Andre Jute wrote:
One of the really irritating things on the usenet is when you write a
long, detailed post full of facts and opinions -- and find everyone so
much in agreement with you that no one replies. It feels like you're
out in a vacuum somewhere, whereas in fact everyone is just sitting
there nodding wisely, saying, "I knowed that," and not posting a
response because they have nothing to disagree with or add. (For the
non-Anglophones, "I knowed that" is not bad English but a schoolboy
joke.) Patrick must feel like that after his two recent long, detailed
posts full of relevant numbers. I especially enjoyed the historical
references, and sure, I agree, the EF86 is a wonderful little pentode.
The doubters of NFB in triodes have ignored me well because when cornered they
know they really cannot support their position on NFB with any proof.
Ian even went beserk and said there wasn't any NFB in triodes, then said today there
was NFB in everything,
so in so many cases where these guys have not a clue they turn around and say
everything is nothing, and nothing is everything to somehow convince us all of how
wise they are about something
and they use lotsa jargon, but I see them coming, and going,
and if they'd stay gone so I won't have to address the weaknesses in their arguments.
But those who try to concentrate on tubes here are a diminishing crew.
But the only question I want to raise on two whole long posts from
Patrick is one so tangential that it needs a subthread of its own.
Yo, Patrick: you say that the inventors of CFB and UL *intended* by
their inventions to get all the advantages of the triode, including
specifically the advantageous harmonic distribution, together with most
of the power of the pentode. Can you cite any documentary proof of a
deliberate intention with reference to harmonic proportions at this
early date, say the late forties or early fifties? Or is your remark
the result of hindsight brought on by your own, much more recent
observations, and simply reflected back upon those guys -- IOW am I
just reading too much into your statement?
I have first to give credit to those who deserve it amoung the pioneers
of better tube audio gear and who actually did worry their brains to peices over
distortion and noise in kit they thought might sell OK.
I'd have to nominate DTN Williamson and Peter Walker.
After having tried dozens of ways of hooking up multigrids in output stages
I always come back to the Quad II Acoustical connection.
Its neither UL nor is it beam or pentode with local NFB in the output stage.
So why not when surely the two guys could have settled for the cheapest option
of high power with triode distortions, ie, UL?
Harold Leak settled quickly for UL; just adding two taps made his amps
much more saleable.
I think Walker must have known that his Acoustical did the business the best.
And if you examine it, with 10% CFB, there is CFB applied in TWO ways around the
KT66. if you examine the actual Quad II circuit,
there will be say +0.9D volts of THD at the anode.
Therefore -0.1D will exist at the cathode. This -0.1Dv appears
as an effective +0.1Dv between g1 and k so it is amplified A times to appear at the
anode
as -2Dv if we were to agree the gain of the output stage between Vgk to anode was
-20.
-2Dv is subtracting from the 3Dv that would be there were there no FB at all.
But it ain't that simple.
The -0.1Dv also exists between k and g2, and effectively the g2 is going +0.1Dv with
respect to
k similarly to the g1 to k case.
The screen transconductance creates some gain and an additional correction signal
is created in the anode current so augment the correction created around the g1
circuit.
The math on this is quite daunting, but the concept is there for our understanding.
The applied NFB around the g2 acts slightly differently to the g1 circuit in that
the tube is tending to be like a triode because of its relative total voltages.
If a KT66 is set up with a simple 20% UL taps, the beam distortion characteristics
change
towards triode substantially.
Applying the CFB to both g1 and g2 preserves the partial triodisation spectra
but because most of the effectiveness of the CFB is around the g1 circuit,
you get the output stage tubes to operate about like trioded KT66 but without the
limitations
of the triode due to the restriction of anode swings due to grid current.
I suspect Walker would have tried to use CFB with separate bypassing of each g2 of
each KT66 to
that tubes's cathode; this would have been dead easy with say 3.3k series R and a
47uF electro,
and thus the output stage could have been true beam tetrode with 10% CFB, but then
the
the spectra of the finished output stage would have the more complex spectra of
pentode thd
left intact, but just reduced perhaps by 12dB.
