[Patrick Turner]

On May 21, 7:03*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 05/20/11 19:21, flipper so wittily quipped:





On Fri, 20 May 2011 06:54:32 -0700, Big Bad Bob
*wrote:

On 05/20/11 03:22, Patrick Turner so wittily quipped:
Baxandall's amp isn't real bad, but the first input pentode must
produce a voltage large enough to drive one 6L6 grid in beam tetrode
mode with fixed Eg2. The other input pentode operates as an inverting
anode follower with unity gain, but any THD generated in V1 is passed
on to be reproduced in V2. In effect, driver amp distortion is double
that of a single tube, and this defect also occurs in any amp using a
paraphase driver.

I hadn't heard the term 'paraphase' until you mentioned it, so I did a
bit of online research, found out that I've seen things like that before
and I never liked it done 'that way', but yeah, using PENTODES just
makes everything worse for distortion and potential imbalance. *Relying
on the circuit gain characteristics of a tube for your design, or
requiring a balance adjustment to make it work, is just silly.

Yes, well, the original Williamson called for a balance control on the
separate triode voltage amps because their gain is what it is. The
Williamson looks balanced but unless the triodes are identical, fat
chance, it isn't. (He later decided GNFB was enough to not need the
balance pot).

The Baxandall is a floating paraphase, which uses negative feedback.

Look at it again. The second pentode is driven by the difference of
the two signal paths, the two 470k summing 'sense' resistors, and is
not operating open loop like the Williamson voltage amp triodes are.
I used a similar floating paraphase on my "Batman" battery guitar amp
but used triodes and since they were low gain I 'predisposed' the
balance to nominal, and let the feedback work around that, but with
pentodes you have lots of gain for NFB around the phase splitter.

curious, which of these designs gives you the best overall gain,
transient response, overdrive characteristics, and lowest cost (or 'part
count')? *Down side for cathodyne is that you have to make sure that the
Rp-Rk never drops below around 50V or so (depending on the tube) in the
worst case voltage swing. *Alternative is to put a 2nd set of triodes
between the splitter and the output tubes, but not sure if that adds
potential imbalance or not (you'd have to match more parts again). *At
that point I'd do the 'long tail' thing and match the Rp's, and use a
constant current source.- Hide quoted text -

- Show quoted text -

My website shows numerous schematics which speak volumes about my
preferences for best-ness.

But in the last few years I have re-engineered a few high end amps
including ARC VT100, Manley Labs Snappers, and an old pair of Dynaco
Mk-VI.

In these amps there were 4 x 9 pin sockets for each channel so to make
good use of the sockets. With the ARC VT100, I did the following....

I stripped out nearly all of what ARC used, leaving only about 5% of
basic heater circuits and PSU. A carpenter's chisel was used remove
the tracks and surplus stuff to the bin. There's a lotta stuff which
does not give more hi-fi, so its best removed.. New tracks were used
using links of wire. 101 other things were don, like soldering copper
wire to bases of 6550 to be able to wire them in so they can't fall
out of the terrible octal sockets ARC have used which have very poor
grip on tube pins.

Then,

Input = LTP with a one twin triode 6DJ8 on each side. MJE340 CCS
cathode tail to a negative rail.

Driver = 2 x 12BH7, one on each side of balanced amp with common Rk to
negative rail. Balance drive was better than 1%.

Output tubes all fitted with individually adjustable fixed bias and bi
colour green/red LED used to indicate bias condition of tubes and with
full protection against bias failures. The original ARC had a total of
5 TO92 tiny j-fets used as CCS, but they are all unsuitable fragile
and unecessary. Just why Big Boys like ARC don't include such things I
routinely do is beyond my comprehension.

I did a similar thing on Dynaco mk-V1 but with EL84 in triode used on
each side of driving balanced amp.

Patrick Turner.