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Old 11th Dec 2019, 9:13 pm   #1
ITAM805
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Default Running EZ81 heaters at 5V?

Hi all, is it doable without losing too much output volts?
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Old 11th Dec 2019, 9:50 pm   #2
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Default Re: Running EZ81 heaters at 5V?

Try it and see.

In times-past I replaced an AZ31 rectifier [ 4V 1.1A heater ] by a 6X5, which supposedly needed 6.3V on the filament but I guess that 1950s indirectly-heated cathodes were a bit more-emissive (despite the lower heater-temperature) than 1930s directly-heated ones.
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Old 11th Dec 2019, 9:51 pm   #3
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Default Re: Running EZ81 heaters at 5V?

Suck it and see?

If you're running the EZ81 at lower HT current (50%?) than its rating it'll probably work OK.
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Old 11th Dec 2019, 9:56 pm   #4
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Default Re: Running EZ81 heaters at 5V?

Thanks guys - there's the rub, I'm wondering whether to buy the tx for a repair of a little 18W amp, it has a 6.3V and a 5V winding, the amp uses an EZ81 hence the question

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Old 12th Dec 2019, 12:54 am   #5
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Default Re: Running EZ81 heaters at 5V?

Is there any room to wind a few turns of suitable wire around the outside of the bobbin, with a bit of trial-and-error for turns-per-volt and phasing, and connect it in series with the 5V winding? 1.3V won't need many turns. As said, it may well work with just 5V but things like forward voltage drop and current capability may not be up to full spec, also it might take a bit longer to warm up.
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Old 12th Dec 2019, 1:05 am   #6
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Default Re: Running EZ81 heaters at 5V?

Another thing!! a 6CA4 only draws one amp, "Most" 5 volt rectifiers I know of are at least 2 amps. As the transformer you quote is fairly small, the compensation of a slight overwind will probably still fetch something very close to 6 volts anyway.
As has been said already, Suck it and see !!.

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Old 12th Dec 2019, 1:32 am   #7
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Default Re: Running EZ81 heaters at 5V?

I had a similar problem on an amp I was building, also using a EZ81. Turretslug's suggestion is the same as the one I used. There was just enough room to wind a couple of turns around the transformer bobbin and I ended up with with about 6.5 VAC just for the rectifier.
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Old 12th Dec 2019, 10:55 am   #8
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Default Re: Running EZ81 heaters at 5V?

Someone please tell me if this is nonsense:
A solid-state bridge rectifier on the output of the 5v winding, plus an appropriate reservoir capacitor could give more than the 6.3v you need. A suitable series resistor then reduces it to 6.3v
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Old 12th Dec 2019, 11:48 am   #9
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Default Re: Running EZ81 heaters at 5V?

Quote:
Originally Posted by llama View Post
Someone please tell me if this is nonsense:
A solid-state bridge rectifier on the output of the 5v winding, plus an appropriate reservoir capacitor could give more than the 6.3v you need. A suitable series resistor then reduces it to 6.3v
Graham

The voltage drop of the bridge would scupper your scheme - I once tried to get a very smooth 4 volts at 1 amp from a 6.3 volt transformer with a bridge rectifier and a very large cap - it only just made 4 volts and no resistor was involved.
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Old 12th Dec 2019, 1:08 pm   #10
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Default Re: Running EZ81 heaters at 5V?

Just power the rectifier from the 6.3V winding. The EZxx rectifiers don't need an isolated heater winding. You could possibly use an EZ80 to reduce the loading on the 6.3V winding if needed; the pin out is the the same.

Underrunning high current valve heaters is not a good idea if you want a long life from these devices.

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Old 12th Dec 2019, 4:36 pm   #11
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Default Re: Running EZ81 heaters at 5V?

Many thanks all. Alas the tx I had my eye on has gone, but interesting replies nonetheless

Quote:
Just power the rectifier from the 6.3V winding...

Is that right Leon? I've found another tx that fits the chassis cut out with a single 3A heater tap. Thing is, this baby amp with the EZ81 coupled to the 6.3V rails (HT not connected) draws a gnats over 3A (3.17A), is that asking for trouble?

Last edited by ITAM805; 12th Dec 2019 at 4:44 pm.
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Old 12th Dec 2019, 5:42 pm   #12
Leon Crampin
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Default Re: Running EZ81 heaters at 5V?

