Here is something from Patrick Turner taken from rec.audio.tubes. This exchange covers anode-anode impedance, class Ab1, Ab2, and near Class B along with a bunch of other stuff that some of you might find interesting. Therefore, I thought I'd past the exchange here. I have learned a lot from going to that group. Beware though, those guys can get pretty vile at times when someone goes off on a limb...
here is the exchange:
On Oct 26, 3:43 pm, Bret L <
ixtarbru...@yahoo.com> wrote:
> > I'm not a fan of the McIntosh circuit because it flogs output tubes
> > hard with a low value RL to get the power, and to get away with the
> > high distortion local NFB is used in the output stage with ß = 0.5.
> > So the driver stage needs to make a max of about 150V at each grid,
> > and then you have the distortion of the driver stage to worry about.
> > So GNFB is used to clean up the whole mess and the final result isn't
> > any better than a regular UL amp with less total applied FB except
> > that the McI gets more power and a slightly better damping factor. So
> > to me the McI is 3 steps forward, and two backward.
>
> The Unity Coupled output stage is simply a buffer.
Well, you could say that about any output stage. The output stage is
what puts amps of current behind a voltage signal.
I recall people marvelled how you could get 50W from apair of 6L6 from
an early McI.
And without a very high Ea voltage, which was the same as the Eg2
supply because screens were cross coupled to force the 6L6 to work as
pure beam tetrodes but with 50% of the signal fed back at the cathode.
So you could have very high open loop THD in the output stage which
was linearized with bucket fulls of local NFB so the output stage then
gave THD about equal to triodes without GNFB but you got 4 times the
triode power.
But where you run a pair of 6L6 in pure class A in beam tet mode, the
THD can be 2% at 25W, with very low THD at low levels even without any
FB at all.
So for 50W I would use a quad of 6L6. Since we want the amp for hi-fi,
the 50W would class AB at the lowest speaker Z envisaged, or at 3
ohms, so that at 8 ohms the PO would be 35W and definateIy all class
A.
If we add a CFB winding of say 12.5%, then its easy to get that 35W at
0.6%, and this is better than the McI, and we don't have to use such a
high drive voltage when the drive amp begins to make more THD/IMD than
the output stage.
Quad-II is an example, and its such a simple design compared to the
McI. Of course Peter Walker saw no reason to make Quad-II amps with 2
x KT88 for 40W, or 4 x KT88 for 80W. There was rarely any need for
such power in 1960.
And after 1960 most makers dived into Sordid State, boots and all.
> The work is in the
> driver stage BUT said driver stage works under ideal conditions in
> some ways,i.e., a constant load.
Indeed it does. And with bootstrapped anode loads which give some
positive FB which luckily increases THD less than the reduction of THD
because the driver tubes see a higher anode load.
I'd prefer not to bootstrap and use a CT choke plus anode resistances
to get DC to the driver stage.
THD is a typical 0.3% at 75Vrms at each anode. And 75V is all I need
for an amp with 20% CFB in its output stage.
The extra benefits by going to 50% CFB don't exist IMHO.
> Some refinement of the Mc circuit circuit is
> possible and Milojub Nestorovic is probably the guy who did the most.
> I have seen schematics for Capt. Catchfire's stuff but never
> Nestorovic's.
>
> Separating the driver supply from the output stage supply is a big
> win in this circuit as is a totally decoupled heater supply for the
> whole works as the supply wavers with output level because they are on
> the same core.
I doubt having separate rail supplies makes any difference at all if
they are well filtered, ie, they use huge values of C compared to the
old PSU designs which were controlled by accountants.
Many old amps gave you the sound of an accountant removing the
quality.
> More work remains to be done but IMO the potential of
> this circuit is still great without the awkwardness of workarounds
> like the Circlotron or various Norman Crowhurst designs. Norm was a
> very excellent writer but his circuits were always not so practical.
> His circuit, the Bereskin and the Circlotron were all workarounds IMO
> of the Mc patent. Bereskin was under contract to Baldwin Piano who
> initially tried to license McIntosh tech for their organs, as had
> Allen Organ, and both were turned down. AO built later, what for hi fi
> use is a superb amp with 6550s that ran them at 600 v B+ and 300 vdc
> on the screens, Baldwin never did build a Bereskin design
> commercially, but there is reputed to be a prototype bass amp built in
> the days when they sold Burns guitars here that ran a pair of small
> transmitting tubes and put out over 250 watts. That's notable because
> the SVT (which is essentially a hi fi amp in its output section and
> which you should study) "set whole new standards' with just under 300
> watts from six 6146s or 6550s. That was two or three years later. Did
> Baldwin take their Big Amp to NAMM and get gazumped?
I hear what you mean about the basic operation of McI and Circlotron
etc. All have 50% CFB in the OP stage and allow near class B op and
still get good measurements. Its all a result of high NFB application.
I had an Ampeg here for a re-design and and instaltion of an Oz made
PT with primary of 240V. The Ampeg surely had its faults. Indeed you
can get 300W from 6 x 6550, ie, at a rate of 100W per pair of 6550.
But it means its nearly all class B power with very low class A
content. So for 300W, I like to use a dozen 6550.
Although having Ea at 600V and Eg2 at 300V does allow a high PO with a
low load if you drive it class AB2, its still a bit nervous nelly time
for the tubes. I have never seen reason to have Ea higher than 500V
and this allows UL or triode operation where Ea = Eg2; the Pd of G2 is
kept within limits because of the Vac applied to G2.
If I wanted to use Ea = 600V, I'd always have the Acoustical
connection with CFB. The load vale for class AB1 is a bit high though
which means Vaa is high so the OPT needs many turns and / or a bigger
than normal core to avoid saturation at too high an F.
There is a point where the use of a high Ea means that Ia dc at idle
has to be kept real low, and the first watts are then created by tubes
which are in their cut off zone and you get early 3H production. With
Ea = 600V, and with idle Pda = 15W, Ia will be only 25mA, and its fine
in a guitar amp where THD/IMD is welcomed. The load line will rise
above the 42W Pda limit to get the high PO, and hence we see many
early tubes fail where high OP stage power way into clipping is
routinely used. For hi-fi I'd feel happier with 500V and Pda can be
25W and Ia can 50mA.
With a high RL you can get 45% efficiency in class A giving about 22W
of class A, about enough for anyone I know where they don't want bone
crushing levels, and they do want the very cleanest sound.
We mainly listen with the first 5 watts which covers all the signal
peaks in a hi-fi situtation.
But the high RL means the open loop tube gain is high, so when one
does use sensible amount of CFB, ie, 20%, this linearizes the tube
nicely, and better than having half the load, and pure tetrode with
CFB.
The fact you have a fixed Eg2 with 20% CFB makes the tube give much
better THD spectra than if you had the screens bypassed to the
cathode; I like the combined effects of CFB in the Acoustical
connection.
Where you have 20% CFB with a fixed Eg2, the stage works similarly in
terms of its open loop gain as youwould have in an OPT stage which has
20% UL taps. Just having 20% UL taps alone reduces Ra and the odd
order spectra very usefully. The 20% CFB combines screen FB and G1
feedback. You end up with an OP stage with closed loop gain of about 4
instead of a McI stage CLG of about 1.6, and needing a very high drive
voltage. ( In a McI, for 250V a-k at an OP tube you need about 150V
applied to its grid. )
Patrick Turner.
