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Old 3rd May 2021, 3:58 pm   #1
qualityten
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Default AZ31 as rectifier for a stereo Mullard 3-3?

Valvepower has kindly given me a power transformer that uses an AZ31 rectifier with a 4V heater winding. It was removed from a Ferguson 289A radio in which it was used to supply an ECH35, 2 x EF39s, an EB34 and 2 x EL33s in push pull.

I happen to have an AZ31 which I've never used and wonder whether I can use it for a stereo Mullard 3-3 built on a single chassis? According to the Valve Museum, the AZ31 is capable of delivering 100mA with 300V on the anodes. The usual rectifier specified with a this amplifier in stereo is an EZ81.

I'd be grateful for advice as I don't know how to calculate the current draw.

David
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Old 3rd May 2021, 4:20 pm   #2
barrymagrec
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

According to the circuit the cathode resistor for the EL84 is 560R and the cathode voltage given is 28 which suggests a 50mA current draw per channel. The EF86 consumption is negligible but two channels is 100mA which is right on the limit for the valve and if the transformer was running a pair of EL33s and a few small valves it is going to be marginal for 100 mA as well
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Old 3rd May 2021, 4:27 pm   #3
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

You might be pushing it a bit since the EL84's in the 3-3 will be drawing about 40mA anode current each, possibly 45mA so the two would be 80 - 90mA. Add the screen grid current at another 5mA or so each and you've reached the maximum.

Don't think it's a good choice!
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Old 3rd May 2021, 5:03 pm   #4
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Thanks for the steer. Interesting that the Ferguson radio was running the rectifier at its limit.

The transformer has two 6.3V windings. In the Ferguson circuit, one supplies the heaters of the EL33s. The heaters of the all the other valves are supplied by a 6.3V winding which is returned via the chassis ground. Could I use the latter for the EF86s and use the other winding for the EL84s and an EZ81? Or is the transformer simply unsuitable for this application?
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Old 3rd May 2021, 5:24 pm   #5
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Hi David, well the combined heaters of the EL33's would draw about 1.8 amp whereas a pair of EL84's and an EZ81 will be around 2.5 amp.
If the EL33 heater winding also supplied a couple of tuning scale lamps at say 300mA each, it would make it a bit nearer to what you need.
But one thing to bear in mind is that it is preferable, though not essential, to have the rectifier heater at the HT level. This means a seperate 6.3v 1 amp winding.

It may be worth looking at other rectifier valves with 4v heaters, just in case something else can be used instead of an AZ31. Example a Mazda UU6 but then the heater current requirement needs to be considered... and different bases.... Hmmm...
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Old 3rd May 2021, 5:29 pm   #6
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Quote:
Originally Posted by Sideband View Post
You might be pushing it a bit since the EL84's in the 3-3 will be drawing about 40mA anode current each, possibly 45mA so the two would be 80 - 90mA. Add the screen grid current at another 5mA or so each and you've reached the maximum.

Don't think it's a good choice!
Yes, for the 3-3, Mullard specify a current demand of 50mA at 310 anode volts. At 2x50mA you would be right at the AZ31's limit. You'd be at an EZ81's limit too

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Old 3rd May 2021, 5:42 pm   #7
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

There is another possibility for the rectifier.... Dare I mention the word silicon?
This would make it very easy to use the transformer you have.
Some experiment might be needed to select suitable resistors to drop the HT, if too high with a pair of silicon rectifier diodes.
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Old 3rd May 2021, 5:43 pm   #8
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

I think the EZ81 should be comfortable. The datasheet http://www.r-type.org/pdfs/ez81-1.pdf rates it for 350V out at 150mA.

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Old 3rd May 2021, 5:55 pm   #9
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

I'd use something else, and save the AZ31!

4V valves on Octal bases are few and far between, and as others have said anyway, it would be hard-worked.
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Old 3rd May 2021, 6:13 pm   #10
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Another vote for the EZ81 if you have sufficient heater current available. There's no need for a separate rectifier heater winding as the EZxx rectifiers are designed to withstand HT between heater and cathode. I have never ever seen a failure in this respect.

AZ31s are a little bit special now, and should perhaps be kept for the bowden cable afficionados who restore the rather splendid Philips sets of old. A directly heated rectifier would not do the capacitors in your amplifier any favours in any event.