A slight overload of the 6.3V winding is no problem if the voltage at this load is maintained. Offloading the 5V winding will reduce the overall transformer load anyway.

Using an EZ80 in place of the 81 would bring the 6.3V winding load to within its rating and is OK if the HT current is < 90mA and the reservoir capacitor and HT and primary winding resistances are within spec. See the Philips data for the EZ80/81.

Using these rectifiers from a common (earthed) 6.3V winding presents no problem whatever - they're designed for this.

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Old 12th Dec 2019, 6:03 pm   #13
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Default Re: Running EZ81 heaters at 5V?

i would just put the EZ81 on the 6.3 volt winding as earlier stated
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Old 12th Dec 2019, 6:43 pm   #14
Leon Crampin
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Default Re: Running EZ81 heaters at 5V?

A few measurements and checks with the valve maker's data never goes amiss if you want to do the job properly.

Leon.
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Old 12th Dec 2019, 9:10 pm   #15
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Default Re: Running EZ81 heaters at 5V?

The current draw of a valve can be off by a few percent, and if the transformer isn't fully loaded on the other windings there should be enough margin to get away with a 3.17A nominal current (below 6% overload for that winding and even less for the primary if it were fully loaded).
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Old 12th Dec 2019, 10:25 pm   #16
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Default Re: Running EZ81 heaters at 5V?

I would, if possible, add a few extra turns to the heater winding as has already been suggested.
The problem with under running the rectifier heater is that it might work one day but not work the next day or not work at a different address due to mains voltage fluctuations.

It might work with 250 volt mains, but fail to work with 220 volt mains. Both figures are well within the permitted tolerance for a nominal 230 volt mains supply.

Upper limit is 253 volts, 110% of 230 volts. Possibly as high as 255 volts if allowing for a 1% rise due to grid tied PV.
Lower limit is 217 volts, 94% of 230 volts, or possibly as low as 205 volts if allowing for about 5% voltage drop within the consumers installation.
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Old 13th Dec 2019, 12:57 am   #17
Leon Crampin
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Default Re: Running EZ81 heaters at 5V?

6.3V is the nominal figure for these valves. Most makers allow a +/-10% margin, but you would need to check with the Philips data for the EZxx devices.

Even in these times of relaxed mains voltage tolerances and difficulties in matching source energy to load on a national basis, if you apply nominal heater voltage at somewhere close to the nominal mains input, you should be well covered.

Valves in car radios functioned adequately during on and off charge operation, sometimes with somewhat erratic generator regulation.

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Old 13th Dec 2019, 2:41 am   #18
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Default Re: Running EZ81 heaters at 5V?

Since the winding is rated at 3 A, rather than 3.000 A, I'd happily run it at 3.17 A.
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Old 13th Dec 2019, 2:47 am   #19
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Default Re: Running EZ81 heaters at 5V?

I've fitted a transformer with a 4 volt rectifier heater winding powering a 6 volt rectifier heater without any problems.

Someone brought me an old radiogram to look at and it turned out that the mains transformer had burnt out. I told the chap that if he could find a replacement transformer then I'd fit it for him. He found a similar one on ebay which was just about the right size, but had what I measured at the time as a 4.5 volt rectifier heater winding rather than the required 6.3 volts for a U150 valve (I think it was). The HT winding on the replacement transformer was higher than the original and I at first thought that the reduced heater voltage on the rectifier would compensate for this. However, it worked much better than I expected and I had to fit a 1K dropper in the HT line from the rectifier to the first smoother to bring it back down to the required level. This was around eight years ago and I happen to know that the chap still has this radiogram and it's still working fine.
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Old 13th Dec 2019, 12:05 pm   #20
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Default Re: Running EZ81 heaters at 5V?

I doubt if there will be a problem either way. Putting on the 6.3V winding might very slightly load that winding more than intended, but against that (as Leon says) the 5V will be unloaded so overall heat generation in the transformer will be fine.

Or, connecting to the 5V winding, the EZ81 will draw less than the average 5V rectifier so the voltage here may be a bit higher than 5V... if the EZ81 is not asked to supply a lot of current, it should be happy. (You could always add a few extra turns, if there's room).
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