Just repaired a tube amp , problem unrelated to this.
But it seemed odd to me to have valves , in inverted
orientation , ie under a pcb with SM resistors on it.

Anyone foresee temperature cycling problems/ dislodged SMD in the future
with such arrangements.


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

"N_Cook" wrote in message
...
Just repaired a tube amp , problem unrelated to this.
But it seemed odd to me to have valves , in inverted
orientation , ie under a pcb with SM resistors on it.

Anyone foresee temperature cycling problems/ dislodged SMD in the future
with such arrangements.

As they say "If it can be done, some fool will do it". Given that heat
rises, I see no benefit whatsoever to putting a tube which converts 70% of
the electricity fed through it to heat underneath a PCB. All it's going to
do is shorten the life of the PCB components and, quite possibly, the tube.
There's a reason why most tube amps have the tubes exposed or covered only
by a mesh grille...

just my 2 cents.

On Jul 23, 9:00*am, "N_Cook" wrote:
Just repaired a tube amp , problem unrelated to this.
But it seemed odd to me to have valves , in inverted
orientation , ie under a pcb with SM resistors on it.

Anyone foresee temperature cycling problems/ dislodged SMD in the future
with such arrangements.

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

What the hell kind of amp was it??? Just so I can refuse repairs on
that kind of garbage.....

boardjunkie wrote in message
...
On Jul 23, 9:00 am, "N_Cook" wrote:
Just repaired a tube amp , problem unrelated to this.
But it seemed odd to me to have valves , in inverted
orientation , ie under a pcb with SM resistors on it.

Anyone foresee temperature cycling problems/ dislodged SMD in the future
with such arrangements.

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

What the hell kind of amp was it??? Just so I can refuse repairs on
that kind of garbage.....



Laney LC15 from 2002

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

"N_Cook" wrote in message
...
boardjunkie wrote in message
...
On Jul 23, 9:00 am, "N_Cook" wrote:
Just repaired a tube amp , problem unrelated to this.
But it seemed odd to me to have valves , in inverted
orientation , ie under a pcb with SM resistors on it.

Anyone foresee temperature cycling problems/ dislodged SMD in the future
with such arrangements.

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

What the hell kind of amp was it??? Just so I can refuse repairs on
that kind of garbage.....



Laney LC15 from 2002

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



Well, I think maybe you've answered your own question there. If it has done
six years so far without a problem of the type you are envisaging, I doubt
that it's going to develop one. Those sm resistors are subjected to a lot of
heat and various levels of thermal cycling when they are first fitted,
that's a lot more intense than anything that the valves are going to
generate in normal service. Many amps that I work on, have the output valves
fitted upside down in a pcb-mounted holder, and it doesn't seem to cause a
problem with either the service life of the valves, or the pcb above them. I
have, however, had bad joints on the output valve holders of several Fenders
recently, that have been manufactured with lead-free solder. I don't know if
this is as a result of thermal cycling or not, but I rather suspect that it
is a 'normal' lead-free production problem that we are getting used to
seeing on all sorts of equipment, now.

Arfa