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[AMPS] 3-500Z amplifiers and grid current

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Subject: [AMPS] 3-500Z amplifiers and grid current
From: (Rich Measures)
Date: Tue, 17 Mar 98 11:14:14 -0800
>On Tue, 17 Mar 98 03:18:37 -0800 Rich Measures <> writes:
>>Only in relation to defective anode cooler welds, not to bent filament 
>The welds caused infant mortality. 

I have seen weld failures in Eimac tubes that were more than a year out 
of warranty.  

>The long term problem was real gas,
>not the vanishing kind. I had several bad ones sent to me and they all
>had that telltale glow...or worse. I was only able to recover one out of
>maybe a dozen. 
I rarely see a tube with bent filament helixes that exhibits gas 
>>By what mechanism does an allegedly defective tube damage the 
>>parasitic suppressor R and blow the grid choke?
>A big bang in an amp without a HV surge resistor.
A "big bang" undoubtedly involves some sort of positive hv arc to chassis 
ground.  The path to the chassis can not be through the grid because the 
grid is already connected to the chassis.  .  How can such an occurrence 
possibly damage the VHF suppressor R, which is paralleled by typically 
less than 0.001 ohm of copper wire?  
-  Since the path of a positive hv arc can not be through the chassis 
grounded grid, by what mechanism could the grid choke possibly be 
damaged, Carl?  
>Real proof would clinch it Rich. How about setting up a SB-220, TL-922,
>etc with a spectrum analyzer and try to create the incident ?
What is "real proof".  Tom Rauch suggested that I faked the photographs 
of the parasite-toasted bandswitches.  OJ claimed that 34 photos of him 
wearing the Bruno Magli shoes were all faked, and that proved there was a 
massive conspiracy againt him because he was black.   .  .  A stock Heath 
sb220 has been tested.  The amplifier reportedly exhibited consistent 
damped ringing at the anode resonance of c. 110MHz when driven with high 
speed dits.  However, the 220 did not sustain a full blown event during 
the test.  Does the damped vhf-ringing magically go away when low vhf-Q 
parasitic suppressors are installed?  Nope, but the peak amplitude 

>Until that occurs I find it a real stretch to believe that a brand new 9
>year old amp and tubes would harbor a dreaded parasitic at turn-on. 
- Perhaps it's not all that stretchy.    During Final Test, a well-used 
pair of tubes are typically plugged in whilst the frisky Eimac beauties 
remain safely tucked away in their factory-sealed boxes.  .  Should one 
expect that tired old tubes have as much gain as new tubes? .  .  .  I 
have a tube that was mfg. in 1967, which still works, upon which I will 
confidently wager a large pizza with 4 toppings and salad bar, that it 
won't oscillate.  .  


R. L. Measures, 805-386-3734, AG6K   

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