( certainly today anyone could bypass each g2 easily to each k in their Quad II amps;
they'd
have to maybe reduce the global FB though ...)
With both g2 taken to a fixed voltage, the open loop gain of the KT66 isn't as great
as the pure beam tetrode
would provide, and the thd reduction with 10% CFB isn't as great but then the
effectively
applied g2 FB made the spectra more acceptable to begin with so the extra reduction
of real beam tet + CFB wasn't needed, so onlt one bypass cap was needed and it could
have the erathy end
taken to 0V, and there was no need for two series R and instead a choke as used to
filter the screen supply
well since that's what helps keep the noise from the PS low in Quad II in addition to
the
common mode rejection which operates over most of the F range with ESL57, since
they are so easy to drive because of their mainly high Z between 50Hz and 1 kHz.
In my experiments with far better OPT than Walker settled for in his Quad II
the operation in pure tetrode with FB gave slightly worse looking THD on the CRO.
A decent PP amp should have a nice clean looking 3H tone of THD right up to clipping
without easily seen 2H, or any other jagged wriggles in the distortion wave which
would indicate the presence
of 4,5,6,7,8,9H etc.
if the 3H trace looks like a clean sine wave to the eye, then any other harmonics are
at least 20dB down and if the 3H is at 0.01%, anything else will be at 0.001%
and who is going to tell me they hear the results of say 0.001% of 5H?
But so often there is a mix of 3H and 2H because of
anything between slight and gross tube mis-matches.
In discussions with Neville Thiele in about 1995 when I was learning about all this
he asked me if I'd tried to use CFB with cross coupled screen taps off the anode
windings to give not just
equal g2 signal to k signal but a boost to g2 signal of about twice the k signal, ie,
positive FB applied to the g2.
Positive FB wasn't sneered at in 1955, and there are schematics in RDH4 with positive
series voltage FB to artificially boost the open loop gain of an amp circuit
and thus increase the applied NFB which depends on gain and thus reduce the N&D and
Rout.
Anyway, I didn't see the point of trying Neville's recomendation
that he and his pals farnarcled around with when they were young fellas in 1955;
these guys were mainly number oriented; if it measured well, they'd just do it, and
numbers guided
most audio engineering efforts, and concerns modern audiophiles have about distortion
and using no loop FB were yet to gain any real prominence.
And PFB always seems to reduce the margin of stability of any amp where its used.
I found spectra with pure tetrode with CFB was worse, and could perhaps get more
worse
with PFB, so I left it out.
I did try CFB with UL taps, and I got 0.7% THD at 40 watts with a quad of 6550,
but when i upped the anode supply to 470v for more AB operation and higher power
the Acoustical was better and without the UL taps with CFB the THD went down an
amazing 10 dB
with both g2 taken to a suitable B+ supply, somewhat lower than the anode B+.
The final preferred schematic is that of my 8585 at
http://www.turneraudio.com.au/8585integrated.html
The output stage in the 8585 is about as blameless as one could hope for,
and if I had used triodes to make the same power and had no CFB, it wouldn't measure
any better of have meaningfully lower amounts of spectra.
Maybe Williamson would be happier if I had used a triode output stage, if he were
around to see it,
but the 8585 has enough redeeming features to excuse it from the obligation
of including triode operation in the output stage.
Typically the THD of the 8585 is at negligible levels at normal listening levels
and it does not have an enormous amount of total FB applied.
It is only slightly better measuring than plain UL with 50% taps with which
I have also pleased some customers.
At some point the measurements don't mean much because they are so low, and so
benign.
At below 0.02%, I doubt I have to worry; the sound is then qualified in terms of
dynamics, and realism, preserved warmth, and those other subjectives that the
engineers
insist may flow on from the good numbers, but which audiophiles think may NOT flow on
from good numbers.
Ive heard changes to sound quality just by replacing input tubes in a line stage
preamp
with different brands; the idea that THD affects the sound does not stack up because
when i measured THd it was below 0.01%, and mainly 2H during such tube rolling.
Was it the subtle effect of microphony resonant F of the tubes chosen?
