Re: A Strawman, Constructed and Destroyed-Williamson's Folly?
- From: Patrick Turner <info@xxxxxxxxxxxxxxxxxx>
- Date: Thu, 21 Jul 2005 05:21:00 GMT
robert casey wrote:
> >
> > The phase inverter is another matter. Because good design demands a
> > "push-pull" power stage, the output tubes must be fed by phase
> > inversion of the driver. Good design mandates that the driver has
> > certain characteristics. The drive should be balanced amplitude wise
> > and phase wise. The careful phase inversion is the most difficult to
> > achieve. The Williamson phase inverter was a split load phase
> > inverter. The plate and cathode resisters of the second section of the
> > 6SN7 were matched at 47 Kohm resisters. This balances the amplitude of
> > the inverted signal (as long as the load resisters don't drift with
> > time), but there is a hidden serious flaw, not dealt with by producers
> > of the Williamson amplifier.
> >
> > The plate impedance and the cathode impedance are not of the same value
> > even though the load resisters are the same. This means that at high
> > frequency, the output of the phase inverter is no longer balanced. A
> > scope sampling the signal between the two driver signals shows the
> > discrepancy. This unbalance causes distortion.
>
> Would an easy fix for this be to insert a resistor in the
> path between the cathode of the phase splitter and the grid
> of the next tube? Once you calculate the impedance of
> the cathode, you would subtract that from the value of the
> impedance of the plate circuit, and use that number to size the
> new resistor. The Rp of the 6SN7 is 7.7K, and with the plate
> resistor the plate circuit impedance looks like 6.6K.
> The cathode follower impedance =1000000/gm roughly, and
> the 6SN7 gets us about 380 ohms. So if I haven't screwed
> it up just yet, the new resistor should be 6.2K to
> make the impedance look like 6.6K to the grid of one of the
> push pull tubes. SO the high frequency rolloff as seen
> by both output tubes' grids should look the same, and
> thus avoid the above distortions. And thus the global
> feedback loop won't have to work so hard.
There is no need to change the Williamson circuit to get better
balance at HF by means of any series R between a or k of CPI and following
stage grids.
The miller C seen by the a or k is virtually the same to some HF, and while
the load stays the same value,
including the capacitance component, the balance of the CPI also stays
substatially the same.
However, stray C seen by the a or k is different, and in practice the anode
response
tends to sag before the cathode response, and as I have been saying for
years, to
make the sag in the balanced response for both a&k signals above 20 kHz, add
a little C
across the Rk of the CPI. I mentioned 15 pF in my last post; it may be too
large a value,
and the actual value has to be determined experimentally.
Phase shift also comes into play here to cause the difference in CPI outputs.
But using a 6H30 with much lower RLa and RLk values will extend the bw of the
CPI outputs to around 400 kHz easily.
There is no compulsion to use 1/2 a 6SN7; a paralleled 6SN7 will
also be a big improvement.
I have always found that any added series resistance anywhere in the tube amp
input/driver stages
always reduces the HF margins of stability because it increases HF phase lag.
Patrick Turner.
.
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- Re: A Strawman, Constructed and Destroyed-Williamson's Folly?
- From: robert casey
- Re: A Strawman, Constructed and Destroyed-Williamson's Folly?
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