On 21 Feb 2004 20:31:34 -0500, (Scott Dorsey) wrote:

Jimmy wrote:
Seems like most of the 6146s I have had to fail was due to shorts. This is
usually sfter many years of service.

Occasionally they fail with screen-plate shorts. If they have not been
damaged by excessive screen currents, you can sometimes bang them hard
against the desk a few time and dislodge the little balls of metal that
cause the shorts.
--scott

have no experience related to 6146's for the 40 years I've been using
them at home and coast radio- as well as TV-transmitters, the only
experience is related to bad design on amplifiers. Particularly bad
control grid or screen supply circuits.
Some cures was described in Radcom Technical Topics by Barry Priestley
G3JGO around 1972 and I have mentioned one on my page for FT901/902
mods http://home.online.no/~la8ak/b21.htm while I believe his
experience was with some Heathkit transceiver, my mods applies for
most similar stages, not necessarily 6146B

73
Jan-Martin, LA8AK
----
Jan-Martin, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/

Ray di Tutto" wrote:

One point you mentioned is very interesting. You wrote that when a tube
gets gassy, plate current goes up to the point the plate can turn red hot.
I'm no specialist and i would have thought that the gas molecules in the
tube would be an obstacle for the electrons so plate current would
decrease.

Vy 73 de HB9SLV

Sorry to be so late getting back to you on this. I had
forgotten I left a thread open over here on .boatanchors...

What really happens is the gas molecules (atoms) become
ionized due to the high plate voltage accelerating the plate-
current electrons to energies above their ionization
potential (typically 10 - 15 electron volts). When
this happens the now-ionized gas becomes a very good
conductor (almost a dead short) and the control grid
is totally ineffective in limiting the cathode-to-plate
current flow. The only thing that can halt the process
is the removal of the plate voltage. Unfortunately,
there is usually enough energy left in the conduction
electrons striking the plate to increase its power
dissipation disastrously.

This process is actually used to good effect in mercury-
vapor rectifiers and thyratrons because that "dead-short"
effect means the losses through the conducting tube are
low. Both of these devices "fire" (ionize) at some
forward voltage and then stay in conduction until the
AC driving them reverses polarity.

Jim Bromley, K7JEB
Glendale, AZ

Usually they don't fail unless abused i.e. not tuned properly, causing
excessive plate dissapation.

Internal elements occasionally fail and short, such as grid to
cathode, or screen to plate, any of these can cause symptoms as you've
described.

Since a new set of finals seems to have cured the problem, I wouldn't
worry about the exact mode of failure, just treat the new tubes with
care and they should last for years.

Gary Hildebrand WA7KKP


Angel Vilaseca wrote in message . ..
Hi

I was recently given a TS 820 and I noticed it behaved strangely:

-output power was only 50 W CW instead of 100.
-one of the tubes plate (the final stage of the TS820 uses two 6146B
tetrodes) glowed red after a few seconds of xmitting.
- on the 40 meter band, when tuning for max RF out while watching the
built-in dial, you could reach 100 W output and even beyond, then the rig
fuse would blow. (I did this only once, before taking out a screwdriver and
opening the rig, realizing that something was really wrong!)

I ordered a new set of 6146Bs and while waiting for them to arrive I
checked all components and voltages in the final stage ( not an easy task
when one knows how the 820 PA is built!). All were OK.
Replacing the 6146Bs with the new ones cured the problem.

Now I should be happy, since my rig is working again right? wrong! I would
like to understand what was going on when running with one of the tubes
bad.

-OK, putting 50 watts out seems only logical if one of the tubes is bad,
but
-why was one of the tubes plate glowing red?
-Was it the good or the bad tube? Which one of the two can I keep as a
spare?
-And why dit it glow red? Could it be that the grid could not stop the
electron flow and the power dissipated in the red hot plate was DC power?

Was it that RF from the good tube went into the bad, dissipating into heat?

A more general question: how do the 6146B tubes fail tipically?
Do they fail quickly or progressively?
Is it because the grid melts?
what about the screen?
what about cathode emission?
what about secondary emission? I read that this was a common problem in the
4CX250B, another tetrode.
What is the best parameter to monitor to see if the final 6146B tubes start
getting tired?

Retrospectively I realize that i operated the rig for quite some time with
only one of the final tubes working! *blush* Could this have caused some
damage to the circuit? or spewed out a "dirty" signal?


73 de HB9SLV