[AMRadio] Amp / Tube question


Jim Candela jcandela at prodigy.net
Thu Nov 24 12:08:32 EST 2005


Hi all, maybe that 3-500Z is gassy, and shorts out in
the presence of HV, OR maybe there is a VHF parasitic
at work here. Check out AG6K's work on the topic. See
link, and excerpt below.

Jim


An excerpt from Richard L. Measures, AG6K:

http://www.somis.org/Oct88qst.html

"I set the unruly amplifier aside for a week and
discussed the problem with some of my
amplifier-builder friends. After some enlightening
technical discussions and a suggestion to have the
amplifier exorcised , I was ready to proceed.

In every HF amplifier design, there is an unavoidable
VHF tuned circuit formed by the anode to ground
capacitance and the total inductance of the wires or
straps between the anode and the output tuning
capacitor. The resonant frequency of this
anode-circuit can be varied only slightly by adjusting
the output tuning capacitor. I measured the
anode-circuit's self-resonant frequency in the unruly
amplifier, with a dip-meter coupled to the wire
between the HV blocking capacitor, and the
anode-choke. I found a very sharp, high-Q dip at
130MHz.

Next, I checked the self-resonance of the
center-conductor of the coax that delivers the input
signal to the cathodes. The input circuit
self-resonated near the same frequency. This was not
good.

Much of the inductance that formed the resonance in
the anode-circuit appeared to be in the 50mm [2
inches] of "U"-shaped #12 copper wire that connected
the HV blocking capacitor to the top of the anode
RF-choke. This innocent looking #12 wire has about
39nH of inductance. At 130MHz this inductance has a
reactance of +j32. I soldered a 5.1 ohm non-inductive
MOF resistor, with "zero" lead-length, across the
"U"-shaped #12 wire to damp the Q of the tuned
circuit. I "fired up" the amplifier on the 14MHz band
and applied drive power. As usual, I saw fire and I
heard a familiar bang. The fuse-resistor exploded
again as did the added 5.1 ohm MOF Q damping resistor
! Thanks to the fuse resistor, the 3-500Zs remained
undamaged and unshorted after this, fifth, full-blown
parasitic-oscillation..

The 5.1 ohm Q-damping resistor's demise was amazing
because it was virtually shorted-out by less than
0.0003 DC ohms of #12 copper wire when it went kaput !
This resistor had an overload rating of 20W for 5
seconds and it had been destroyed in milliseconds. The
only thing that could have so quickly blown away a
tough, essentially DC and HF shorted resistor like
that was VHF current in the multi-ampere range.

I concluded that the anode-circuit self-resonance of
130MHz was probably the culprit due to the 3-500Z's
110MHz+ rating and the fact that the input resonance
was tuned to almost the same frequency. If I could
increase the self-resonant frequency of the
anode-circuit to a higher frequency, where the
3-500Z's excellent amplifying ability was waning, I
suspected that it might reduce the chance for a
parasitic-oscillation.

I also decided that, because of the extremely sharp
dip at 130MHz, the high Q of the anode-circuit was
probably another contributing factor. This problem
seemed to be exacerbated by the fact that high VHF-Q
silver-plated strap had been used for the combination
anode-suppressors/anode-leads. It did not seem logical
to use the highest Q material to build a circuit that
obviously requires a low-Q to prevent the creation of
a transient- induced VHF seed-voltage that could start
a parasitic-oscillation."

--- John Lawson <jpl15 at panix.com> wrote:

> 
> 
> On Wed, 23 Nov 2005, Patrick Jankowiak wrote:
> 
> > Probably a bad tube, any 3-500Z should be fine in
> there. Sometimes, an abused 
> > one will have a deformed grid from overdriving,
> and it's too close to the 
> > plate, or one of the grid wires is broken and
> sticking out towards the plate, 
> > and it can arc. Or the tube could have some gas in
> it, causing an arc. Had 
> > this happen with a few old used tubes before. I
> even blew a fuse on a bias 
> > supply with a 304TH that had a grid-filament short
> once.
> 
> 
>    That is likely what is happening - it was from
> the estate of a silent 
> key by way of a friend of mine - I saw it a his
> place right after he'd 
> unloaded it - I made him an offer he couldn't refuse
> (traded it for a 
> small debt) - but it was as-is...    I've checked
> the tube for gross 
> shorts, none - the fils work brightly so I don't
> thing the tube's very 
> gassy, if it is.  But as sson as the B+ hits the
> plate it shorts 
> internally.
> 
> 
>    Right now I'm scouting for a new 3-500Z - NIB,
> NOS, or Known Good Pull.
> 
> 
> >
> > If you are worried, you may try next time to put
> perhaps a 2 to 5K/100 to 200 
> > watt resistor in series with the plate of the tube
> and the plate cap of the 
> > amplifier, just to check, for a test only. You can
> lay the resistor on a 
> > piece of wood or other insulating material, and
> use 10KV test probe wire, or 
> > just route the leads so they are away from other
> things. This is very 
> > dangerous and must be done with great caution of
> course, and only for a few 
> > moments. -but then the arc in a bad tube will only
> be pretty, and not 
> > destructive (hopefully). -and you can just shut
> off the amp if bad things 
> > happen.
> 
> 
> 
>    Maybe if I also run a small set of Jacob's Lddder
> wires up from the 
> tube, when it shorts I can shut the lights off and
> laugh evilly whilst 
> exclaiming "It's ALIVE!!!   It's ALIIIIVE!!!"
> 
> 
>   Damn electronics anyway. O well - need to get the
> Ranger and the Valiant 
> working - hopefully this weekend.
> 
> 
>    Cheers, thanks, and Happy Thanksgiving!
> 
> 
> John  KB6SCO
> 
>
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