Perhaps the sensible approach to this is extend the rubber feet by half an
inch as this is a known cure. There is no obvious place to put a secoind fan
at the air outlet.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N_Cook wrote:

Perhaps the sensible approach to this is extend the rubber feet by half an
inch as this is a known cure.

A simple solution at least.


There is no obvious place to put a secoind fan at the air outlet.

You CAN 'gang' 2 fans in series to increase the air pressure but I've never
needed to do it.

Also, fitting a 'thicker' fan will increase the air pressure. You may find 12,
19 and 25mm versions of certain axial fans. The thicker they get, the more
pressure they generate and that helps achieve good airflow. The CFM figures are
largely just a guidline since there are with zero pressure gradient.

Graham

Meat Plow wrote in message
...
On Wed, 28 May 2008 18:31:00 +0100, N_Cook wrote:

Perhaps the sensible approach to this is extend the rubber feet by half
an
inch as this is a known cure. There is no obvious place to put a secoind
fan
at the air outlet.

So every XR 886 overheats?


Not to the extent of tripping any of the thermal switches but enough to
concern the owner into trying to improve airflow - admirable thinking for a
non-techie.
Just giving greater clearance to the airflow under an amp should not
normally affect the amp temperature though. I would suggest that it is due
to forcing the air to follow a tortuous path so a generic design flaw. I've
heated up the fan motor, etc and there seems nothing wrong with this fan as
well as no change of fan noise reported by the owner.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N_Cook wrote:
Meat Plow wrote in message
...
On Wed, 28 May 2008 18:31:00 +0100, N_Cook wrote:

Perhaps the sensible approach to this is extend the rubber feet by half
an
inch as this is a known cure. There is no obvious place to put a secoind
fan
at the air outlet.
So every XR 886 overheats?


Not to the extent of tripping any of the thermal switches but enough to
concern the owner into trying to improve airflow - admirable thinking for a
non-techie.
Just giving greater clearance to the airflow under an amp should not
normally affect the amp temperature though. I would suggest that it is due
to forcing the air to follow a tortuous path so a generic design flaw. I've
heated up the fan motor, etc and there seems nothing wrong with this fan as
well as no change of fan noise reported by the owner.


What say if an amplifier was designed in such a way that the thermal
protection was intended to operate at some given device temperature, say
the safe working temp of the output transistors, and the cooling
arrangments designed accordingly? Output transistors get hot, heatsink
gets hot, thermal devices trip, amplifier shuts down or whatever it`s
designed to do.

But, by increasing the airflow from the fan[1] or improving the cooling
to the heatsink in some other way, results in the heatsink remaining
cooler when the actual output devices themselves are still running at a
high temperature. Bearing in mind that the temp sensors may not be
directly monitoring the devices temp, but some other area on the
heatsink, that might lead to the thermal protection failing to operate.

By changing the design of the cooling, you could be blowing cool air
over the temp sensors and not the output devices. Result = a trip to
silicon heaven.

I have seen Peavey mixer amps shut down in hot weather when placed in
direct sunlight and driven hard, that`s the protection doing what it`s
designed to do tho. Generally speaking, if a decent quality amplifier is
overheating, it`s operator error, not a design fault. (IMO)

Ron(UK)


[1] This is hypothetical of course, I realise you havent changed the fan
or it`s cooling path in this instance, but I think it`s something to be
aware of. Maybe someone with experience in 'designing' power amplifiers
could comment?

Ron(UK) wrote in message
...
N_Cook wrote:
Meat Plow wrote in message
...
On Wed, 28 May 2008 18:31:00 +0100, N_Cook wrote:

Perhaps the sensible approach to this is extend the rubber feet by
half
an
inch as this is a known cure. There is no obvious place to put a
secoind
fan
at the air outlet.
So every XR 886 overheats?


Not to the extent of tripping any of the thermal switches but enough to
concern the owner into trying to improve airflow - admirable thinking
for a
non-techie.
Just giving greater clearance to the airflow under an amp should not
normally affect the amp temperature though. I would suggest that it is
due
to forcing the air to follow a tortuous path so a generic design flaw.
I've
heated up the fan motor, etc and there seems nothing wrong with this fan
as
well as no change of fan noise reported by the owner.


What say if an amplifier was designed in such a way that the thermal
protection was intended to operate at some given device temperature, say
the safe working temp of the output transistors, and the cooling
arrangments designed accordingly? Output transistors get hot, heatsink
gets hot, thermal devices trip, amplifier shuts down or whatever it`s
designed to do.