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Old 3rd May 2021, 6:16 pm   #11
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Yes I'd go for an EZ81: it ws used in loads of 50s/60s stereo radiograms to provide HT to a pair of single-ended EL84 alongside [typically] an ECC82 double-triode audio-amp, an EABC80 detector, EF89 IF amp, ECH81 frequency-changer and ECC85 FM front-end. Add in an EM84 magic-eye for an extra few milliamps and the trusty EZ81 still wouldn't worry.

I'd happily use an EZ81 to power a pair of EL84 and some trivial milliamps for the low-level audio stages.
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Old 3rd May 2021, 6:22 pm   #12
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Thanks for this prompt guidance. I did not know that valves like the AZ31 are rather scarce. I only remembered I had one when I saw that it had been used in the Ferguson circuit and thought it would look rather nice!

Reading the Ferguson service manual, I see that it has a total HT current of 94mA, so just under the 100mA limit of the AZ31. Out of interest, the four tuning scale lamps are powered by the heater supply to all the valves except the EL33s.

Noting Andrew's comment in post #5, if I use an EZ81 can I power all the heaters via the winding intended for the EL33s? If not, it'll have to be silicon. Or the transformer might be better used in another project in the pipeline that is also SS rectified and uses just four ECL82s.

Last edited by qualityten; 3rd May 2021 at 6:30 pm.
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Old 3rd May 2021, 6:35 pm   #13
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

If my calculations are correct, and assuming the four scale lamps were all 300mA types, then you have two 6.3v heater windings capable of providing 1.8 amp (EL33's) and the other 2.1 amp for all the rest.

I would say it is quite do-able. The EL33 winding for the EZ81, and the other for the EL84's and EF86's.
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Old 3rd May 2021, 9:38 pm   #14
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Thank you Andrew.
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Old 4th May 2021, 6:36 am   #15
Robert Gribnau
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

The Mullard 3-3 needs a B+ of 310 V (measured after the resistor coming from the EZ80). I do not have a schematic of the Ferguson 289A but since it is running a pair of EL33's in push-pull at probably Vak = 250 V, I doubt if the B+ in the Ferguson 298A is 310 V.

But maybe you will come close to the 310 V using silicon rectifiers instead of a valve rectifier.
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Old 4th May 2021, 6:12 pm   #16
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Unloaded, the transformer is 310-0-310. The Ferguson 289 manual gives the unsmoothed HT at 290V.

My transformer on the mono 3-3 has 300-0-300 unloaded and gives 330VDC unsmoothed. After a dropper resistor, I get 292V on the EL84 anode, which is spot on.
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Old 4th May 2021, 8:52 pm   #17
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Hi.
I use an EZ81 in my stereo 3-3, its been in it for some years now and is well used never had any issues.
The AZ31 is a filamentary valve so will conduct a long time before the EL84's warm up so the HT will rise significantly. Go for the EZ81 off a separate 6.3v winding. or a GZ34 off a separate 5v winding.

The heater insulation is good enough on both the above valves to run off the normal heater supply and the GZ34 will run via suitable dropper resistor of 1.46r or 1.5ohms happily
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Old 4th May 2021, 10:29 pm   #18
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Thanks Trevor.
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Old 5th May 2021, 7:39 am   #19
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Without prolonging this thread, one last question. What is the best way to wire up the heater winding that is 6.3-0, rather than 3.15-3.15? I’ve looked in my go-to references and can’t see an example of this. The Ferguson circuit diagram shows each heater 0 going to a symbol like this __. I.e. not to the common ground of the circuit. Just a twisted pair as with the 3.15-3.15 winding, which will get a virtual CT? As neither winding has a centre tap, I wonder if the 6.3-0 winding is more suitable for the EZ81 heater?

Last edited by qualityten; 5th May 2021 at 7:54 am. Reason: Clarification.
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Old 12th May 2021, 10:52 am   #20
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Default Re: AZ31 as rectifier for a stereo Mullard 3-3?

Neither of my 3-3s have centre tapped heaters, one side is grounded at one point only and the valves are fed via twisted cables. Hum is negligible, you need your ear to the speaker before being able to detect the hum.
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