On Dec 6, 11:30*pm, "GRe" wrote:
"Patrick Turner" wrote in message
...
On Nov 29, 8:31 am, "GRe" wrote:
"Patrick Turner" wrote in message
....
[...]
The answer to such uneven
matching amoung a given batch of supposedly NOS Red Lion KT88 is to
have some additional bias balancing adjust method as seen in another
pair of Quad-II I rewired where I used EHKT90 instead of KT66, see
http://www.turneraudio.com.au/quad2p....htmandproceed to
about 1/2 way down the page to the 2005 schematic.
What's the reason you chose 560K for R11 while R10= 470K?
Rgds,
Gio
To achieve Vac drive balance to each OP tube where there is an LTP
with common Rk resistance, the RLa seen by each gain tube of the LTP
should be slightly dissimilar to compensate for the unbalancing effect
of the common Rk value. If Rk was a CCS, then the total cap coupled
RLa on each side of the LTP needs to be the same for perfect balance,
even if the tube types used are quite different, eg, see the Radford
schematics. But where you have two pentodes as LTP gain tubes, gain is
close to gm x RL, and quite high, and the current change in one equals
the current cjange in the other plus the change of current in the
common Rk. You should be able to do the analysis of all working
voltages and currents in my circuit as easily as in any other.
In fact, the use of 560k and 470k is a bit high for Rg bias values,
because reverse grid current in ageing tubes can cause Eg1 to become
several volts more positive than the actual bias voltage at the end of
Rg away from the grid. In my example, if that happens, it unbalances
the Idc of the OP tubes thus causing the balance monitoring LEDs to
change from normal OK with both equally bright to having one brighter
than the other. An owner will turn the adjust pot with a thumbnail or
a dinner knife until Idc balance is restored, but if the problem of
positve grids gets so bad the balance pot cannot be adjusted to get
equal LED brightness, then he knows he has a tube problem, and needs
new tubes. For this reason, the high Rg values are not so bad, bearing
in mind the amp has cathode bias with individual R&C networks in each
OP tube cathode circuit.
In more recent rewiring of Quad-II-Forty amps completed last week, I
used 6SH7 in triode for V1, then used a 6SN7 for LTP with a CCS with
MJE340. RLa to B+ was 50k per 1/2 6SN7, and Rg = 120k taken to to the
OPT CFB tranny winding to partially bootstrap the Rg to have it appear
to to the 6SN7 as about 300k. The dc balance will remain fair but not
excellent with individual R&C bias networks, and with Rg at 120k, the
reverse positive grid current in ageing tubes won't cause too much
trouble. *In two samples of the Red Lion KT88 in one Quad-II-Forty
monobloc, Ek = +38Vdc in one tube and +44Vdc in the other. Rk = 630
ohms, so Il for the two OP tubes is 60.3mAdc and 69.8mAdc, a
difference of 9.5mAdc, a considerable difference. But effects of the
Idc imbalance in Chinese made OPTs is minimal because although the OPT
cores seem large enough, the number of turns is dissapointing. But the
paucity of primary turns means the Np x Idc product in the equation
governing the Bdc within the OPT core remains fairly low, thus
allowing a high ammount of additive Bac before saturation occurs at
full power at about 25Hz, good enough. But Quad-II amps from the 1950s
have more Np turns and smaller core, and the use of a balancing pot
for Idc in each OPT tube isn't a bad idea.
My customer who had me re-wire his Quad-II-Forty amps has given me a
dozen Red Lion KT88 to test, all with unknown condition. I should be
able to find close pairs among them all, and then if he had an uncle,
his name would be Bob.
Generally, whatever I do in amps is done to help the tubes give their
best in terms of distortion, bandwidth, output resistance and
stability. I don't include ALL techniques in every pair of amps; not
all amps of mine have the same range of techniques. They don't need
to.
Providing the loading of tubes in tube amplifiers is technically
optimised, and the other 4 items done properly, with optimal DC
biasing, music will flow well without it becoming insiduously grubby
as the tube age, which is so common in Quad-II amps where their Uncle
Scrooge owners cannot bring themselves to replace 20 year old KT66.
Patrick Turner.
Hi Patrick,
It took me some time to digest the comprehensive info, especially when I
started reading the QuadII mod's article on your site, hence the delayed
answer.
I don't think I digested it all, yet I learned something, thanks.
B.t.w., do yourself a favor, leave the narcissists doing what they do best;
gazing at their own belly button.
Gio
If ppl ask many more questions after reading my website then all is
very well indeed. But if they just throw rocks at me, what's the
point?
I just done a pair of Quad-II-Forty amps with a basic 5050 schematic
instead of the original Quad horror.
6SH7 is a really good pentode when Ia becomes high enough, ie, much
more than in any Quad. But the 6SH7 becomes a really super dooper
triode with high gain and low Ra, and it works with 6SN7 LTP very
well.
During the same months I did tha last Quad mods, I also did a re-wire
of an Earl Weston amp which had about the worst damn schematic I have
ever seen in any amp, but after my singing lessons with a 5050 type
schematic, the owner said it sounded better than the Quads. Earl is a
bloke in Victoria making more amps than I ever will, but man, although
his work looks so pretty, there's a mess under the bonnet. My customer
got lotsa smoke instead of music, I could not just repair it; it
needed complete gutting and re-wiring. Earl's transformers were quite
OK and I could give the amp unconditional stability with just the
right phase tweaking networks. Tube line up ended up 1/2 6SN7 for
input SET and then 6SN7 for LTP, then chinese KT66 with 40% UL
operation, it gave about 30Watts, very nice.
When I get time, all the schematics of these mods to amps will be
posted at my site.
At the CERN accelerator, they are looking for a Higs Boson or somink
like that, and today I read a whole page in the newspaper about it. I
didn't understand a word. But, maybe it'd be nice to know what
particle causes Time, and Mass, as then we might manipulate it, and
fly to elswhere at a speed of C squared, or C to the power of
infinity, and we might amble up to God, and ask him how the universe
doth go, and he might tell us he's busy, because he has to look after
an infinite number of universes besides our little one we find
ourselves within. It would not seem unreasonable to think that if some
God does exist who we might comprehend, then He, She, or It might just
know NOW what's going on, and that means light must travel infinitely
faster than it does, and just why it seems like it is stuck at about
300,000km/sec and restricted to this speed is a deep question indeed.
I also read today about nuclear power and how Green Politics is often
so backward in thinking, like they never wanna give up old beliefs
based on BS rather than answers to questions based on facts observed.
As a result of much questioning, blokes have come up with IFRs,
Integral Fast Reactors which use up leftover waste from old fashioned
nuclear power plants. With IFRs, we could avoid greenhouse effects,
and get off fossil fuels, thus be able to use fossil deposits sensibly
without burning them for transport, hot water, cooking etc.
Of course, if we had cheap electricity and no greenhouse effect,
nothing would stop economic growth and population growth so we'd have
to farewell the environment anyway. Food production has to double in
35 years, well, its what ppl will want, and anyway, lots more
questions will need answering as we muddle along, fixin' one problem,
only to find we have another.
But I digress.
Patrick Turner.