Current Values for Power transformer Phono Pre Tube amp

[MadhaviPrem]

Hello
I just got a new Little Bear T10 Pro 6n2 Valve Phono Amplifier Turntable RIAA HIFI Preamp Ver2.5.
https://www.ebay.co.uk/p/Little-Bea...-Turntable-RIAA-HIFI-Preamp-Ver2-5/2133754858

This has three 6N2 or 12AX7 ( By a a switch) and one 6Z4 Rectifier tube with plate voltage 200V-0-200V .
It has a toroidal power transformer.
I want to upgrade the power transformer. I live in Greece and here there is a very good transformer maker.
My transformer is toroidal 50VA Primary 0- 230V and Secondary 0-15V , 0-6.3V and rectifier 200V-0-200V .
My question is where I can find current requirements?
Does anybody know how much current capacity is needed at each secondary? 1A ,2A or 50mA ?

Thanks in advance

 

[piano3]

Why do you think changing the transformer will be an upgrade?

 

[MadhaviPrem]

I have made transformers before for my power amp from this manufacturer and the sound improved. Better iron core and very good copper winding work. Now is toroidal and I want to make Ei because I heard they are better for audio quality.

 

[jazbo8]

Moved to Power Supplies.

 

[jan.didden]

My question is where I can find current requirements?
Does anybody know how much current capacity is needed at each secondary? 1A ,2A or 50mA ?

Thanks in advance

If your current xformer is 50VA you should replace with the same. You can check the current for the heaters on the data sheets and thus calculate the VA this represents. The rest is for the high voltage, so that gives you the high voltage current.

Or you can just measure all currents with the xformer you have. ;-)

Jan

 

[MadhaviPrem]

Thank you , can I measure the currents when the Amplifier is running?
What is high voltage current ? its usually low like 450 mA? forgive me I am new in DIY.

 

[jan.didden]

Well it's not much use to measure current when the amp is off ;-)

Look up the heater current for the tubes, that's easy, no measurement needed.

For the high voltage, you have two choices. You can directly put your multimeter (set to DC amps) in series with the high voltage line after the last rectifier cap, when the high voltage goes to the tube stages. That is a direct measurement.

The other way is to put a resistor of say 100 ohms in series with the high voltage after the big capacitors, going to the tube stages. The high voltage current will go through that resistor. Measure the voltage across (between the two pins of the R, so not to ground!) that resistor with the multimeter set to DC volts, then you can calculate the current, because Mr. Ohm found out a few centuries ago that I = V / R.
So if you measure 4V across that 100 ohms resistor, you know that I = 4 / 100 = 40mA.

BTW Do you have a schematic of that preamp?

BTW2 you could also try just to ask the seller.

That's real diy! ;-)

Jan

 

[jan.didden]

What is high voltage current ? its usually low like 450 mA? forgive me I am new in DIY.

Look at the tubes in that preamp, check the max anode current in the data sheet. It will only be a few mA so that whole preamp is probably drawing less than 20mA.

Edit: Duncanamps says 8mA max for the 12AX7:

TDSL Tube data [ECC83]

So that's absolute max 24mA if all are running at max which they are not.

Jan

 

[piano3]

Toroidal transformers leak less magnetic flux than EI transformers. From the linked Ebay picture I would think that you might get rather more hum if you make the replacement.

 

[MadhaviPrem]

Thank you Jan, Yes I have a schematic. The seller wants to sell me another same transformer he does not give the specs.
But now you cleared it for me I can look in tube data sheet for 12AX7 and 6N2

 

[jan.didden]

If you could post the schematic we can probably just see the currents.
3 x 12AX7 heaters is about 1A at 6.3V so say less than 7W.
The high voltage probably about 2mA per tube, but there are 2 x 3 tubes sections so say 6 x 2 = 12mA at 250V = 3W.

So even without measuring we are looking at 10W, even if it is 3 times larger a 50VA xformer seems perfectly OK.

Jan

 

[MadhaviPrem]

Thank you Jan
for 6.3 V 2A or 1A
for 15V 2A or 1A?
for 200-0-200 2VA or 10VA can it be 0.05A current ?
somebody suggested above values
Here is the schematics please let me know your values..

An externally hosted image should be here but it was not working when we last tested it.

 

[PRR]

...post the schematic
...high voltage probably about 2mA per tube, but there are 2 x 3 ...

Now that we have a schematic, assuming <360V supply,

Four sections have 330k plate loads and will be more like 0.5mA each, 2mA altogether. Two sections have 62k cathode loads and may run near 2mA each, 4ma altogether. So 6mA all three bottles.

Assuming the AC of 6mA DC is 10mA, and assuming the raw B+ figures as 450V before rectifier and filter losses, we need <4VA for the HV.

Three heaters is 6 Watts, taken as DC, so we need 8VA of AC capacity to cover that.

I make it as 12VA total. 12VA is a very small lump of iron and a 50VA looks more impressive. Me, I'm cheap, and might look for 24VA.

 

[jan.didden]

Can't say I disagree. Isn't it amazing how far you can get by plain old thinking?? ;-)

Jan

 

[MadhaviPrem]

So what final values should I give to my Transformer maker? I am not as good in power supply calculations as you Power Supply Gurus!!
Are following values good?

50 VA
Secondaries
15V-->2A or 1A?
6.3V-->1A
200-0-200V-->13,7VA, 0.05A
I read on another forum following..
The 6.3V AC heater supply is just for the rectifier valve
6.3 volt, 340 mA

12AX7 may be operated from either 6.3 or 12.6 volts, at 300 mA or 150 mA
The 6z4 needs 6.3V with 600mA for the filament.

Please adivice. Thank you

 

[jan.didden]

Do you have a schematic? Are the triode heaters all in parallel, 6.3V circuit or 12.6V?

And the rectifier on a different one? You have the unit, why not just measure ??

Jan

 

[PRR]

Do you have a schematic? ...

He posted one, but as an external link, which often confounds browsers.

Raw image Attached.

Looking closer, there is a trick. The three heaters are in Series. So 18.9V 0.3A of DC. Simple rectifier and cap-input. So the AC winding is about 15VAC full load at around 0.5 Amps AC.

 

[MadhaviPrem]

Thank you very much this makes it very clear... Yes I will try to measure it today.

 

[jan.didden]

So in the left image the heaters are in parallel, in the right they are in series. How do we know which he has?

Ohh yes, we will know after the measurement ;-)
"To measure is to know".

Jan

 

[kodabmx]

The 6Z4 datasheet lists 8uF as the max for cap input filter... 100uF will most likely destroy the tube unless you put in a rather large series resistance...

https://frank.pocnet.net/sheets/095/6/6Z4.pdf