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Guest
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 Posted:
Wed Sep 28, 2005 4:43 am Post subject:
Unsual PP Output Topology? |
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Has anyone ever seen this topology covered in any text?
I would like to learn more about how to use it.
What are the benefits, and what's required of the transformer??
I like the idea having the transformer in a feedback loop with the 6550.
Any comments? anyone used it before?
view schematic here:
http://www.nashaudio.com/Public/6550_OUT.pdf
Thanks
RonL
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Phil Allison
Guest
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| Posted:
Wed Sep 28, 2005 4:43 am Post subject:
Re: Unsual PP Output Topology? |
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<r.laury@comcast.net>
| Quote: |
Has anyone ever seen this topology covered in any text?
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** What: Ultralinear with 100% Cathode Feedback ?
| Quote: | I would like to learn more about how to use it.
What are the benefits, and what's required of the transformer??
I like the idea having the transformer in a feedback loop with the 6550.
Any comments? anyone used it before?
view schematic here:
http://www.nashaudio.com/Public/6550_OUT.pdf
|
** See:
http://www.kbapps.com/audio/schematics/tubeamps/leslie/147poweramp.html
Leslie amps used the same basic idea decades ago.
............ Phil |
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Guest
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| Posted:
Wed Sep 28, 2005 10:04 am Post subject:
Re: Unsual PP Output Topology? |
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Well.. I've worked on 1492 different 147 and 122's in my life, but I never
noticed It!!
100% cathode feedback. It that the offical name for it?
Thanks!
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:3puk0aFc2edqU1@individual.net...
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Ian Iveson
Guest
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| Posted:
Wed Sep 28, 2005 10:09 am Post subject:
Re: Unsual PP Output Topology? |
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<r.laury@comcast.net> wrote
| Quote: | Has anyone ever seen this topology covered in any text?
I would like to learn more about how to use it.
What are the benefits, and what's required of the transformer??
I like the idea having the transformer in a feedback loop with the
6550.
Any comments? anyone used it before?
view schematic here:
http://www.nashaudio.com/Public/6550_OUT.pdf
|
Yes. It is a form of distributed load that is widely used but more
often the speaker winding is separate from the cathode winding. This
makes a big difference to how it looks in the drawing which is
perhaps why you don't recognise it.
The ins and outs of distributed loading are many...used in different
proportions by Quad, Mackintosh, van der Veen, and various current
manufacturers. There may be a paper on Plitron's site. It is quite
well covered in Menno van der Veen's "Modern High-End Valve
Amplifiers".
Using the speaker winding as shown means that the ratio of anode to
cathode winding, and hence the amount of feedback, depends on the
winding ratio between primary+secondary and secondary. It is
unlikely that both can be optimised at once. Handy if you only have
simple transformers to hand though.
This limitation can be overcome by using the optimum ratio for the
cathode, and then tapping that winding at the best point for the
speaker.
The common alternative is to use separate cathode windings
altogether. I think this makes it easier to minimise leakage
inductance between anode and cathode, and also avoids attaching the
cathodes to the speaker terminals, which could pose problems for the
average user and makes global feedback a little more awkward to
implement.
For an illustration of how cathode feedback alters the
characteristics of the output valves:
http://www.ivesonaudio.pwp.blueyonder.co.uk/modal.htm
For the circuit of my amp, wired for the "UL30CF-10" option:
http://www.ivesonaudio.pwp.blueyonder.co.uk/cfb6ch6schema.gif
cheers, Ian |
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Ian Iveson
Guest
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| Posted:
Wed Sep 28, 2005 10:23 am Post subject:
Re: Unsual PP Output Topology? |
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<r.laury@comcast.net> wrote
| Quote: | 100% cathode feedback. It that the offical name for it?
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Not really. "Distributed load" is a better term than cathode
feedback, and it is not 100% because it is normally given as a
percentage of the anode voltage. 100% wouldn't work...
cheers, Ian |
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Phil Allison
Guest
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| Posted:
Wed Sep 28, 2005 10:58 am Post subject:
Re: Unsual PP Output Topology? |
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<r.laury@comcast.net
| Quote: |
Well.. I've worked on 1492 different 147 and 122's in my life, but I never
noticed It!!
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** How idiotic.
Must be the result of top posting all your experiences.
| Quote: | 100% cathode feedback. It that the offical name for it?
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** Dunno - but 100% of the speaker voltage is being used as negative feed
back to the cathodes in both in these examples.