I doubt that because you only get microphony effects with phono and microphone
amplifiers,
unless the chosen tubes were really badly micophonic, and none I have selected have
been.
So there is something inexplicable about tubes.
And in the 1970s ppl in hi-fi mags talked about the sound of various transistors
making a big difference, even though ALL the circuits were drenched in NFB
wheras the triode preamp i fiddled with had zero loop FB. Just what made the
difference
in 1970? maybe it was LSD by 1975...
I cannot point to any particular written material which prooves
that Wlliamson amd Walker understood that UL was better than pure beam with either
shunt FB or series CFB, but I cannot help thinking they must have given
THD and the spectral purity a lot of thought.
What else was there to think about in 1940 to 1955 apart from a terrible world war,
and frightful distortions in all audio gear except in the FB amps they contrived?
There were guys who tried many of the techniques of output stage contrivances
well before they were officially invented by prominent ppl, some of whom
were well off and could afford to patent their ideas.
Just about every single damn connection of OPT to tubes that could be tried
had been tried by 1950 by somebody.
Things got a lot worse for awhile with germanium transistors.
They just applied MORE NFB.
Most consumer electronics continued to be the results of eager
dumbing down efforts by bean counters....
Very few ppl stuck to using their triode amps.
The 1950s and 60s were a regressive period in human history imho.
If we are bright enough to see that history repeats itself, we are more likely
to keep our faith and ignore the BS, and sweep out only what is
definately inferior, and avoid the traps of fads.
Patrick Turner.
TIA.
Andre Jute
Twenty-twenty hindsight
Patrick Turner wrote:
Andre Jute wrote:
John Byrns wrote:
In article .com,
"Andre Jute" wrote:
I'm reading this part of the thread with great pleasure; it reminds me
of the glory days of RAT when high theory was everyday meat on the
table.
There's some confusion here about which tubes we're talking about.
Perhaps I am to blame for it, in phrasing the thread headline too
bluntly as "triode superiority". But the results of tests and my actual
opinion, which comes as no surprise to you, is not the unqualified
"triodes are superior" but that "ZNFB or low NFB Class A1 triodes and
trioded pentodes are superior on the ear of qualified listeners to any
other form of reproduced music". My best amp ever, in the ears of my
test groups of professional performers, is PP EL34 Class A running ZNFB
or less than 6dB of NFB; I usually test it against expensive 300B gear
either of my own decising or commercially available and *behind the
curtain* it usually wins.
Next, the problem with pentode sound is not so much the pentode itself
but the amount of NFB required to get it to deliver its power with any
pretense to quality, the NFB creating recombinant harmonic artifacts
which are very disturbing to qualified listeners. Thus, if we use any
NFB at all in the levelling process, as in the CFB example, the
question is opened up of the recombinant harmonic effects of NFB *which
we are then adding to only one of the contestants*, the pentode, to
disadvantage the pentode yet again.
Hi Andre,
Please cut me a little slack if I have missed something, as I wasn't in
on this thread from the beginning.
The necessary information is spread over many threads and my netsite.
I don't understand your apparent
thesis that the triode is superior, if indeed your "PP EL34 Class A
running ZNFB" is your best amp ever, as judged by your "test groups of
professional performers"?
The headline is a provocative outgrowth of a bad-tempered set of
threads on whether a triode has native or built-in negative feedback. I
have always said that best amp I ever built was the trioded EL34 PP. In
fact I spelt it out earlier in this thread as well, but nobody took the
slightest notice.
Are the EL34s in this amp triode connected
with the screen grids strapped to the plates?
Yes.
If this is the case, the
implication is that an EL34 with the screen tied to the plate makes a
better triode than the 300B used in several of your other amp designs?
Yes. The comparison is ZNFB or very low NFB triode strapped EL34 PP
against SE ZNFB 300B. In level matched tests with a third, usually
solid state placebo amp also behind the curtain, a sophisticated
listening panel will choose the PP EL34 pseudo-triode amp every time.
(The SS amp commonly gets no votes.)