But, by increasing the airflow from the fan[1] or improving the cooling
to the heatsink in some other way, results in the heatsink remaining
cooler when the actual output devices themselves are still running at a
high temperature. Bearing in mind that the temp sensors may not be
directly monitoring the devices temp, but some other area on the
heatsink, that might lead to the thermal protection failing to operate.

By changing the design of the cooling, you could be blowing cool air
over the temp sensors and not the output devices. Result = a trip to
silicon heaven.

I have seen Peavey mixer amps shut down in hot weather when placed in
direct sunlight and driven hard, that`s the protection doing what it`s
designed to do tho. Generally speaking, if a decent quality amplifier is
overheating, it`s operator error, not a design fault. (IMO)

Ron(UK)


[1] This is hypothetical of course, I realise you havent changed the fan
or it`s cooling path in this instance, but I think it`s something to be
aware of. Maybe someone with experience in 'designing' power amplifiers
could comment?


s.e.d added

All very true but you seem to be missing the point.
An amp that works perfectly all right should not suddenly run noticeably
cooler doing exactly the same job in the same gig but just placing fag
packets, lighters etc , whatever was at hand, under each of the feet on the
base.

2 th sw on 2 of the o/p TO3 and one on the heatsink in this case and yes the
air blows over the those 2 th sw as much as the cans of the TO3, in fact
they are directly in the ducted flow of air probably reducing the passing
volume/area by a 1/3 at each end of one of the heatsink ducts, compared to
no th sw in the path.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N_Cook wrote:

An amp that works perfectly all right should not suddenly run noticeably
cooler doing exactly the same job in the same gig but just placing fag
packets, lighters etc , whatever was at hand, under each of the feet on the
base.

You don't understand equipment cooling adequately. It's PERFECTLY obvious to me.
It's all to do with the effect of back pressure on air flow.

Adding more space underneath the unit reduces the back pressure. It's a truly
cruddy design that they did it that way though.

Graham

"Ron(UK)" wrote:

Maybe someone with experience in 'designing' power amplifiers
could comment?

I have done !

There's something worth adding though.

To optmise air flow you need a high 'static pressure'. There's a huge variation
in this characteristic. The best examples I've found are NMB (formerly Boxer) and
Panasonic's Panaflow fans. Be careful to order the high pressure version. They
make versions with various motors speeds. In the case of NMB, the 'best' ones
used to have 'B50' as part of the part number. I forget Panasonic's method of
marking.

DO NOT use cheap 'off brand' fans for applications like this.

I see Farnell have NMB but only as a special US import. You didn't say what size
it is but the popular 80mm size would be for example ....
http://uk.farnell.com/8235414/electr...kl-05w-b50-e00

Read the data sheet here
http://www.eminebea.com/content/pdf/...3/I/3112kl.pdf

to see just how radically. the airflow varies with the Bxx model number.

Graham

N_Cook wrote:

Just giving greater clearance to the airflow under an amp should not
normally affect the amp temperature though.

Yes it will by reducing the back pressure.

Graham

N_Cook wrote:

I would suggest that it is due
to forcing the air to follow a tortuous path so a generic design flaw.

That's what causes the high back pressure (and hence low air flow). For good
cooling the airflow path should be in a striaght line with air vents in and out
as large as possible (consistent with safety regs) and there are some other cute
aspects which are even less well known too.

Graham

Eeyore wrote in message
...


N_Cook wrote:

I would suggest that it is due
to forcing the air to follow a tortuous path so a generic design flaw.

That's what causes the high back pressure (and hence low air flow). For
good
cooling the airflow path should be in a striaght line with air vents in
and out
as large as possible (consistent with safety regs) and there are some
other cute
aspects which are even less well known too.

Graham


Agreed , I'll take a photo later to show how tortuous this path is.

Even more dramatic was an Ashdown amp I had to repair recently for another
fault.
Supposedly fan assisted cooling but the fan "design" location was just
circulating air inside the casing as no baffles etc to guide air in and out
the single "vent" port.
Owner much appreciated the amp running much cooler and being able to feel
air coming out of the vent.

s.e.r added as design issue

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

Meat Plow wrote:

N_Cook wrote:

Perhaps the sensible approach to this is extend the rubber feet by half an
inch as this is a known cure. There is no obvious place to put a secoind fan
at the air outlet.

So every XR 886 overheats?

Would depend how they're operated, load impedance and the style of music
(especially so). I have researched this area extensively and the variations are
enormous.Stnadardised tests such as 1/8 or 1/3 power noise are especially useless.

Graham