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Do any manufacturers (either of old-timey tubes or new tubes or amps)
offer specific numeric limits on allowable grid current in a beam
tetrode in class A or AB1? I'm thinking specifically of tubes in the
6L6GC/807/6146 class.

I'm finding that the best of my older tubes have a grid current of a
little less than a microamp when biased at -20 or -25V on the grid,
250V or 300V on the screen, and 500V or 600V on the plate for a nominal
plate current of 30 or 40mA, most are in the low microamp range, and
some skyrocket into the hundreds of microamps or even milliamps within
minutes.

It's possible that the bad/worst ones would be perfectly acceptable in
a transformer-input class B modulator/audio amp or in a class C RF amp.

Tim.

[AuroraOldRadios]

wrote:
Do any manufacturers (either of old-timey tubes or new tubes or amps)
offer specific numeric limits on allowable grid current in a beam
tetrode in class A or AB1? I'm thinking specifically of tubes in the
6L6GC/807/6146 class.

I'm finding that the best of my older tubes have a grid current of a
little less than a microamp when biased at -20 or -25V on the grid,
250V or 300V on the screen, and 500V or 600V on the plate for a nominal
plate current of 30 or 40mA, most are in the low microamp range, and
some skyrocket into the hundreds of microamps or even milliamps within
minutes.

It's possible that the bad/worst ones would be perfectly acceptable in
a transformer-input class B modulator/audio amp or in a class C RF amp.

Tim.
I'm no expert on this subject so I'll refer you to the Radiotron
designer's handbook. In the third edition, it talks about grid current
on pages 243 and 275-277. In brief, it says power valves should not
exceed 2-5mA. With the type 50, 10-15mA is the limit. You should read
it yourself and see if that's how you interpret it. It appears that the
grid current under discussion involves the relative presence of gas
with more gas resulting in more grid current.

[Phil Allison]

"AuroraOldRadios"

I'm no expert on this subject so I'll refer you to the Radiotron
designer's handbook. In the third edition, it talks about grid current
on pages 243 and 275-277. In brief, it says power valves should not
exceed 2-5mA.


** The OP is talking about * grid leakage current * ( ie a tube fault)
NOT "grid current" caused by the operating mode.




......... Phil

[Phil Allison]

wrote in message
ups.com...


** Groper Alert !!

Do any manufacturers (either of old-timey tubes or new tubes or amps)
offer specific numeric limits on allowable grid current in a beam
tetrode in class A or AB1? I'm thinking specifically of tubes in the
6L6GC/807/6146 class.


** You mean grid leakage - not "grid current".


I'm finding that the best of my older tubes have a grid current of a
little less than a microamp when biased at -20 or -25V on the grid,
250V or 300V on the screen, and 500V or 600V on the plate for a nominal
plate current of 30 or 40mA, most are in the low microamp range, and
some skyrocket into the hundreds of microamps or even milliamps within
minutes.

It's possible that the bad/worst ones would be perfectly acceptable in
a transformer-input class B modulator/audio amp or in a class C RF amp.



** It is possible - depending on what is the cause of the * LEAKAGE*.

In many cases it is nothing more than surface contamination on the plastic
base of the tube causing LEAKAGE current to flow from the screen pin to the
ADJACENT grid pin when the surface is hot.

Fixed a whole bunch of new EL34 tubes that did that by repeatedly washing
the bases in de-natured alcohol and drying in hot air. Other cases proved to
be the plastic material itself - the Chinese and Ruskies have had some
batches of very crappy plastic.

The only reason such leakage is a problem is that the tube base gets very
hot in use ( ie with tubes hanging inverted) PLUS the negative grid bias is
applied via a high value resistor. Smart designers keep that resistor value
low as possible and provide ventilation around all output tube bases. Using
ceramic sockets is a damn good idea too.

Amps that use transistors to drive the grids or cathodes of output tubes
also avoid the issue.

So could an amp that use a coupling transformer.





....... Phil

[Patrick Turner]

wrote:

Do any manufacturers (either of old-timey tubes or new tubes or amps)
offer specific numeric limits on allowable grid current in a beam
tetrode in class A or AB1? I'm thinking specifically of tubes in the
6L6GC/807/6146 class.

I'm finding that the best of my older tubes have a grid current of a
little less than a microamp when biased at -20 or -25V on the grid,
250V or 300V on the screen, and 500V or 600V on the plate for a nominal
plate current of 30 or 40mA, most are in the low microamp range, and
some skyrocket into the hundreds of microamps or even milliamps within
minutes.

The ones that develop milliamps of bias grid current and hence maybe
several
+ volts across a 470k bias resistor are stuffed tubes, with gas or emitting
grids
or some leakage fault.
The makers usually specifiy the idle grid current, but seeing +0.05Vddc
across
a 470k bias R is OK. New tubes usually always have a slight -ve vdc across
the bias R but as tubes age the
grid tends to go positive, and the tube can sometimes run away thermally.





It's possible that the bad/worst ones would be perfectly acceptable in
a transformer-input class B modulator/audio amp or in a class C RF amp.

Just make sure that the grid current is low and the anode current remains
under control of the
grid bias voltage regardless of grid current.
Use slow blow fuses in each cathode to 0V path = about 3 times the idle
current.

Patrick Turner


Tim.

[Ned Carlson]

wrote:

Do any manufacturers (either of old-timey tubes or new tubes or amps)
offer specific numeric limits on allowable grid current in a beam
tetrode in class A or AB1? I'm thinking specifically of tubes in the
6L6GC/807/6146 class.

Some do, some don't, usually they at least give a maximum
DC grid circuit resistance spec depending on how the tube
is used (cathode or fixed biasing). With the correct value
of grid resistor, there shouldn't be much grid current,
although there's some exceptions, but not for the tube
types you're talking about.


I'm finding that the best of my older tubes have a grid current of a
little less than a microamp when biased at -20 or -25V on the grid,
250V or 300V on the screen, and 500V or 600V on the plate for a nominal
plate current of 30 or 40mA, most are in the low microamp range, and
some skyrocket into the hundreds of microamps or even milliamps within
minutes.

That sounds bad, but what value of grid resistor are you using?
If it's too high, you'll have problems. Also, are you getting
any parasitic oscillations? Sometimes getting rid of that
is a headache. I've seen amplifiers where the physical layout
of the circuit itself (You've worked on RF stuff, so
I'm preaching to the choir here) was a problem.
Also, is this in an amplifier or a test rig? If in an amp,
is the output loaded and the input shorted?

BTW, 300V is too much for 6146 screens.

It's possible that the bad/worst ones would be perfectly acceptable in
a transformer-input class B modulator/audio amp or in a class C RF amp.

Modulator, possibly, as the DCR in the grid circuit would
be very low, but you're talking about zero-signal, the
problem should be worse if the grid is being forced
to draw current. RF? If it's not stable at DC, probably
less so at RF. Pretty much, a tube that's a dud for AF
shouldn't be any good for RF, either.


--
Ned Carlson
SW side of Chicago, USA
www.tubezone.net