There are complications, in that when you double up the tubes on each,
to compare PSE and PPP, all other factors equal, except that now you
can use better speakers, specifically ESL63, the results get a bit
closer simply because the speakers offer more resolution to a test
panel accustomed to listening to details, and that big broadcasting DHT
like 845 and SV572-xx whip little EL34, but for practical,
level-matched tests at the entry level -- not meaning money, for all
these amps are expensive, but the entry level of very high resolution
-- sure as hell triode strapped PP ZNFB EL34 hold the upper hand. You
have to be in PP 300B at umpteen-eleven times the price, with a lot of
high-ticket high-carriage iron on board, before the natural triodes
make as good a showing as the humble, inexpensive, ultra-versatile
EL34; by then it is clearly no longer a fair test (but then, in terms
of high street hi-fi prices even the base EL34/300B test has only a
tenuous connection to what most people would regard as reality).
I have heard music via a pair of made in HK 18watt Vincent PP amps using two
300B
which to me did all the business really well.
it wasn't an expensive amp in1995, when I used to attend the NSW Audiophile
Society's meetings
to see what other folks got up to.
For PP triodes and 18 watts you need about 400V and 150mA,
and an OPT with about 8k : 8 ohms, and your'e away.
The same PS could be used for a pair of EL34, and get less power, or a pair of
6550 in triode
for maybe slightly more power. But the iron need only be about the same and not
cost a lot'
the cost is mainly in the higher costs of the 300B and perhaps in its needed
pair
of 5V filament supplies.
I personally don't see any point to using EL34 in triode when for such a small
extra outlay
6550 or KT88 in triode could be used and which would have lower Ra-a
and more power which is very much worth that slight extra outlay.
I give you the additional information because we need to be clear that
what I have proved isn't that PP amps are better than SE, merely that
the EL34 is a stonking good tube, especially at the price.
Also don't forget the EL34 replacement called the 6CA7, a beam tetrode, also
an underated tube.
I had to do a minor repair on a 'THD" branded SE guitar amp last week
which had a Russian 6L6GC running with 45 watts of anode dissipation with the
anode only
a tiny bit red in one place. It was fine at 35 watts.
EL34 will glow red with less Pda than this 6L6GC.
So I it seems the humble 6L6GC now made with its actual higher pda capability
well above the
original 22 watt rating could produce good triode power in a PP circuit.
The test
only incidentally pitches a PP against an SE amp; I had long before
this discovered that SE or PP matters not when both are made with
triodes running in Class A and you have a very light or absent hand on
the NFB; what I set out to prove in these later tests with the PP
trioded EL34 was something about Class A sound, and something about the
composition of distortion, and something about the lowest imperceptible
level of loop or anyway additional NFB.
I tried PP EL34 in triode in a pair of Quad II amps for 13 watts with 6db of
global NFB
and the owner said it lacked the sparkling
dynamics of a 10 watt amp using 6GW8 in class AB1 and about 17dB of GNFB.
Only when i went to KT88 in triode with 12 dB of GNFB did the owner say that
the
Quads then sounded as good as the UL amp and that the bonus was a slighty
higher ceiling
because the KT88 in triode with Ea at 400V and fixed bias the amp gave 20 watts
AB1.
There is lots about modding Quad II at
http://www.turneraudio.com.au/quad2powerampmods.html
I conclude from my observations and from what customers tell me that 12dB of
NFB is fine in a
triode PP amp. Nobody I know can tell the difference between
a normal triode amp with 12db of GNFB and the same amp with its screens taken
to UL
taps and with the FB left the same.
You also seem to be saying that the negative feedback that is inherent
in the operation of a triode is not responsible for its superior sound,
On the contrary, it seems to me very likely that the negative feedback
inside a triode is responsible for the quality of its sound. To me it
is a very persuasive argument that to bring a pentode back even partway
towards the silence of a triode you have to use the NFB that was taken
away to make it a pentode!
Well, yes, but impossible....
However, it is clear to me that external forms of NFB, as in loop
feedback, UL, CFB, etc, act differently to the native or internal NFB
of a triode. This is easily seen in comparative harmonic output
analysis of an EL34 amp with a pentode/UL/triode switch and a pot or
switch to alter the amount of loop NFB, as in my T113 "Triple Threat"
PP EL34.
