RadioBanter - View Single Post

June 18th 06, 06:12 PM posted to rec.audio.tubes,rec.radio.amateur.homebrew

wrote:

OK, I'm trying to understand the failure of a 807 beam tetrode in a
homebrew push-pull audio amp.

Tubes are GE, about 45 years old, bought as new old stock several weeks
ago.

Being run with 400V on the plate and 255V on the screens.

Both behaved very nicely for a few weeks in my amp, and plate current
at idle followed the curves in the books just fine. They were
well-balanced.

But tonight under some listening stress the B+ fuse blew.

Turning the bias (supplied from a bias supply through a 100K resistor)
down to -40V, the good tube is drawing a small amount of plate current
while the other draws 40mA after warmup and current goes up and up and
up until it hits 100mA or so and I turn the amp off before blowing
another fuse. Check the grid current and as the bad tube warms up grid
current is increasing and increasing as well, to the point where the
-40V is being entirely dropped in the 100K resistor (I guess that means
about half a mA of grid current).

The tube with high current always had some violet-blue spots that
danced on the top but they were on the glass and mica so I figured that
was normal but now I don't know. There was also this strange orange
spot that was on a mica insulator there too.

Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

The violet - orange colouring only ever occurs as a tube dies from gas
intake. Normally
the tubes should have a slight "royal blue" low level flurescence usually
visible in darkness.
the bright blue flickering is the end for the tube; its only good for the
bin unless you
salvage the tube base plug for a multi wire plug.

After turn on with the output tubes in you should measure the voltage
across each bias resistor
from supply to grid pin. Usually the gassy tube draws grid current at idle
with the grid becoming
positive with respect to the bias supply.

Just what has caused the gasiness in anyone's guess but to test the
coupling caps remove the
output tubes and measure dc voltage across the biasing resistors to the
output tubes.
There should be less than a mV after leaving the amp on for an hour.
Using 807 from WW2 NOS could be dodgy since gas leaks could occur; not
enough to turn the getters white
or even slightly brown edged but enough to cause rapid failure well before
the usual 5,000 hrs you should get from a hi-fi output tube.
But then I have just repaired an ARC VT100 and out of 8 brand new Ei 6DJ8
there were two
which were quite faulty because one of the twin triodes in each tube became
gradually saturated
and fully turned on for no reason other than they had been poorly
manufactured in Yugoslavia.
Nice gold lettering on the outside, crummy work inside.

I've burnt up/blown up a lot of 6146 and other variant beam tetrodes
over the years but now I'm trying to get some understanding of what's
happening :-).

Will a certain chunk of NOS 807's simply be this way from age?

Youse takes your chances with ancient 807.
They usually were renowned for their ruggedness but heck, anything ancient
must surely be less reliable than had it been made yesterday to the same
spec.

Would you fly in a NOS Mustang P51 that was NOS?

Not me, that's fo sure.

Maybe I got 20 x 807, I dunno their history, and I ain't got around to
using any; maybe I never will....

Patrick Turner.

Tim.