Quad II Modification
Eric Calderwood
How feasible would it be to modify a pair of Quad II's
to use KT88's instead of the original KT66's ?
This is intended purely for an increase/ doubling of the power output.
Thanks
Eric
Patrick Turner
Eric Calderwood wrote:
I did this with a pair of monoblocs about 2 years ago.
The amps run fine.
I removed all the power supply parts.
then in went silicon rectifiers, and pair of 100 uF electro caps,
and a choke to replace the existing screen supply choke.
Tis got the anode supply up to +400v.
Then I created a fixed bais supply, with a two transistor diff amp to work
two LEDs so that bias balance can be adjusted with a pot, and no voltmeter
required.
The KT88 were set up in triode, and I got 20 watts, and the sound was just
delicious.
I didn't try tetrode mode, but I guess about 40+ watts is available.
There is extra power drawn by the KT88 heaters, but the extra losses
in the heater windings are not high.
Meanwhile, the 5 volt heater winding isn't used, and less idle current
in each KT88 means the existing power and output transformers
are under less stress than the original condition.
I also removed all the EF86 drive circuitry, and replaced that with a
12AU7
LTP driver with a CCS tail using one MJE340,
and the input tube is a 12AT7.
The global NFB is about 12 dB, and of course the CFB in the OPT
is retained, but provides only 3 dB of NFB which basically only
compensates
for the appalling OPT winding losses when connected for 8 ohms.
In tetrode, this CFB would peform better.
Should the radical changes mentioned not be wanted,
then indeed, KT88 could be used instead of KT66,
and as long as the tube idle currents were not more than the
KT66, then the only extra power drawn will be the heaters, which
are 1.8 amps instead of 1.3 amps, so there is 1 extra amp of heater power,
and if the tranny losses are 15%, then the extra tranny heat is just 1
watt.
I would at least get rid of the common cathode resistance and single
bypass cap, and ground the CT of the OPT CFB winding. Then I would use
individual cathode resistors and 1,000 uF bypass caps, and choose the Rk
for
about 60 mA.
This will assist in balancing the bias currents of the tubes, which tend
to become unbalanced with the shared R&C, since when one tube decides
to conduct more current, the other tube tends to get cut off,
so its not uncommon to find old Quad II amps
with 40 mA in one tube, and 100 mA in the other,
which ruins the tubes, and the sound.
I have always thought a pair of 6CM5/EL36 could be used
instead of KT66, and at a lower idle current, and probably
you'd get more power.
These need only 1.25 amps of heater current, and
have higher gm than KT88 or KT66.
Also, 6FW5 which is just like 6CM5, but without the anode top cap,
would also be good.
Patrick Turner.
Choky
nah---- no free sonic lunch there........
except that you will have mebbe free sonic gadget launch...........
--
Choky
Prodanovic Aleksandar
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"Eric Calderwood" <eric.ca...@btinternet.com> wrote in message
news:arkvan$22t$1...@pheidippides.axion.bt.co.uk...
Patrick Turner
Choky wrote:
> same OPTs and PTs ?
> nah---- no free sonic lunch there........
> except that you will have mebbe free sonic gadget launch...........
And for such a Launch, a bottle or two of champagne
wouldn't go astray.....
To my mind the KT88 might sound better than the KT66.
But what determines the output power is the supply voltage
and the load, and the available voltage swing,
and perhaps the KT88 would only offer a marginal increase
in voltage swing, if the supply voltage stayed fixed.
I have never seen a Quad 40 schematic, but getting 40 watts from a pair
of KT88 into a 4 k load means an anode to anode voltage swing of 400 vrms,
and in Quad II, at 22 watts into 4 k, there is 300 vrms,
which is 67 peak volts less per anode than the KT88.
so maybe the supply voltage for the 40 is higher than the 22.
If you carried out the mods I listed by raising the supply voltage from the
standard
360 v to 400v, then indeed you would get far more power AB1 from 2 x KT88
used in tetrode.
I got 20 watts even in triode, AB1......
It sounded better than the original circuit.
Patrick Turner.
Choky
in other way-changing to KT88 can be interesting -just like in your last
year's case.
--
Choky
Prodanovic Aleksandar
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"Patrick Turner" <in...@turneraudio.com.au> wrote in message
news:3DDE2D41...@turneraudio.com.au...
Denis
More reasonable mod: Not to increase output power but refine
linearity.
For this purpose you need to provide reduced screen grid supply for
KT88. It is feasible to inset a series voltage regulator between +350V
supply for plates and screen grids of KT88. You'll probably need 200
or 230V of Eg2. Also different cathode bias network will be needed. I
don't recommend to switch to fixed bias. The modified amplifier also
will provide a considerable output current headroom (original QUAD II
with KT66 has almost none). Afer such mod you'll feel that new 15W are
way more clean and powerful.
The next step will be replacing the output transformers. Today we know
much more about them. Indeed the original CFB topology of QUAD II
power stage is a real masterpiece and should be preserved anyway.
Regards
Denis.
Patrick Turner
Choky wrote:
> hehe,I meant on 40W from that supply and that OPT.
> in other way-changing to KT88 can be interesting -just like in your last
> year's case.
Trying to get 40 watts AB1 from the Quad II OPT
isn't all that wonderful an idea, since the saturation F is going
to be higher than it already is, but nevertherless, there would be greater
headroom
for most music, and probably better fidelity during the first 5 watts.
I settled on trioded KT88, which gave 20 watts, and fitted the client's
expectations.
It was much better than when EL34 in triode had been used.
