Re: Cathode Bias PushPull

Eeyore wrote:
> 
> Phil Allison wrote:
> 
> > "Eeysore"
> > >
> > > I've seen it in a VOX AC30 for example. FOUR EL84s sharing a single
Rk. I
> > > modified it to use 2 resistors.
> >
> > ** What I do is double the value of Rk to 100ohms,  add  3 x 5W 4.7
volt
> > zeners in series  ALL  in parallel with Rk  to limit the cathode bias
> > voltage to about 14 volts.
> >
> > Cools the poor,  over-heated EL84s off and increases power out by
moving the
> > stage further into class AB.
> >
> > Works a treat.
> 
> Yup, good idea.
> 
> Graham

In a guitar amp like the Vox AC30, Phil's idea does 
work where distortion does not matter and power does.

However, I have had to service quite a few amps with 4 x EL84 
with a variety of cathode biasing methods, all wrong,
because none used individual tube biasing.

In all cases the disgruntled muso with his unhappy amp has turned up
here
and an examination reveals the amp has one or more EL84 going into
thermal runaway before the others,
and the usual huge amount of overdriving has unmatched the quad of
output 
tubes so badly that some run with 15 watts at idle and some with 5
watts.
All over the ****ing place.
Installing individual Rk&Ck regulates the bias of each tube, and the
problem is much
reduced. 

All this about Vox AC30 has been discussed at length before here within
the last 4 years.

Patrick Turner.