[Bar Nash]

Hi all

I need to order a SE EL84 transformer .
Does any one knows the ratio of turns primary/seccondary for 8ohm load ?

Thanks
EC

[Phil Allison]

"Bar Nash"

I need to order a SE EL84 transformer .
Does any one knows the ratio of turns primary/seccondary for 8ohm load ?


** Yep - it's the square root of the optimum plate load impedance divided
by 8.

Kinda depends on the DC plate supply voltage and whether you are going to
use pentode or triode mode.




....... Phil

[John Byrns]

In article ,
"Bar Nash" wrote:

Hi all

I need to order a SE EL84 transformer .
Does any one knows the ratio of turns primary/seccondary for 8ohm load ?

25:1

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[Patrick Turner]

Bar Nash wrote:

Hi all

I need to order a SE EL84 transformer .
Does any one knows the ratio of turns primary/seccondary for 8ohm load ?

Thanks
EC

The impedance ratio you want is 8,000 : 8 ohms, ie, 1,000 : 1 .

The turn ratio is the square root of the impedance ratio, in this case,
31.6 : 1.

So when you have 12 watts at the 8 ohm load, the load voltage = 8.9Vrms.
The primary voltage anode to anode = 8.9 x 31.6 = 282Vrms.

The voltage ratio of primary to secondary = turns ratio.

However, when testing to see if this is true, you may find the voltage
at the anodes is 300Vrms, and at the output load is 8.9Vrms
which is a ratio of 33.7 : 1, and this occurs because the current in the
the winding resistances cause an additional voltage to appear in series
with the voltage which would appear if the transformer had winding
resistances = zero ohms.

To measure the turn ratio accurately, use a signal of 500Hz and without
a secondary load connected. If your voltage meter is an accurate one at
different voltage levels, you can use say 12.6V filament voltage at
50/60Hz applied across the primary and you should see 0.398Vrms at the
secondary.

Patrick Turner.

[Patrick Turner]

Phil Allison wrote:

"Bar Nash"

I need to order a SE EL84 transformer .
Does any one knows the ratio of turns primary/seccondary for 8ohm load ?

** Yep - it's the square root of the optimum plate load impedance divided
by 8.

Kinda depends on the DC plate supply voltage and whether you are going to
use pentode or triode mode.

Correct, and whether he wants pure class A or almost all class AB with
little % of class A.

8k:8 is about right for a starting point.

Patrick Turner

...... Phil

[Patrick Turner]

flipper wrote:

On Sun, 7 Dec 2008 14:13:17 +0200, "Bar Nash"
wrote:

Hi all

I need to order a SE EL84 transformer .
Does any one knows the ratio of turns primary/seccondary for 8ohm load ?

Thanks
EC


Depends on how you want to operate it.

For typical values and curves get the tube datasheet from

http://www.mif.pg.gda.pl/homepages/frank/vs.html

If you want typical pentode mode then about 5k PP with 250V B+ and
screen for 5-6 Watts out, less OPT losses.

If you want triode mode then 3.5k PP with 250V B+ for about 2 Watt
out, minus OPT losses.

Turns ratio is the square root of the impedance ratio. I.E. 5K/8 is
25/1.


Correction to my replies to the OP.

I mis-read the original post and gave load values for PP applications.

For SE applications The load value for a power pentode = 0.9 x (Ea / Ia)

The tube will always operate in class A and lets assume the anode idle
disipation = 12 watts.
So Ea x Ia = 12 watts.

If you have Ea = 300V, then Ia = 12 / 300 = 40mA, so RL for maximum
pentode power = 0.9 x ( 300 / 0.04 ) = 6,750 ohms.

The transformer has an impedance ratio of 6,750 : 8, ie, 843:1, so the
turn ratio = 29:1.

But you may find that such a transfromer is unavailable and you can only
find one rated for 5,000 : 8 ohms.

Working backwards, 5,000 = 0.9 x ( Ea / Ia ).

Pda = 12 watts = Ea x Ia, then Ea = 12 / Ia.

Then 5,000 = 0.9 x [ (12/Ia)/Ia ] = 0.9 x ( 12 / Ia squared ).

Ia = sq root of ( 0.9 x 12 / 5,000 ) = 46mA.

if Pda = 12 watts, then Ea must be 12 / 0.046 = +258Vdc.