Patrick Turner
Denis wrote:
Indeed, a lower Eg2 might help the fidelity.
But Ea is already low in a standard Quad II.
The power stage in Quad II is a masterpiece of an idea,
but in practice, the OPT needs to have more interleavings,
more weight in the core, and is only a Morris of OPTs,
and certainly not a Jaguar.
Quad II are toys, but very chic toys for the era.
What really made Quad famous was the ESLs, not their amps.
Some folks get sad, or even angry that someone would butcher up
old Quads in an effort to improve the sound. I have no feelings of
guilt about trying to give these old bangers some singing lessons.
As long as three samples of mint Quad II amps remain safely
in museums for general display, I will be happy.
If only Quad had used 1/2" larger dimensions all round, and put more iron
and copper in the amps then they could have been a lot better.
Maybe the Quad 40 incorporates the improvements.
I hear the 40 uses the 6SH7 of 6SJ7 for the input tubes,
and either of these pentodes would make far better drivers than the EF86.
Has anyone got a schematic of the Quad 40???
Patrick Turner.
Choky
;)
ps.last known is that you are somewhere in USSR net area..........
?
--
Choky
Prodanovic Aleksandar
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"Andre Jute" <squi...@echelon.alias.net> wrote in message
news:629b11fde96feff4...@anonymous.poster...
| Eric:
|
| *************************
| Andre Jute
|
Patrick Turner
Choky wrote:
> ZM welcomes one nice and polite gentleman.
> ;)
Indeed, but just how long can we expect this guy to stay tolerant and
nice?
He didn't last long last time.
Patrick Turner.
Eric C
The amps in question are at the moment with my local
valve guru, who reckon's that he has a contact in Quad who
may be able to help.
At the moment I am waiting for him to get back to me.
Regards and thanks.
Eric
in article 3DE02FC7...@turneraudio.com.au, Patrick Turner at
in...@turneraudio.com.au wrote on 24/11/02 1:47 AM:
--
Choky
guru!
--
Choky
Prodanovic Aleksandar
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"Eric C" <eric.ca...@btinternet.com> wrote in message
news:BA06AA4E.77B7%eric.ca...@btinternet.com...
ÖIS
t.
ÖIS
you buy a new, good KT88 amp and become very happy
mvh
t.
Patrick Turner
"ÖIS" wrote:
> how many screws was left?
> t.
Quite a few screws were left over, and a bunch of
other junk,
like a tube rectifier, a screen choke, poor quality
electrolytics,
coupling caps, and carbon comp resistors, many
of which had changed value.
There must be hundreds of old junky Quad II amps
sitting
unused in the back sheds on farms all around Oz,
bought at a time when the produce prices were high
enough
for young farmers to pay the high prices of the day
for fancy audio gear.
Now the farmers are mostly old, and poor, and
perhaps deaf, and the prices they get
are very low for a bag of wheat.
Their city slicker counterparts with some knowledge
built their
own kit amps with transformers wound in Oz,
and these were often far better and cheaper than the
imported British junk.
Patrick Turner.
Patrick Turner
"ÖIS" wrote:
My customers want me to upgrade their old Quads, and I don't refuse them.
They might buy some old pair for US $100, and bring them to me for a fix.
This costs far less than a new amp.
I am happy, and so are they.
Its extremely rare that ppl spend more than US $700 on any audio amp.
The same folks whinge about prices for the one thing in life they appear
to really admit to liking.
Some will spend $30,000 on a car which depreciates at $10 per hour,
and think nothing of paying $20,000 for the kitchen to be upgraded.
I don't ever find old Quads lying around that I would sell cheap.
But sometimes customers do.
But if I did find a bunch of old Quads, I might sell, sure,
but at similar prices to the average Ebay figure.
If junk brings money, so be it.
1925 tube radios in mint condition bring HUGE prices.
Its just old junk.
My own DIY superhet with low distortion and wide
bandwidth features and SET feedback amp on the output
is probably worthless, maybe $10, but it outperforms the 1925
models by a mile.
Owning old radio junk just to look at it, or owning a Picasso, or Jackson
Pollock
painting, seems silly to me.
But to the rich man who likes such things, its all fair enough.
To me, the Quad II and old Leaks remind me of a bygone age,
which was certainly not the golden good old days of old times.
Life was a hard struggle.
The old amps are like the early Morris and Austin motor cars used by
very average upper middle class city folks. They are stodgy.
I am not stuck in Nostalgia.
Patrick Turner.
LiloLee
to 220ohm and that was it. I reasoned that since the Quad was designed to
run a Quad 22, FM1 and AM then if it is being used as just a monoblock the
current draw should be acceptable.
Lee
"Eric Calderwood" <eric.ca...@btinternet.com> wrote in message
news:arkvan$22t$1...@pheidippides.axion.bt.co.uk...
Patrick Turner
LiloLee wrote:
> I have a pair of Quad amps with 6550 in. I just changed the cathode resistor
> to 220ohm and that was it. I reasoned that since the Quad was designed to
> run a Quad 22, FM1 and AM then if it is being used as just a monoblock the
> current draw should be acceptable.
The 220 ohm commom cathode R should make the 6550
draw about the same current as the KT66, ie, about 70 mA each tube.
The other things that could be run from Quad II would have considerable
extra heater power.
The power increase will be negligible, since the voltage swing is only
marginally greater, but methinks the higher Gm of the 6550
will not go astray as it will increase the gain of the output stage, and the thd
could
be half that of KT66.
I've used KT88 as KT66 replacements, but in triode mode,
and of course keeping the CFB, which cannot be removed.
Patrick Turner.