Dudes, see the post at ABSE.

I have just posted thd test results from a single ended amp using
4 x 6CA7EH output tubes, cathode biased,
class A1, with 12BH7 driver, and 12AU7 input tube.
Eg2 is fixed, and 12.5% of the P windings are for cathode feedback.

With 5 ohm load, output stage FB is about 10 dB, and global FB = 4 dB.
Ro at the sec = 0.62 ohms, bandwidth 10 Hz to 68 kHz,
perfect stability and capacitor load capability.

Patrick Turner.

"Patrick Turner" a écrit dans le message de news:
...

Dudes, see the post at ABSE.

I have just posted thd test results from a single ended amp using
4 x 6CA7EH output tubes, cathode biased,
class A1, with 12BH7 driver, and 12AU7 input tube.
Eg2 is fixed, and 12.5% of the P windings are for cathode feedback.

With 5 ohm load, output stage FB is about 10 dB, and global FB = 4 dB.
Ro at the sec = 0.62 ohms, bandwidth 10 Hz to 68 kHz,
perfect stability and capacitor load capability.

Patrick Turner.


Oh Yes !

Now tell us how you drive that and how the OPT is wound ;)

(I'm looking to make one OPT for 2 x 6550 SE with 10% cathode feedback,
20 watts expected, and I plan to use 3.8 Kg iron and near 1Kg copper.
Does yours look similar ? )

Cheers, Yves.

Yves wrote:

"Patrick Turner" a écrit dans le message de news:
...

Dudes, see the post at ABSE.

I have just posted thd test results from a single ended amp using
4 x 6CA7EH output tubes, cathode biased,
class A1, with 12BH7 driver, and 12AU7 input tube.
Eg2 is fixed, and 12.5% of the P windings are for cathode feedback.

With 5 ohm load, output stage FB is about 10 dB, and global FB = 4 dB.
Ro at the sec = 0.62 ohms, bandwidth 10 Hz to 68 kHz,
perfect stability and capacitor load capability.

Patrick Turner.


Oh Yes !

Now tell us how you drive that and how the OPT is wound ;)

The driver is 12BH7 with 12AU7 input as I said above.

OPT has core = 60mm stack of 44 tongue GOSS, gapped at
about 0.25 mm
Pri has 16 layers at 119 turns each, of 0.45 Cu dia.
Anode winding is 3 sections, 4L, 6L and 4L
CFB sections are two layers between two double S sections.
Secs are 6 layers of 60 turns of 0.9 wire.
These are interleaved so that 4 S sections are used
with the middle two sections having two 60 turn layers, with CFB windings
between.
So finaly we have a wind up of

S PPPP S FB S PPPPPP S FB S PPPP S

Between S and P there is 0.7mm insulation,
between P and P, 0.05mm,
and between S and FB 0.05mm

Series pairs of 60 windings give 1.23k to 5 ohms.
So each tube sees 4.9k for a nice load to drive.

I have also wound two of the sec layers each split into two windings of 30
turns,
so the sec can be 6 @ 60 turns, 1.23 k to 1.25 ohms,
4 @ 90t, 1.23 k to 2.8 ohms, 3 @ 120 t, 1.23k to 5 ohms.
There are never any wasted sections in the transformer.


(I'm looking to make one OPT for 2 x 6550 SE with 10% cathode feedback,
20 watts expected, and I plan to use 3.8 Kg iron and near 1Kg copper.
Does yours look similar ? )

Use 3 x KT88/6550 with the above tranny, or 4 x EL34/6CA7.
Or 4 x KT88 with same op conditions as EL34, for long tube life.

Two KT88/6550 could dissipate 60 watts, and if plate efficiency is
a maximum of 45%, then expect 27 watts max.
4 x KT88/6550 would allow 120 watts of Pd, and 54 watts of class A po.
Awesome.

