EL156 SE Ra of transformer

[peedros]

Hi all,

I would like to ask you regarding the single ended amplifier with EL156. I already have output transformer with Ra 3k5 Ohm instead of recommended Ra 3k8 in the EL156 datasheet from Telefunken / Ua=450V, Ug2=280V, Rk=90 Ohm, first page/. source: https://frank.pocnet.net/sheets/128/e/EL156.pdf
I try to plot the both Ra to the characteristic, but I am little bit confused from the plate current on the characteristic in area where the Ug1~0V. Is it okey, when the anode current exceed the datasheet limit for the cathode current Ik=180mA? Because the cathode current in the area where Ug1~0V will be about 210mA Ia + circa 12mA Ig2. Can be the cathode current limit exceed? Can I use the output transformer with Ra 3k5? If yes, should i increase the value of Rk?

Thanks guys!

PS: Sorry, I am not strong in English 🙂

 

[Ketje]

Can be the cathode current limit exceed? Can I use the output transformer with Ra 3k5? If yes, should i increase the value of Rk?

The 180mA is for DC, no problem here.
And the cathode resistor 120Ω (>3W).
Mona

 

[pieter t]

Hi all,
I already have output transformer with Ra 3k5 Ohm instead of recommended Ra 3k8 in the EL156 datasheet from Telefunken

No, you don't have an output transformer with Ra 3k5.
When the 3k5 is related to a secondary of 8 ohm, the winding ratio of the transformer is about 21 : 1.
The tube "sees" the reflected impedance of the load.
When this load is an 8 ohm loudspeaker with a perfectly straight impedance curve, yes, the EL156 will "see" 3k5.
When your loudspeaker happens to be 8.7 ohm, the tube will see 3k8.
So, it depends on the impedance of your loudspeaker which primary load is seen by the tube.
Not many loudspeakers have flat impedance curves.
Take that into account, and it becomes "relative".