Peavey XR 886, 1997, mixer amp

[N_Cook]

In for intermittant loss of one channel but this query is about overheating.
There are 5/8 inch high feet on the base but unless the owners prop up on
blocks the whole base gets noticeably hot.

I will be replacing the mains fan as 10 years of much use but any other
observations from anyone familiar with them ?
Dusty inside but not blocking any air throughput and no reported changes in
fan noise and currently seems the correct sort of air flow for 14W fan.
Design fault concerning airodynamics? There are 2 full width flutes of the
heatsink that go up to the fan face. I would expect these bits of aluminium
to be milled out normally in such situations.

 

[Eeyore]

In for intermittant loss of one channel but this query is about overheating.
There are 5/8 inch high feet on the base but unless the owners prop up on
blocks the whole base gets noticeably hot.

I will be replacing the mains fan as 10 years of much use but any other
observations from anyone familiar with them ?
Dusty inside but not blocking any air throughput and no reported changes in
fan noise and currently seems the correct sort of air flow for 14W fan.
Design fault concerning airodynamics? There are 2 full width flutes of the
heatsink that go up to the fan face. I would expect these bits of aluminium
to be milled out normally in such situations.

Is that the model that has a fan mounted horizontally blowing down onto a skimpy
heatsink ?

Graham

 

[N_Cook]

Is that the model that has a fan mounted horizontally blowing down onto a skimpy
heatsink ?

Graham

Blowing up into an adequate , for fan blown, heatsink with a bank of 12
motorola 70483080 TO3 . As far as I can see the 2 uncut-down heatsink vanes
that go across the fan do little functionally for cooling and probably
disrupt the air flow.

The air has to come in horizontally under the base , up the one inch of fan
axis and then have to abruptly divert horizontally again down the 2
(separated by these vanes) relatively small channels in the heatsink. I'll
take a pic perhaps

 

[N_Cook]

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.

 

[N_Cook]

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.

 

[N_Cook]

half
an secoind
fan


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.

 

[Eeyore]

Blowing up into an adequate , for fan blown, heatsink with a bank of 12
motorola 70483080 TO3

How do you define adequate ? I design my own heatsinks for amps you see and most
others I come across are very inadequate, notably recent Peaveys.

Graham

 

[Eeyore]

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

 

[Eeyore]

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

 

[Eeyore]

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

 

[Eeyore]

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

 

[N_Cook]

How do you define adequate ? I design my own heatsinks for amps you see and most
others I come across are very inadequate, notably recent Peaveys.

Graham

For fan blown o/p devices the heatsink surely does not have to be much more
that a metal matrix for holding the devices to. Does not have to be
multifluted or with those chisseled multi-vanes etc (sorry don't know the
generic term , end up curved ) or a great mass of metal for convection
cooling

 

[N_Cook]

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

 

[Eeyore]

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/electrical-circuit-protection/product.us0?sku=nmb-3112kl-05w-b50-e00

Read the data sheet here
http://www.eminebea.com/content/pdf/TMP00123/I/3112kl.pdf

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

Graham

 

[Eeyore]

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

 

[Eeyore]

For fan blown o/p devices the heatsink surely does not have to be much more
that a metal matrix for holding the devices to.

The larger area the better.

Does not have to be multifluted

That certainly helps.

or with those chisseled multi-vanes etc (sorry don't know the
generic term , end up curved )

Those increase surface area which is a GOOD THING.

or a great mass of metal for convection cooling

It most certainly DOES NOT need to have great weight. But almost none of the
colling in SR amp is classic convection anyway. What typically helps is maximum
surface area of the fins/vanes to create the best opportunity to let the heat
transfer to the air flow, but you can improve even that by 'slotting' the
'vanes' to create a slightly turbulent air flow.

Trust me I could write a lecture on the subject.

Graham

 

[N_Cook]

The larger area the better.



That certainly helps.



Those increase surface area which is a GOOD THING.



It most certainly DOES NOT need to have great weight. But almost none of the
colling in SR amp is classic convection anyway. What typically helps is maximum
surface area of the fins/vanes to create the best opportunity to let the heat
transfer to the air flow, but you can improve even that by 'slotting' the
'vanes' to create a slightly turbulent air flow.

Trust me I could write a lecture on the subject.

Graham

Anyone know the generic name for this type of what I assume is
chisel-action-cut vaned heatsink.
Is it made using a shaper type machine with some sort of chisel or plane
blade?

http://home.graffiti.net/diverse:graffiti.net/chisel_vane_heatsink.jpg

 

[N_Cook]

This is the amp in question
http://home.graffiti.net/diverse:graffiti.net/peavey.jpg
Yellow ring stands in for the 4 inch fan removed (position marked with
Fs )along with metal sheet that encloses the heatsink. The view of this amp
is upside down as the fan is at the base. So drawing air horizonatally
theough a "designed" 5/8 inch space under the amp. Air then goes through the
1 in of fan vertically and then has to sharply divert horizontally to flow
either way through the 3 ducts in each direction. The central duct contains
no active ducts and as far as I can see only interferes with air flow as
they are right up against the fan housing. Air then vents mainly at either
end when the metal closure in place with not enough space to add a fan or 2
at those points and nowhere near the mains transformer and main air outlet
vent . A lot of venting must go up and out the mixer pannel slider holes etc
as well. The 2 TO3 mounted thermal switches are marked S and restrict the
air flow through the uppermost duct in the pic, the leads and spades as much
as the switch body

 

[Eeyore]

You can btw see this in many 'CPU cooler' designs. However it can cause dust and
fluff to collect.

I recall many years ago that there was a 'manufacturers mod' for some
power amp or other ( dont recall which) where you had to bend all the
vanes of the heatsinks - which were like combs - away from the vertical
in various different directions to improve the marginal cooling just
that tiny bit to prevent the thermal protection tripping. Looked damn
ugly but did the trick.

Yes I can believe that. It would help cause the 'turbulent airflow' effect. I first
learnt about this around 30 years ago from data supplied by the IERC heatsink company
and have widely used the technique ever since with measurable success.

You see with purely vertical or horizontal fins widely spaced, the air going 'up the
middle' between them never actually extracts much useful heat. You need to get the
airflow to 'mix' to get the best effect.

What works best Graham, a thick ally heatsink with many thick fins, or a
thick ally plate with many convoluted thin sheet fins?

Within reason, the fins don't need to be that thick at all. Their minimum width is
normally a limitation of the extrusion technique. What's best it have lots of them,
consistent with not creating a dust trap by having them pitched too fine..

Don't make the fins TOO convoluted as this will actually impede airflow. I have my
own 'proprietary' technique that avoids any need to twist the fins which most people
would find counter-intuitive but in a calibrated test produced a 20% improvement in
thermal resistance with a lower weight of aluminium in 2 (superficially to the casual
viewer) identical designs.

As it seems no-one has yet twigged my method, I'm reluctant to give away my 'secret'.

As for the 'plate' its needs to be thickest near the devices and can taper further
out. Thermal resistance calculation are little different to electrical resistance
after all.

Graham

 

[Eeyore]

Anyone know the generic name for this type of what I assume is
chisel-action-cut vaned heatsink.
Is it made using a shaper type machine with some sort of chisel or plane
blade?

http://home.graffiti.net/diverse:graffiti.net/chisel_vane_heatsink.jpg

I'm afraid I found the pic too unclear to be sure what I was looking at.

Is it formed from sheet aluminium ?

Graham