The load would want to be about 2k to 6 ohms,
so 333 : 1 ZR.
So 18.2 :1 TR, so 1904 P turns total A + K to 104 S turns,
which cannot be realised with my design above.
However 1.23 k to 2.8 ohms is available,
which could allow 2k to 4.5 ohms.
The 1.23k to 5 ohms also allows
2k to 8.13 ohms.
So adjusting the B+ to suit the RL needs to be worked out.
The higher the Ea and lower the idle current means the
most power is obtained into a high value sec load,
and the lower Ea and high Ia means RL will be lower to get full po.

The OPT No1 design at my website could also be used for an SE
amp with the core gapped, and a couple of P layers
used for CFB windings.
The CFB windings for use with OPT No1 would be two
that are located between anode P sections and S sections, which are just one
layer of wire.
The CFB needs to be insulated from the anode sections with 0.7 mm insulation,
but
between CFB and S layers, the distance can be 0.05mm, because you want
good coupling between S and FB layers, and both are earthy in potential.

BTW, bypassing the cathode R of the 12BH7 driver tube dramatically
increases the thd of the 5 & 6 ohm loads, whilst only reducing
thd of 4 & 3 ohm loads marginally.

The choice of driver needs thought to obtain natural
maximally useful 2H cancelling to obtain the figures I have
without deliberately and erroneously setting up a driver tube with an adverse
load
to create unwarranted extra 2H, which is always then accompanied by
other harmonics.

The thd in my latest comprises mostly 2H, and where it has been reduced by
natural easy cancellation in loads of 5 and 6 ohms, one then can see
the prresence of the other harmonics when the thd is displayed on the CRO.

The 5&6 ohm thd at 1 watt is around the 0.025% mark, about 20 dB lower than
most other SE amps I measure, and the 2H is low, so the remaining other
products
can be viewed as a low level motley collection.
I doubt its important to know what these products are because
they exist at such a low level where the amp is intended for use.

I only measure down to 0.5 vrms of output, which is 50 milliwatts of power,
but able to produce 70 dB SPL in 90 dB/W/M speakers.

0.02% of thd in 0.5 vrms of output is 0.1 mV of distortion signal,
and utterly inaudible, imho, because the noise voltage from an amp like mine
would be about 0.5 mV at least.

There are those who would begrudge me the use of CFB and multigrids,
reckoning that only 300B are suitable for SE, but methinks that the thd I have
in my amp
is no worse than in a 3 stage SE amp using real triodes with the same amount
of applied FB, in order to obtain the same Ro, which means only slight global
NFB be used.

The presence of the 2H in SE amps is not very objectionable, its the muck of
the higher order
harmonics which are the worser problem, and if one were to inspect the thd of
many 3 stage real triode SET amps on a CRO, one may find surprising levels of
odd and high numbered even order harmonics.
Has anyone got thd test figures for an SET amp using 3 stages with 300B output?

I figured that the dips in the impedance of "8 ohm" speakers often appear
in the main power range in the crossover region between bass and midrange,
ie, from 150Hz to 500Hz.
If the thd character of the amp was trained to
produce less than usual thd into 4 to 6 ohms loads, then the dips in impedance
would not cause the distortion troubles that otherwise would occur.

Patrick Turner.





Cheers, Yves.

"Patrick Turner" a écrit dans le message de news:
...

[ . . . ]

Mny thanks !
Quite detailled .... as usual !
Just the gap seems lo, unless you mean 0.25 mm on each leg ?
But good idea to squeeze the FB between two SEC layers !

I've to read twice ... at least !

Cheers, Yves.

Patrick Turner wrote:
Dudes, see the post at ABSE.

I have just posted thd test results from a single ended amp using
4 x 6CA7EH output tubes, cathode biased,
class A1, with 12BH7 driver, and 12AU7 input tube.
Eg2 is fixed, and 12.5% of the P windings are for cathode feedback.

With 5 ohm load, output stage FB is about 10 dB, and global FB = 4 dB.
Ro at the sec = 0.62 ohms, bandwidth 10 Hz to 68 kHz,
perfect stability and capacitor load capability.

Patrick Turner.

Have a schematic handy?


Adam