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Old 6th Oct 2015, 8:55 pm   #1
ian_rodger
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Default Valve PSU transformer rating

Hi All,

I’m presently putting together a phono pre-amp based on the EAR 834P design. I do realise that this could be done using a few opamps btw

The PCB is available on Ebay for (in my opinion) reasonable cost and comes as a single board comprising power supply and signal section. The board is perforated to allow these sections to be split, which I have done and I am now trying to juggle the bits to fit into a case that a bit on the small side. See illustrations below (photos show the board as it arrives, components are placed but not soldered).

The circuit diagram provided by the seller, however (poor copy below) does not resemble what is on the board so I’ve traced it from the PCB. The purpose for doing so is to try and identify a transformer that I can fit into the space left.

If you are still reading, here is the point of the post; It looks to me as if the majority of power is dissipated on the PSU board drawing around 10 mA total for the raised heater supply divider and through the zener chain. The signal board I estimate will only require a few mA.

I can just about fit in a toroidal transformer rated at 30mA 250v which should be ok for steady state operation but what should I consider for the transient condition whilst all those capacitors are charging? Note that there are another 6 x 47 uF caps on the signal board. Is such a small transformer likely to survive?

Thanks,
Ian
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Old 6th Oct 2015, 9:43 pm   #2
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Default Re: Valve PSU transformar rating

I don't think you need to worry about the current taken to charge the capacitors. It's only for an instant at startup, and in any case the HT line has series resistors in it which will limit the current. The biggest threat to an overloaded transformer is overheating, and that simply won't happen in the few tens of milliseconds while the capacitors are charging.

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Old 6th Oct 2015, 10:12 pm   #3
ian_rodger
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Default Re: Valve PSU transformar rating

Hi Chris,

Thanks for the fast reply. I'm probably over analysing this, but I suspect that a fairly large overload may persist for several seconds. I've tried simulating an approximation to the circuit in PSU designer II (the free app from Duncan amps). See graphs below. Red line is theoretical current from Tx. Anyway next stage is build it and see!

Ian
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Old 6th Oct 2015, 10:50 pm   #4
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Default Re: Valve PSU transformar rating

Quote:
Originally Posted by ian_rodger View Post
I can just about fit in a toroidal transformer rated at 30mA 250v....
Hi Ian, is this the rating of the primary or secondary winding?
The circuit diagram you have attached shows a voltage doubler formed by D1, D2, C11 and C12.
Applying 250Vac will result in an HT voltage approaching 700Vdc
I estimate you require a transformer with a secondary voltage of 90 - 100Vac.

Cheers, Rob.
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Old 6th Oct 2015, 10:58 pm   #5
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Default Re: Valve PSU transformar rating

Quote:
Originally Posted by ian_rodger View Post
I've tried simulating an approximation to the circuit in PSU designer II....
Hi Ian, which circuit are you using?
This one, or the diagram in your opening post?
This circuit looks completely overkill for pre-amp that draws 10 mA or so.

Rob.
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Old 7th Oct 2015, 9:10 am   #6
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Default Re: Valve PSU transformer rating

Hi Ian, the transformer uses copper wiring which requires an energy input to heat it up; it also has a positive temp co that will cause its resistance to rise and limit the current.

All these things are second or 3rd order effects. The 10mA or so of continuous current is the thing that determines the heating.

If it is a stereo amp with 3 twin valves and is running on an HT of, say, 200v.
The HT sec of the transformer will therefore be about 100v. As it is a voltage doubler the sec current will be 2*10mA, 20mA; or 2 watts.
The largest power consumption is from the valve heaters, 3*12.6*0.15 (approx.), or 5.7W

Forget simulating a circuit as simple as this, use analysis by inspection, as it is all too easy to get the wrong figures in a simulation.

If you are really worried (and there is no need to be) you can always add an NTC resistor in series with the HT winding to limit initial start up.

Note that if you have a 200V output transformer you do not need the voltage doubler circuit.

Ed
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Old 7th Oct 2015, 9:21 am   #7
ian_rodger
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Default Re: Valve PSU transformer rating

I should probably have been clearer on which circuit I am using. Also just realised the sketch is rather hard to spot in my opening post - its the first of four diagrams.

