Line stage question

[yagas]

My line stage consists of a grounded cathode amp cascaded into a cathode follower.
The tube is ECC99 and the operating point is 150V@18 mA each section.
B+ is 360V, load resistor and CF cathode resistor are 10k.
My measurements showed that the GC section draws 18mA but the CF section draws 16,5 mA.
Why ?
Should I reduce the CF cathode resistor at 9,7K to achieve equal current draw and cathode
to plate voltage ?

Thanks
Mic

 

[PRR]

> achieve equal current draw and cathode to plate voltage?

Why? What possible advantage could there be?

 

[audiousername]

Why? What possible advantage could there be?

Perhaps to cancel some distortion by operating both valves (of the same type) at the same operating point?

 

[slowmotion]

Hmm, I seem to remember something about this from way back.
Wasn't it something about this on the Audio Genesis website? ( now gone )
Something to do with the power supply and subjective dynamics, I believe.
I may be wrong.

cheers

 

[mcs]

Something to do with the power supply and subjective dynamics, I believe.

I remember reading something like that in an old issue of the "Tube CAD Journal" also... I think it was the cathode follower issue.

Best regards,

Mikkel C. Simonsen

 

[slowmotion]

Hi mcs

You're right it was in "Tube CAD Journal",October '99, page 3.
http://www.tubecad.com/october99/page3.html

cheers

 

[Sch3mat1c]

Pff... sounds like BS to me...

Besides, a CF doesn't produce distortion so it can't cancel any.

Tim

 

[SY]

Tim, the idea was that there is a cancellation at the supply end; at AC, as the inverting amp's current drops, the CF's current increases by a near identical amount. So it's a valid way of optimizing a circuit to use with weedy supplies and inadequate bypassing. Of course, it begs the obvious question...

 

[yagas]

As I'm not an expert, I would like to know if this non-symmetrical operation between the two sections of the same tube is something to worry about.
Normally, each triode shares the same load resistance and current draw. So, in my case I expect to see each section to draw 18mA.
But no, they don't. As I said before the GC section draws 18mA@153V and the CF draws 16.5mA@158V.
Maybe it is not such a big issue and to my ears it performs quite good.

Thanks for your replies.

 

[SemperFi]

Besides, a CF doesn't produce distortion so it can't cancel any.

Good one Tim. You are very wrong though...

 

[Sch3mat1c]

Good one Tim. You are very wrong though...

Ok. So tell me, when's the last time you could measure any distortion produced by a typical CF such as the circuit in question, at line (listening) levels such as 1Vp-p or less?

...Yes, it is technically measurable. But using an instrument capable of resolving 0.001% or less is cheating.

Tim