I wouldn't believe it either if it weren't for years of servicing RF amps. Some
of the old G-G sweep tube amps were so prone to self oscillation, that the
cathodes were killed all together on idle to prevent it. Some killed the plate
voltage at the HV transformer lead with a relay The ones most prone to it had
high interelectrode capacitances. When you look at some of the larger tubes,
their interelectrode capacitances are higher than that with greater potentials.
Keep in mind, they didn't do it all the time, and some of the same model never
had the problem either. The ones that did though, you could tell that the
parasitic supressors resistor had been hot. So to me saying that I've owned one
model for years and never had any problems don't quiet mean that your neighbor
down the road didn't. What causes it on one model from the next is beyond me, I
sure couldn't say, just that I've seen it with my own eyes.
Best,
Will
*********** REPLY SEPARATOR ***********
On 8/16/05 at 3:31 AM R.Measures wrote:
>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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