........... Phil
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J.Koning
Guest
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| Posted:
Wed Sep 28, 2005 12:01 pm Post subject:
Re: Unsual PP Output Topology? |
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<r.laury@comcast.net> wrote in message
news:NLydnR_dZYGBiqfeRVn-qw@comcast.com...
| Quote: | Has anyone ever seen this topology covered in any text?
I would like to learn more about how to use it.
What are the benefits, and what's required of the transformer??
I like the idea having the transformer in a feedback loop with the 6550.
Any comments? anyone used it before?
view schematic here:
http://www.nashaudio.com/Public/6550_OUT.pdf
Thanks
RonL
I
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Menno van der Veen published an article about
several topologies and their properties some
time ago amoung which is "your" circuit.
See:
http://www.plitron.com/PDF/PB/Article/Atcl_4.pdf
Refer to circuit #7, "Super-Triode".
I'd prefer to name it "UL with CFB", but who am I?
Maybe I'd have to read the entire article to
understand the authors remark "..newly invented..".
For the moment though I'm missing the point of it.
I guess answers on your questions are covered
in the article, so I won't repeat them here.
Rgds,
Jan. |
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Bob
Guest
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| Posted:
Wed Sep 28, 2005 1:01 pm Post subject:
Re: Unsual PP Output Topology? |
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IIRC, impedance is turns ratio squared. That would mean that he should
tie the "4 ohm" tap to ground instead of the 8 ohm tap. I would
presume that he wanted the same voltage levels (but 180 degree out of
phase) on both cathodes). |
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Phil Allison
Guest
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| Posted:
Wed Sep 28, 2005 2:29 pm Post subject:
Re: Unsual PP Output Topology? |
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"Bob"
| Quote: |
IIRC, impedance is turns ratio squared. That would mean that he should
tie the "4 ohm" tap to ground instead of the 8 ohm tap. I would
presume that he wanted the same voltage levels (but 180 degree out of
phase) on both cathodes).
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** Correct - the posted ( hypothetical) diagram is wrong on that.
http://www.nashaudio.com/Public/6550_OUT.pdf
.......... Phil |
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Guest
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| Posted:
Wed Sep 28, 2005 6:01 pm Post subject:
Re: Unsual PP Output Topology? |
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Thanks All for putting me on track
RonL
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:3pv646FcevuaU1@individual.net...
| Quote: |
"Bob"
IIRC, impedance is turns ratio squared. That would mean that he should
tie the "4 ohm" tap to ground instead of the 8 ohm tap. I would
presume that he wanted the same voltage levels (but 180 degree out of
phase) on both cathodes).
** Correct - the posted ( hypothetical) diagram is wrong on that.
http://www.nashaudio.com/Public/6550_OUT.pdf
......... Phil
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Phil Allison
Guest
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| Posted:
Wed Sep 28, 2005 6:18 pm Post subject:
Re: Unsual PP Output Topology? |
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<r.laury@comcast.net
| Quote: |
Thanks All for putting me on track
RonL
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** Yeah - just slightly removed form being " rail roaded" !!!
............ Phil |
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Patrick Turner
Guest
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| Posted:
Wed Sep 28, 2005 7:47 pm Post subject:
Re: Unsual PP Output Topology? |
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r.laury@comcast.net wrote:
| Quote: | Has anyone ever seen this topology covered in any text?
I would like to learn more about how to use it.
What are the benefits, and what's required of the transformer??
I like the idea having the transformer in a feedback loop with the 6550.
Any comments? anyone used it before?
view schematic here:
http://www.nashaudio.com/Public/6550_OUT.pdf
Thanks
RonL
I
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As Phil suggests, the schematic is almost identical to the Leslie amp.
The extra cathode feedback from the speaker secondary is a rather
ineffective use of NFB.
Let us suppose the OPT Z ratio is 6k to 16, then the TR = 19.36:1.
then if the typical signal voltage from cathode to anode is 250Vrms per
tube,
then we would have +237.1Vrms at an anode, and -12.9Vrms at the cathode,
so the amount of NFB applied is 4.84%.
The 4.84% = 100% of the secondary winding voltage.
However it isn't as simple as this.
There is also some NFB applied via this connection between the cathode and
screen.
So its a very complex equation indeed to work out exactly what the
final amount of applied NFB is in terms of dB of FB, because the amount of
applied FB due to the
UL connection has to be taken into consideration, and we don't know the
% tapping in this case.
I have some notes in Radiotron booklets of 1955, and indeed
equations for gain/feedback are beyond the scope of this email for me to
explain
further.
