Distortion query on the K5881

[acorn]

Hi All


I read an artical on the K5881 , refering to the fact that it was designed for the lowest distortion figures posible at the cost of power output. My question is what does the artical mean when refering to quote :- loading and biased for least distortion, is that refering to the 560R cathode bias resistors and the 4.7K resistor which drops the voltage from thev main HT to the 5881 screens. I would be gratefull for any enlighten expressed.


Regards Acorn

[Richard]

Hi Acorn

It's referring to the load as the TX primary load and the bias as setting the operating point.

Have a look here, http://www.tubedepot.com/whisbipo.html

Not got a 5881 one to hand but here's a data sheet illustration of how load/power/distortion varies using a graph from the GE6550A valve. You can see the stage can be set for more power, 6k load, or less distortion 10k,



Rich

[acorn]

Thanks for that Richard

Has I understand it, the lower the primary windings ie 4.5K and 5.6K used in the more power full amps like the 5660 and KT66 are implemented to realise more power at the expense of distortion, hence some designers recommend 8K - 10K primary for the likes of EL34s to obtain a more linear response, likewise the cathode bias current sets the best working conditions for the valve, that means I am right out of headroom with my 470R cathode bias and 275 HT owing to the fact that I dont have more HT because of my mains transformers HT windings limations. I surpose 330R would be more closer to the mark but then my HT would drop to accommadate the extra current the valves would draw.

Acorn

[Richard]

Hi Acorn,

If you have approx 6K OPTX primaries I'd use lower value cathode resistors in Pentode to get around 60mA current as per this 250V plate set up for 5881s.

This one is done for fixed (grid) bias but with approx 20V cathode bias voltage it will be 270V -20V = 250V plate and 60mA will keep the stage in class A.

V/R = I (current) so if you got 20V bias with 330R cathode resistors it would be 60mA for example. Tweak cathode R value to get 60mA. (As usual, sorry if you know this!!)



Rich

[acorn]

Hi Richard


I replaced the 470R with the 330R and the first reaction was more bass and overall cleaner sound,and is sounding quite resonable, but because of the increase in current through the cathode resistors, the HT volts dropped down a bit I can see that the transformer isn,t man anough for the job so will have to look for a replacement,hence my request in the wanted colum.

I saw an advert for EL84 Push - Pull transformers both rated at 10 Watts, but one was at 8K primary and the other at 10 K primary I assume that the 10K one would give you a better distortion figures or have I got that wrong?

Acorn