>November 7, 2003
>AL-80B reversing its grid current
>de VO1TV
>Our club station, VO1CRC, uses the AL-80B amp with the 3-500Z tube. Quite
>suddenly, the amp stopped transmitting and the Ig meter actually reversed
>itself and pinned on the bottom of the plastic case. I shut off the amp and
>then turned it on again. All OK... Sure enough, after a short time the amp
>stopped transmitting and the Ig meter reverse-pinned its little self.
*** The likely cause is a bent filament helix. (see Sep.- Oct., 1990
*QST*) When things get hot and expand, the bent filament touches the grid
cage and a filament-to-grid short occurs. When the tube cools, the
short may disappear.. // The hot (1820ºK) tungsten filament/cathode is
apparently bent by the perpindicular EM force which accompanies the large
burst in cathode to grid electron flow during an intermittent osc. at c.
155MHz. If you have a dipmeter, you measure the freq. of the resonance
that supports the oscillation condition by coupling the dipmeter to
either side of the DC blocking cap.
-- [note: this resonance is formed by the anode-C (5pF), in series
with the anode lead L, in series with the DC-blocker cap., in series with
the Tune-C, in series with the chassis-L. The total L is c. 600nH and
the net C is a bit under 5pF]
- The feedback path that supports the oscillation condition is the
3-500Z's anode-cathode C of 0.15pF (c. 7000-ohms of XC), One fix is to
reduce tube amplification c. 150MHz and/or provide a series-resonant
lossy path to ground at the cathode-input to attenuate the150MHz feedback
signal.
>
>Now, the amp will do the reverse-pin routine even before we transmit. The
>problem is getting worse. Because the amp stops transmitting, I conclude the
>Ig meter is accurately telling us that there is a problem. I donít think ití
>s just a metering problem.
** I agree.
>Any idea what the problem might be?? Iíd like to get some collective wisdom
>before opening the case and fiddling with the innards...
>
** Some people will tell you thatVHF parasitic oscillation in a MF or
HF amplifier is absolutely an impossibility. I am not one of them. Mr.
Murphy was right.
>I do have an old tube tester, the Heathkit IT-17. Unfortunately, the
>Heathkit tube manual I purchased in the 1980's was written in January 1978
>and does not list the 3-500Z tube, so I donít know how to test it.
*** I test 'em initially on the bench with a high-potential tester. A
3-500Z with an OK vacuum and a straight filament helix will withstand at
least 5kV between the filament and the grid (2uA point). New tubes test
at c. 8kV.
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