The original has the voltage doubler - that is not the one I am using. The hand drawn sketch is my trace from the actual PCB supplied. Its a series of RC stages followed by a zener chain & emitter follower to give a stabilised 286 v supply. It does seem overkill for such a current limited supply.

Ed - from what you say I should focus on the heating effect and not worry to much about transient conditions? My concern was how thin the secondary winding rated at 30 mA would be that it may fuse rather quickly.

Last edited by ian_rodger; 7th Oct 2015 at 9:27 am.
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Old 7th Oct 2015, 10:25 am   #8
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Default Re: Valve PSU transformer rating

In my view... whenever you are building something that is "unknown" or has unknown parameters, lash it up.... test it.... de bug it... build it... simples.
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Old 7th Oct 2015, 10:25 am   #9
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Default Re: Valve PSU transformer rating

The thing you should be concerned about is whether the 30mA quoted for the transformer secondary is the AC RMS current rating or the 'likely DC output from a capacitor input PSU' rating. In the old days the latter was likely, for a transformer intended for a valve PSU. Nowadays the former is more likely - so you need to find out what RMS current is produced in the winding by the charging pulses used by the PSU.

PSUD2 can tell you, although I am never quite sure it gets this right. Otherwise, assume a factor of 2 or 3 (the exact value depends on lots of things, like winding resistance, reservoir capacitor value, and DC current draw) for something which draws constant average current like a preamp or a Class A power amp.
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Old 7th Oct 2015, 10:26 am   #10
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Default Re: Valve PSU transformer rating

Quote:
Originally Posted by ian_rodger View Post
My concern was how thin the secondary winding rated at 30 mA would be that it may fuse rather quickly.
As my grandfather would say: "look at how thick a fusewire is. Now look at the wire in the transformer. Think it'll hold up?" Hint: you can't wind a transformer with even 1A fusewire, it's too thin!
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Old 7th Oct 2015, 10:36 am   #11
ian_rodger
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Default Re: Valve PSU transformer rating

That makes a lot of sense. I don't have the transformer yet but will order it now.

Thanks all.
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Old 7th Oct 2015, 9:28 pm   #12
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Default Re: Valve PSU transformer rating

Hi Ian, transformer wire is often run at 3A per mm sq, so transformer wire for this current will be 0.01mm sq, or about 0.113 mm dia.

Do not forget in your simulation that surges are reduced by the reflected primary impedance of the transformer as well as the pretty low mains impedance.

Most transformers will operate at 50C (just about keep your hand on it) after some time; some will operate at much higher temps as they have better insulation.
As I said earlier, the biggest current/ power consumption is the heaters, not the HT

Ed
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Old 8th Oct 2015, 9:32 am   #13
ian_rodger
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Default Re: Valve PSU transformer rating

Ed,

Thanks, very useful. I had actually forgotten to include the reflected primary impedance, but given that copper wire of that diameter has a fusing current of around 3A (best data I can find) then it looks like I should be safe.

Ian
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Old 10th Oct 2015, 9:00 am   #14
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Default Re: Valve PSU transformer rating

Ian, a couple of comments on the PSU design:
1. The LT will have problems with regulation and temperature stability. You would be best to use a regulator if you really want DC.
2. The HT design includes a lot of high voltage Zener diodes which generate noise. The circuit design will be pretty tolerant of HT volts so I would drop the regulator here and just use a bridge rectifier with RC filter.
3. Why is the LT tied to a potential divider on the raw HT? You are using DC for the LT then tying it to a rail that will have some ripple on it.
4. I have not checked but I suspect a single RC filter with 2xC and 2xR is much more effective than the RCRC arrangement.
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Old 10th Oct 2015, 12:13 pm   #15
ian_rodger
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Default Re: Valve PSU transformer rating

PJL,

I have no answer to your point (3). I've double checked and that is definitely how the PCB is configured. Connecting the divider to the stabilised rail would seem sensible and it would also provide a discharge path for the last capacitor. I'll see how it works in practice.

Ian
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