Don Pearce wrote:
On Wed, 08 Aug 2007 07:58:46 +0100, Nick Gorham
wrote:


Phil Allison wrote:

"Nick Gorham"


Here is a link to some pictures of the GM70 amp that I have been working
on for the past eight months.

http://www.lurcher.org/nick/images/gm70/

Circuit is a C3g Pentode voltage amplifier with VR tube screen regulator,
then 300b driver to 1:1 amorphous interstage to GM70 grid. Output TX 7k
amorphous. Driver and 300b supply, series regulator using 6080, ecl82 and
0d3 (Steve Bench inspired). TJ-Meshplate 300b, running 330v @ 55ma. Copper
plate GM70 running 1020v @ 110ma.




** The GM70's heater is rated for 20 volts at 3 amps - means it
dissipates 60 watts, mainly by radiating it to the surrounding plate
structure. The plate dissipates 112 watts on its own - adds up to around
170 watts.

So the tube glows red all the time - right ?





...... Phil





Almost, there is a lot of light from the fillament, but at that op
point, the anode is just below the temperature at which it starts to
glow. The graphite plate version is a dull red at that point (only
visable when the fill is turned off).

if the diss was taken to (say) 130w then it starts to glow visably. As a
experiment we tried to see just what a GM70 would take, we got to 1500v
and 140ma, at 210w it is was glowing red in the center of the plate and
we left it like this for ten minutes until we decided not to push our
luck :-).

That particular GM70 is still going strong, the old USR certainly knew
how to make robust triodes.

There is a lot of ventilation in the base for that very reason, the
convection from below the valve carries a lot of the (non radiated) heat
away from the amp, the TX's near get warm, but not worryingly so.


In my earliest work days I designed colour TVs. We had an X-Ray
detector permanently covering the bench, because if the line output
valve got a few too many volts, and a bit too much current, its anode
would start to glow red, and X-Rays would come flooding out.

I really wouldn't be sitting close to this thing for ten minutes if I
were you.

d


Why?

The x-ray generation is caused by the impact speed of the electrons on
the anode, this is only running at 1kv, I think you need at least 5kev
to start worrying about x-rays.

The red-ness is neither here or there.

--
Nick

On Wed, 08 Aug 2007 09:29:32 +0100, Nick Gorham
wrote:

Don Pearce wrote:
On Wed, 08 Aug 2007 07:58:46 +0100, Nick Gorham
wrote:


Phil Allison wrote:

"Nick Gorham"


Here is a link to some pictures of the GM70 amp that I have been working
on for the past eight months.

http://www.lurcher.org/nick/images/gm70/

Circuit is a C3g Pentode voltage amplifier with VR tube screen regulator,
then 300b driver to 1:1 amorphous interstage to GM70 grid. Output TX 7k
amorphous. Driver and 300b supply, series regulator using 6080, ecl82 and
0d3 (Steve Bench inspired). TJ-Meshplate 300b, running 330v @ 55ma. Copper
plate GM70 running 1020v @ 110ma.




** The GM70's heater is rated for 20 volts at 3 amps - means it
dissipates 60 watts, mainly by radiating it to the surrounding plate
structure. The plate dissipates 112 watts on its own - adds up to around
170 watts.

So the tube glows red all the time - right ?





...... Phil





Almost, there is a lot of light from the fillament, but at that op
point, the anode is just below the temperature at which it starts to
glow. The graphite plate version is a dull red at that point (only
visable when the fill is turned off).

if the diss was taken to (say) 130w then it starts to glow visably. As a
experiment we tried to see just what a GM70 would take, we got to 1500v
and 140ma, at 210w it is was glowing red in the center of the plate and
we left it like this for ten minutes until we decided not to push our
luck :-).

That particular GM70 is still going strong, the old USR certainly knew
how to make robust triodes.

There is a lot of ventilation in the base for that very reason, the
convection from below the valve carries a lot of the (non radiated) heat
away from the amp, the TX's near get warm, but not worryingly so.


In my earliest work days I designed colour TVs. We had an X-Ray
detector permanently covering the bench, because if the line output
valve got a few too many volts, and a bit too much current, its anode
would start to glow red, and X-Rays would come flooding out.

I really wouldn't be sitting close to this thing for ten minutes if I
were you.

d


Why?

The x-ray generation is caused by the impact speed of the electrons on
the anode, this is only running at 1kv, I think you need at least 5kev
to start worrying about x-rays.

The red-ness is neither here or there.

I can't remember the anode voltage we were running back then, but I'm
pretty sure it was closer to 1kV than 5.

d

--
Pearce Consulting
http://www.pearce.uk.com

"Don Pearce Posturing Pommy Prick "


I can't remember the anode voltage we were running back then, but I'm
pretty sure it was closer to 1kV than 5.



** Irrelevant which - neither can be responsible for x- ray generation.




........... Phil