Well indeed there IS a difference in FB transfer between external and triode
internal transfer.
Local or global NFB loops are normally employed in linear mechanisms; ie the
CFB of an OPT with CFB is where
the cathode voltage is magnetically locked to the energy elsewhere generated.
The fed back voltage is exactly the same in spectral content at the cathode as
at the anode'
only the phase is opposite.
The local CFB with screen fully bypassed to cathode acts to reduce all the
spectra of the
pentode without NFB, and most pentodes suffer double digit THD at clipping when
tested
to gain the published data for SE use; 13% THD is about what is quoted, and a
horrible mix compared to
the 5% of the trioded case, with most only 2H.
Note that these figures are for the rated load use stated for the tube;
Loading means a heck of a lot with output tube distortions.
Unloaded, the pentode is abysmally horrible.
But when CFB or external GNFB is added, the higher the load, the higher the
gain
so for a given value of ß, the greater tha amount of NFB applied, so although
the
THD rises as load increases with no NFB, it is limited in rise when a given
amount of NFB is applied.
The triode THD reduces to almost nothing when load value is very high,
indicating
its inmternal FB is maximally effective, similar to the case od the unloaded
pentode with a given amount
of CFB as i calculated in a previous case for the EL34.
When loaded though, the gain of the triode goes lower, and the internally
applied amount of NFB becomes lower
but still present, and the transfer of the NFB becomes maximally affected by
the nonlinear 3/2
voltage effect on Ia.
A similar thing occurs in UL, but intermediate between triode and pentode,
and somewhat unique, ie, different to either triode or pentode.
A UL amp with 43% taps and EL34 may make a maximum of say 32 watts,
and in triode its 14 watts, but the UL amp will have no more THD than the
triode amp at 5 watts,
and be spectrally as clean.
Trouble is the Rout. The UL amp has Rout about = RL, and GNFB must be used to
further reduce Rout.
In the past many makers competed with each other to get the highest possible
amount of GNFB applied around their UL
amps to be able to say they had 0.03% THD at 45 watts.
30db of applied GNFB was not uncommon in top brands using
well made OPT.
Meanwhile all the makers with lesser quality OPTs had to settle for no more
than the standard 20dB
of GNFB and 0.3% THD at clipping for class AB1.
Now 17db is a lot of GNFB around any tube amp.
In actual fact, few could really tell the difference between samples of all the
above
amp recipes at average normal listening levels of 86db, or 1/2 a watt average
to each of two speakers.
Nobody is going to win any prizes for applying more than just enough GNFB
around any tube amp these days.
Sanity has prevailed; we have done away with the uneccessary.
A UL amp with 50% taps and in class A PP with 6550 typically makes 1% THD of
mainly only 3H
at about 30 watts and without GNFB.
At 3 watts, output voltage is 1/3 of clipping, and THD is about 0.2%, and at 1
watt
its THD is about 0.11%, and THD is already so low that it isn't of any concern
and would
not be discernible by 99% of the population.
However, the Rout would seriously colour the sound, depending on the vaguaries
of speaker Z
so if 15dB of NFb were applied, the typical Rout without NFB of about 5 ohms
if the secondary winding was a match for 5 ohms is reduced to about 0.7 ohms.
All is well if the OPT has wide BW to start with and the driver amp is suitably
set up.
people knock UL amps with 6550, but it has been my experience to seriously dent
all the
egos of other makers of amps with triodes and low GNFB with samples of mine in
demonstrations with the
audiophile club, and while using 1/2 the output tube count.
I have nothing against trying to use zero GNFB and rely on just the natural low
Ra of the triode.
But I would say that then one is compelled to use wide BW OPTs, lest the OPT
unecessarily act as a filter,
so don't try an old Leak Point 1 TL12 without its GNB in triode; the OPT has
50mH of leakage inductance!
And one wants low winding loss OPTs, and with a nice high voltage ratio
to give the output tubes a higher than normal AB1 loading.
