On Aug 15, 2005, at 10:57 PM, Steve Thompson wrote:
> R. Measures wrote:
>
>>
>> On Aug 15, 2005, at 2:04 PM, Steve Thompson wrote:
>>
>>>
>>>> - I have observed that serious parasites in 811s can put so much
>>>> EMF
>>>> on the filament, that it shatters into pieces and falls to the
>>>> bottom
>>>> of the glass envelope.
>>>>
>>>>
>>> Even with the amp in standby?
>>
>>
>> I was not there when it happened, I saw the 3 casualties after a Ham
>> brought the AL-811 over for a post mortem. As I recall, he said he
>> heard a pop after concluding the power measurement, and the three
>> filaments stopped glowing. .
>
> The original post described resistor damage when the amp was in standby
> and with no rf applied. In that situation parasitic, or any other,
> oscillation seems highly unlikely.
>
Steve -- At one time. I used to make this argument, but not any more.
Then - In the days of radio yore, spark transmitters crudely made RF
by rapidly switching DC on and Off through a high-Q resonant circuit.
Now - Apparently, the event that starts the snowball rolling downhill
is the damped-wave ringing potential developed across the anode
circuit's natural VHF self-resonance. Since ringing V is directly
proportional to Q, mo' Q is not mo' betta -- except in spark
transmitters, of course. This signal can be seen on a spectrum
analyzer - at roughly 100MHz in this case - each time the DC anode
current increases or Decreases. Thus, when the tubes cut-off, a 100MHz
or so signal briefly appears at the anode. Such a signal would not be
a potential problem if there was no feedback between the (anode) /
output and the (cathode-filament) / input. The anode-cathode C in an
811A is 0.7pf, so three of them provide 2.1pF of feedback-C. At
100MHz, 2.1pF has c. 800-ohms of XC.
- Is there anyone who would like to argue that 800-ohms of XC in a
feedback loop could not support an oscillation in a high-Mu triode?
How about it, W8JI?
- I have heard a number of reports of HF amplifiers that produced a
big-bang as the tubes were being cut-off -- and especially in TL-922s.
On the surface, this looks quite impossible since the cathode's bias
relay contacts are opening at this instant - so the tube should cut off
almost immediately, However, if the tube was beginning to oscillate
while the current was falling, current would try to increase and a
metal-vapour arc could appear across the opening contacts and
substantial current could eventually flow -- albeit briefly - from the
HV PS filter caps.
end
> Steve
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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