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And still yet further - this matching issue seems to be all the more fluff
in the cadence arena - where distortion is embraced with the utmost
affection 
--
JAMES RUGGIERI
"Patrick Turner" wrote in message
...
"James Angelo Ruggieri, P.E." wrote:
and the price of GE6550A has gone far too higgh to be worth buying,
at over US $400 for a matched quad, ( which in my experience
means little, because they don't stay matched ).
And further - "matched" on a production jig often does not mean
"matched"
in your amp!
I might add that matched does not mean matched for
Gm right across the operation range.
This means that in an AB amp, which most PP amps are,
the there is quite a bit of difference in the transfer curve
for the top half of the wave compared with the bottom half, since
each tube only does its own half of the wave.
The result is lots of 2H in PP in a PP amp.
Musos should really like this trait with unmatched tubes.
But purist hi-fi ppl might find that the thd of the PP amps they own could
have
up to 12 dB less thd if only the tubes were matched for Gm, and not
just for a given bias current for the same grid bias voltage.
The use of adjustable signal drive to the output grids allows an owner to
reduce 2H distortion in PP amps to a minimum if he wishes.
Many PP amps, even class A types, where the maximum of cancelling of
2H distortion occurs, could have their thd further reduced by deliberately
trimming the drive voltages to the PP grids.
Unfortunately, a CRO and distortion analyser is needed to do this, and
when
tubes are
renewed, the old setting is wrong, and the thd may be worse.
So equal drive voltages to each op grid is what most folks settle for.
But in fact, users of stereo amps may find that
one channel makes 3 times the thd of the other.
I guess that providing thd is below 0.2 % at 10 watts no matter
what the lack of matching is, then the music isn't spoiled.
Trimming is sometimes hard to do; it is in the Quad II amp.
In a pair of these I serviced this week, thd at 5 watts was able to be
reduced
about 10 dB max by simply swapping the EF86 and KT66
around for lowest thd.
Amps with an LTP driver stage are easier to deal with.
Patrick Turner.
--
JAMES RUGGIERI
"Patrick Turner" wrote in message
...
Look, NONE of the import 6550's have what GE did to theirs. After
all,
THEY raised the
standards for what was commonly thought of as a 6550 here in the
states.
The import
companies have THEIR *Versions* of what they THINK a 6550 should be.
Published families of curves for each should represent EACH of these
6550's, and SHOW
exactly what the electrical differences ARE!!!!
There is this mistaken notion that all 6550's SHOULD *BE* the
same!!!!!!
(snip some interesting info about "polished turds", et all)
I did a search on the GE6550A.
It appears that the 16 pcs I bought in 1996 are GE6550A, the good sort
of
tube
you are raving about.
I looked at
http://www.tubeworld.com/6550.html
and the price of GE6550A has gone far too higgh to be worth buying,
at over US $400 for a matched quad, ( which in my experience
means little, because they don't stay matched ).
I couldn't place my hands on the spec sheets for the GE6550A,
to compare it with other versions to confirm your claims
about these tubes superior ratings.
perahps you could be so kind as to post to the group the
list of maximum design ratings of the GE6550A,
and others that you regard as inferior.
Patrick Turner.
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