So when thinking of EL34 in triode I'd be thinking of 10k : 6 ohms OPT ratios,
using Ea = 425V, Ia = at least 50mA.
The Ra-a of the triode = 2.5k, and this is transformed to 1.5 ohms by the OPT Z
ratio
of 1,666:1.
Then one adds the effective winding resistance of say 0.4 ohms at the sec and
you still get
Rout at about 1.9 ohms.
This is one reason why not to use EL34 in triode when 6550, KT88, or KT90
will all give you 1.5 ohms under the same conditions.
But the larger tubes allow a higher safe Ea at the same Ia, and so RLa-a can be
higher,
say 12k : 6 ohms, or 2,000 : 1 so the Ra-a of 1,800 ohms is reduced to only 0.9
ohms,
and plus Rw, Rout = 1.3 ohms. 300B will do slightly better since the Ra is
slightly lower than
any of the larger octal power tubes in triode.
The benefit of having each output tube seeing a class A load of 6k each is that
the THD also is quite low, and negligible at ordinary listening levels.
do you have any clues yet as to exactly what it is that is responsible
for the superior sound of the triode and the triode connected pentode?
Sure. The difference in sound between a triode (natural or screentied)
and a pentode is caused by the different makeup of the harmonics.
Especially the IMD caused by the varying gain with load changes.
In the pentode, these are high, in the triode low, and its a function of the
Ra....
Regardless of the absolute level of distortion, the triode, however
made, has a lower proportion of odd and higher harmonics than the
pentode. It is known that very low percentages of odd and higher
harmonics can cause uneasiness in listeners.
This very true, and when i built a pair of SET 4 watt amps using 2A3
they sounded cleaner than the same power from an EL84 in pentode with global
NFB.
However, I do have a pair of SE amps with EL84 with about 10% CFB and slight
NFB
and these are remarkably triode like, and very listenable.
I think the 2A3 gives the best 4 watts possible out of all devices in the
universe.
It is strong supporting
information that the difference between a good 300B amp and a bad one
can be read in the proportion of 3rd harmonic left on the bad amp (as
you yourself showed in a table when you compared my HVHCHL "Hedonist"
to the distrastrous BobC/LaFevre/Magnequest "Bubbaland" amp roundabout
1998 -- that was a big clue on the way to my present understanding --
thanks!).
The easiest way to prove the importance of the proportions inside the
THD to yourself is to put a pentode/UL/triode switch and an NFB pot on
a pentode amp and to study the makeup of the distortion in each mode at
various levels of NFB including zero NFB. Of course, it took me a bit
longer than a single sentence, because I had to eliminate all the
common factors between the various topologies (I wasted years on AC
balance in PP amps) before I isolated the odd factor and could examine
it closely. There is no other parameter where the change is so striking
and so inexplicable. Even at zero NFB, the third and higher harmonics
fall through the floor when you turn the triode switch on a pentode
amp.
Well, switching that switch from pentode to triode is connecting the triode FB.
Ra falls heaps, the iron distortions and intermods are much suppressed,
and sure, the THD reduces along with IMD.
Some ppl may not initially like the triode sound if the pentode with FB gave
increased bass and treble due a
speakers higher Z below 100Hz and above 500Hz, and this was the case in old
radios,
where the FB was omitted not only to save having to have another tube for the
extra
open loop gain, but to allow the high pentode Ra to compensate the miserable F
response
from the AF detector of the radio, typically 150hz to 2 kHz.
But with hi-fi speakers powered by current sources, which is what a pentode amo
is without its GNFB,
the outcome is a bit tricky indeed.
Of course, before you even get there, you notice that in the trioded
pentode, the THD is lower than in the other two modes (pentode, UL) at
any (low) level of NFB. It is difficult not to conclude that the
effective removal of the pentode's screen grid adds additional NFB
which works in some internal way. It is equally difficult to see (and
hear) the different result of this internal NFB and not to conclude
that it works differently from external feedback.
I have posulated that the triode FB works along paths which act on a 3/2
voltage to current rule.
