has something to do with taking stress of the heater-cathode insulation of the rectifier (or any of the other valves). The Philips BX624A ( https://www.radiodatabase.nl/toestel...24A/BX624A.pdf ) also shows this biasing of the heater voltage but has a directly heated AZ41 as the rectifier which is (ofcourse) being fed from a seperate heater winding.

So like you I assume it is there for noise (heater hum) cancellation.
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Old 9th May 2021, 8:29 am   #4
valves4amps
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Default Re: Philips 624A/544A/644A heaters biased to +30V ?

It's a technique used in some guitar amps to reduce hum. Basically biasing the heaters with a positive supply makes them look like an anode and therefore any hum signal from the heater cannot pass to the cathode. This was noted by the op when disconnecting the bias as the hum increased.

Dave
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Old 9th May 2021, 9:10 pm   #5
PJL
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Default Re: Philips 624A/544A/644A heaters biased to +30V ?

Happy to be corrected! Given the circuit it seemed to make sense to be heater/cathode insulation.
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Old 9th May 2021, 10:42 pm   #6
unitelex
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Default Re: Philips 624A/544A/644A heaters biased to +30V ?

Thanks for all the feedback, much appreciated.

For the record, the benefit of this feature on this particular set I have is not very significant. I might try adding similar heater bias to other parallel heater sets to see if they benefit
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Old 9th May 2021, 10:54 pm   #7
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Default Re: Philips 624A/544A/644A heaters biased to +30V ?

This is often done to relieve heater cathode voltage when there are cathode followers in the circuit. If the cathode follower is running high voltage on the cathodes the insulation between heater and cathode can break down. Lifting the heater supply sets the heater somewhere nearer the cathode voltage to avoid this problem.

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Old 10th May 2021, 2:54 am   #8
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Default Re: Philips 624A/544A/644A heaters biased to +30V ?

If the heaters are floating there will be hum.
If the rectifier has its own heater winding the remainder of the heaters are normally earthed via a centre tap or a pair of resistors.
I would guess that the voltage has been selected for least hum with the rectifier heater sharing the winding.
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