But its all been thought about a lot 50+ years ago, and the best minds
did try to work it all out.
But the 4.84% of applied CFB is not a very significant amount of FB.
One would definately find that since UL gain is rather low, applying some
5% of CFB isn't going to have any significant effect
on Rout, or thd, and in fact it amounts to only about 2db of NFB.
For any real benefit with CFB in the output circuit the
% of the anode to cathode total signal must be over 10%
And then one finds the tubes work best when the screens are taken to a fixed
supply voltage.
Quad found all this out before they released the Quad II amp,
which is why they use a dedicated tertiary cathode winding which equals 1/9
of the turns of the
anode winding.
I have followed in their footsteps and provided ppl
with amplifiers using say Ea = +470V, Eg2 fixed at +330V, and
12.5% of the primary turns devoted to a CFB winding, and with a
separate speaker winding, all somewhat better implememnted than
the bean counters at Quad would allow.
My most recent 300 watt amps using 12 x 6550 use 20% of the primary
as CFB.
The use of the speaker secondary for FB results in an insignificant
further improvement, and makes it very awkward to implement
with regard to arranging series/parallel windings on the speaker secondary
to allow for impedance matching to 4,8,&16 ohms while
retaining symetrical windings each side of a centre tapping,
and the same current density in each wire of the speaker secondary,
and no wasted or unused sections of windings in the speaker secondary.
I think the Leslie connection is BS, and the above website pdf
also is a case of poor design.
Good design includes for plenty of interleaving, low losses,
and scope for load matching without waste or any increase in
winding losses if the amp is to be matched to say 4 ohms instead of 16 ohms.
Sadly, many amps have much increased winding losses when selecting
the 4 ohm tapping.
Its a lazy crummy way to build an amp.
Musical Reference also used a centre tapped speaker
secondary as a CFB winding in an amp which used a pair of EL84
with Ea = 700V, Eg2 = 350V, and it put out 36 watts
into a 14k a-a load.
The output tubes were working in nearly pure
CFB, but because the voltage gain of the EL84 is much greater than many
other larger output tubes, the small amount of CFB was slightly more
effective than
the above mentioned case.
This amp worked in nearly class B conditions, and sure it made impressive
power,
but it wasn't as good a class A amp which
made 36 watts.
Patrick Turner. |
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Patrick Turner
Guest
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| Posted:
Wed Sep 28, 2005 7:58 pm Post subject:
Re: Unsual PP Output Topology? |
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Phil Allison wrote:
| Quote: | "Bob"
IIRC, impedance is turns ratio squared. That would mean that he should
tie the "4 ohm" tap to ground instead of the 8 ohm tap. I would
presume that he wanted the same voltage levels (but 180 degree out of
phase) on both cathodes).
** Correct - the posted ( hypothetical) diagram is wrong on that.
|
Indeed, the grounded CT of the 16 ohm whole speaker winding should be for
4ohms
from either end, and the 8 ohm connection should be between the shown 4&16
points.
Don't expect diagrams posted on the web to contain correct information
which has all the operating voltages and information relevant.
Patrick Turner.
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Bob
Guest
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| Posted:
Thu Sep 29, 2005 4:31 am Post subject:
Re: Unsual PP Output Topology? |
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On Wed, 28 Sep 2005 00:04:43 -0500, <r.laury@comcast.net> wrote:
| Quote: | Well.. I've worked on 1492 different 147 and 122's in my life, but I never
noticed It!!
100% cathode feedback. It that the offical name for it?
Thanks!
|
Probably not - since there is no guarantee that the feedback is 100%... because
of the transfer of the transformer, I have no idea what the feedback percentage
would be...it would depend on the ratio... but I would guess it's a lot lower...
you would have to develop a voltage in the secondary to negate the cathode
voltage developed , or something... which I doubt is happening! And who wants a
zero gain output stage?? The only 100% here is the amount of current, not it's
effect. |
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Guest
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| Posted:
Thu Sep 29, 2005 4:43 am Post subject:
Re: Unsual PP Output Topology? |
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Thanks for the info..I just had the idea for trying this
to keep from ordering an output trannie with dedicated
cathode windings. It is a rather complicated thing to try to
figure out with the ultralinear connections as Patrick pointed out.
I was un-aware that Leslie had already done it!
Again, thanks for kicking this around.
RonL
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:3q13r9Fcc53nU1@individual.net...
| Quote: |
"Chris Hornbeck"
** A snipping psychopath AND serial topic rapist.
........ Phil
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