But where very small changes level occur, the changes in THD are virtually
linear because
one acts along what is such a slight amount of bend in the Ra curves.
When plotting the THD for a given class A amp on linear voltage output
versus THD % scales the THD rises slightly faster than a straight line.
So a given amp may make 5% at 20Vrms into a load,
and 2% at 10V.
One might draw a curve through the 0.0V and the 10v and 20V points and find
that's the average for several samples of that amp.
But wide variations in THD occur between samples; tubes are not always matched.
In Quad II samples i have worked on with older tubes it isn't unusual to find
5 times the rated data THD. Swapping positions of the KT66 and or the EF86
can drastically reduce the THD.
Usually they measure a whole lot better when all R&C are replaced,
but still its worth final swapping of known healthy tubes to get the
THD to measure low; and voila, usually the sound is blameless
while every effort has been made to accomodate the imperfections of the items
within the system.
Class AB usually gives a slowly rising THD profile at first which is virtually
the same as full class A, but when the threshold of AB commences the THD rises
much more rapidly,
and graphs of THD with kinks showing the THD increases rapidly with AB action.
pentode amps are worst in this regard because their gain is prportional to
load,,
since gain = gm x RL, and RL reduces to half the class A load ( or to 1/4 RLa-a
) when one tube cuts off.
Triode gm tends to rise and Ra get lower with increasing anode current so
the AB transition produces less odd order harmonics; principally less 5H.
The principle reason why PP triode output tubes in class A are so linear up to
clipping
where 1% THD is not uncommon is that the decline in gm with lessening Ia is
about equal to the increase in
gm with increasing Ia.
The actual load seen by one tube of a PP pair affects the load seen by the
other, and in fact the loads
are curved lines for each tube.
The PP amplifier is so inherently linear and majestically simple that it has
stayed in favour
for all these years............
But its hard to get a PNP transistor to act just like its NPN companion.
They just don't come in twin pairs; more like distant cousins.
Thus my fear that, if you use any form of external feedback to make the
triode and the pentode "electrically equivalent", you will disadvantage
the pentode because the makeup of its harmonics will be adverse *and
will be heard by the panel*.
This is the fundemental problem with comparisons of pentode with loop FB
and triodes, arranged for the same Ra.
However, I have used CFB in my SE35, and with screens bypassed to 0V
and with Eg2 much lower than Ea to minimise Ig2, and got only about 1.5% THD
without
GNFB at 35 watts from a quad of 6CA7.
With such a small amount of 2H at the output stage, it was easy to cancel it by
the
triode driver stage without setting up the driver with an adverse load to
deliberately
create 2H that will cancel in the output stage.
The amp is designed to give maximal THD cancelations in the 4 to 6 ohm load
range
and the benefits seem real sonically during the tests I have done with SEUL
amps using
13EI for a comparison.
I only had to apply a small amount of GNFB in the SE35 to "complete the
picture".
The owner now uses NOS EL34 instead of the Sovtek or EH6CA7,
and perhaps the harmonics produced are even more benign than they were
when supplied with the Russian 6CA7.
I don't actually see any alternative way
which won't turn the pentode into a triode, which defeats the purpose
of a straight-up challenge, so I think you and Patrick are on a wild
goose chase. But that has never stopped us speculating before!
One can only compare two different things to be more aware how each works.
I sometimes wonder what the folks on different planets in the many universes
out there thought when they
discovered thermionic emission in vacuum tubes, and farnarcled around with 2
electrodes, then 3 electrodes,
then 4, then 5, then 7 then 9..............
And we can't advise those who have not yet discovered tubes, and them out there
can't advise us
of what to look out for next.
We are profoundly alone so far, scratching at the surface of the very hugely
unknown.............
Patrick Turner.
Regards,
John Byrns
These, incidentally, are the same test results I interpret as meaning
that there is no such thing as an SE sound, that what we hear which so
appeals is a Class A ZNFB or very low NFB sonic signature. People just
confuse Class A with SE because only a tiny, tiny minority even of the
ultrafidelista have ever heard pure Class A PP amp.
HTH.
Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
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