For triode use of most negatively biased pentodes, the load for maximum
useful power and symetrical clipping = ( Ea / Ia ) - ( 2 x Ra ). The
same reasoning for pentodes can be used for triodes, but with EL84, you
don't want to use an Ea which is +/- 33% from the usual 250vdc voltage
mentioned in the data pages for this tube.

Triode connection works best with ea about 33% higher than the centre
value of 250Vdc, and with Ia at 36mA. From this you can work out
RL.
For SE pentode use, Ea from 220 to about 300V is the range.

Maximum power output with symetrical clipping = ( 0.707 x Ia )squared x
RL.

So for the 6,750 ohms expect 5.4 watts of audio power at the anode.
At the secondary load you might only get 5 watts because of power lost
in the transformer winding resistances.

Patrick Turner

[Ian Iveson]

Bar Nash wrote

Hi all

Hi Bar

I need to order a SE EL84 transformer .
Does any one knows the ratio of turns primary/seccondary
for 8ohm load ?

Some variation in answers so far. It would be worth checking
with winders to see what they offer from stock. Sowter, for
example, do 5k-4,8,16 specifically for EL84 SE triode, but
the secondary is not best suited to the 8.

To add to the uncertainty, you may wonder if the speakers
are actually 8 ohms.

Ian

[Iain Churches[_2_]]

"Ian Iveson" wrote in message
...


Bar Nash wrote

Hi all

Hi Bar

I need to order a SE EL84 transformer .
Does any one knows the ratio of turns primary/seccondary for 8ohm load ?

Some variation in answers so far. It would be worth checking with winders
to see what they offer from stock. Sowter, for example, do 5k-4,8,16
specifically for EL84 SE triode, but the secondary is not best suited to
the 8.

To add to the uncertainty, you may wonder if the speakers are actually 8
ohms.


Sowter will also wind you a non-standard "special" at no extra
cost, if you tell them *exactly* what you need.

[Ian Iveson]

Iain Churches wrote

Sowter will also wind you a non-standard "special" at no
extra
cost, if you tell them *exactly* what you need.

Yes, they made a good job of mine, 5k:6 including bifilar
10% cathode winding, and 40% UL taps, for 4 EL84 although
I'm using them with with 6CH6, which I will swap when they
die, except they seem to be living forever.

The bifilar bit was Sowter's idea, and they went to some
trouble to ensure that insulation between the anode and
cathode windings is adequate. They are also huge and very
heavy, so good down to a few Hz, and don't seem to have
suffered at the top end. Giving themselves extra work for no
additional charge seems extraordinarily noble.

Price depended on core size regardless of winding details,
within reason. Extra for stuff like shielding or potting.
They provide a form to guide you in specifying in the terms
they need to know. You do have to wait for a special...it
took them several months to do mine.

If you want top quality EI for a medium price, mains or
output, off-the-shelf or bespoke, Sowter is the place to go.

I'm flirting with the idea of their nickel/iron cored OPTs
if I get round to making my headphone amp.

Do you know what kind of reputation their line level and
small signal transformers have in pro audio circles, BTW?

Ian

[Ian Bell[_2_]]

Ian Iveson wrote:

snip

If you want top quality EI for a medium price, mains or
output, off-the-shelf or bespoke, Sowter is the place to go.

I'm flirting with the idea of their nickel/iron cored OPTs
if I get round to making my headphone amp.

Do you know what kind of reputation their line level and
small signal transformers have in pro audio circles, BTW?

Ian



They supplied Neve in the 70s when I was there -mic/line input and line
output transformers. I would say their reputation in pro circles is
excellent.

Cheers

Ian

[Iain Churches[_2_]]

"Ian Iveson" wrote in message
...
Iain Churches wrote

Sowter will also wind you a non-standard "special" at no extra
cost, if you tell them *exactly* what you need.

Yes, they made a good job of mine, 5k:6 including bifilar 10% cathode
winding, and 40% UL taps, for 4 EL84 although I'm using them with with
6CH6, which I will swap when they die, except they seem to be living
forever.

Do you know what kind of reputation their line level and small signal
transformers have in pro audio circles, BTW?


Very good indeed. They were used in BBC equipment, and
by Decca Studios, which had extensive in-house engineering-
Types 3575e and 5069e were ubiquitous. The "e" style
package is a circular pot with fixing via a threaded collet
and nut.

Decca had two analogue Neve consoles.which also
used Sowter transformers. If they are good enough for
Rupert........

Iain