Re: Trafo for PPP



"Blazej Czeladzki (ByCZy)" wrote:

Patrick Turner <info@xxxxxxxxxxxxxxxxxx> wklepal(a) i opublikowal(a) co
nastepuje:
If you don't have active protection against bias failure,
a 100mA fuse soldered between each EL84 cathode and 0V is wise.
I like to use 0.3mm absolute minimum dia.

Could you draw it on schematics?

How should I adjust bias on finished monoblok?

Could you give me some hints for first powerup?

--
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Pozdrawiam:
Blazej Czeladzki

You want to have have 4 output tubes per channel
If I was doing this with EL84, I would have cathode bias RC network for
each EL84, about 270 ohms plus 1,000uF, 35V rated.
And I would have the 100mA fuses between the bottom of the R&C to
ground,
so that if a cathode cap shorts and / or a tube saturates with more than
100mA,
a fuse blows.

If you go to the page at
http://www.turneraudio.com.au/8585-amp-october-2006.html
and go about 2/3 the way down the page, you will find an elaborate
active protection schemaic for a stereo amp with the 8 tubes of the 8585
I have.
Read all about how it works and analyse what you'd need to do to apply
the idea to your amp.
To get this right, you need to know a heck of a lot!

But if all that is too hard, and it it would be for most ppl here,
then just use the cathode bias and 4 fuses.

If one tube out of 4 has a blown fuse, the amp will still work
without you suspecting anything is wrong at lowish levels.

Another schematic for active shut-down prorection and bias balance
indication
is shown at
http://www.turneraudio.com.au/leakampmods.html
Go to the bottom of the page.

Where one does have multiple output tubes with fixed bias,
another schematic for active shut-down protection is at
http://www.turneraudio.com.au/100w-monobloc-2004.html
Go 1/2 way down the page to see the schematic which
turns off the amp if one or more tubes conduct too much dc cathode
current.

With fixed bias, tubes suffer sudden death with bias failure.

I have just re-designed and re-wired a pair of Manley Labs Snappers
and have two leds mounted near each of 4 output tubes with fixed bias
which is adjusted with with 4 individual bias adjust pots.
Always ONLY have separate bias adjust pots, one for each tube, or a way
of balancing bias.
But with the Manleys, the red and yellow LED beside each output tube
is adjusted for equal brightness when the cathode is within 5% of the
wanted figure.
if the tube conducts too little Ik, the red goes out and yellow stays
bright,
indicating its a bit weak or sick, and if too much Ik flows, the
yellow goes out indicating too much Ok.
And if Ik goes to twice the wanted bias Ik for longer than 4 seconds,
the mains tranny
is turned right off, and separate red LED turns on, whaile all other
yellow LEDs are on, and no other red LED.

I don't like Ik meters with a switch,
I don't like reliance on owners having to use a screw driver and turn a
pot to adjust bias,
read a voltmeter, and make sure they adjust the right pot for the right
tube.
If technically illiterate owners can *** an amp up, they will very
easily find a way,
and in the case of the Manleys, the previous owner had mismanaged the
amp badly,
couldn't figure out why smoke happened so often, and sold these amps at
a low price.
The Manley Snappers have VERY GOOD OUTPUT TRANSFORMERS, and were able to
survive trouble when it happened.
Ive done lots more to fix these two amps, but that's a story for another
day.

So all sorts of faults and problems are indicated by the 8 LEDs poking
up
through the chassis so the owner can see if there's a problem in a
glance.
If one tube of 4 goes dead, then the other one in that side of the PP
circuit will show a rising red LED signal while the dead one just shows
the yellow one on.
So just which tube is stuffed becomes obvious without delay.

There is still a fuse of 0.5A between OPT CT and the PSU caps, so that
if a tube
arcs internally, the amp is well protected.

All these protection measures are best designed so a 10VA tranny with
240V : 12V winding
provides independant power to the protection circuit, and if a fault
happens,
the main power tranny is turned off by a relay generously rated for
mains AC and current,
so I use 240V @ 6A, and interupt both active and neutral lines.
The little auxilliary tranny is connected after the mains fuse, but
before the mains switch,
so the protection remains operative if the main PT is off.

Resetting an amp with such protection measures is easy
with a turn off, wait 3 seconds, then turn back on.
If the amp trips off again soon, and you see LEDs telling you something,
heed what they say to you.

With my way, one NEVER has any disasters.

I've had an EL34 develop a dry joint in one heater wire in its base,
and the owner soon saw he had a problem, and came to me asap,
and in ten minutes I found why and soldered the heater, and all was
fixed so easily.

Patrick Turner.
.

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