>Luck doesn't matter so much here.
>Tubes like 8877 are inherently good under this perspective and do not
>require VHF suppressors, other tubes aren't stable and suppprssors must
>be installed, that's all.
** Not quite, Mauri. The 8877 has about 2/3 as much feedback-C as the
3-500Z, but it has about 2x as much gain at 100 - 150MHz. 3-500Zs have a
history of vhf parasite problems, so the 8877 should not be absolutely
immune to oscillation at vhf/uhf.
- About half of the kaput 8877s I have awaiting autopsy have evidence of
loose gold meltballs, a condition that causes anode to grounded-grid
arcs. Eimac's Specifications Engineer W. Foote told me this is caused by
an "oscillation condition" that causes thin layers of gold to boil off of
the grid and create current leakage. [see Figure 24 on my Web site].
>If the amplifier electrical design, material layout and casting is done
>properly, an amplifier using tubes like a 3CX1500A7 is stable.
>Of course this requires that critical points external to the tube are
>known an managed properly, compromises or mistakes (i.e. choke
>resonaces, XL in grid connections to chassis, etc.) lead always to
>symptomatic cures that sooner or later damage something, possibly the
>most expensive devices that "fuses and protect" the most cheap resistor.
>In analog circuits and maximally in RF I trust experienced designers
>rather than occasionals or "pure academy" engineers.
>
>73,
>Mauri I4JMY
>
>
>
>> -
>> The point is you might have just gotten lucky not
>> including VHF suppressors. Although you had
>> good results, could you be sure there would be
>> no surprises in various tube circuit combinations..?
>> -
>> I would expect the only real proof of a stable circuit
>> would be a track record over time. I would also
>> consider the addition of suppressors a practical
>> addition and some measure (no pun intended)
>> of prevention. Pictures of damaged tubes make
>> me nervous.
>> -
>> cheers
>> skipp
>> -
>> -
>> From: "John T. M. Lyles" <jtml@lanl.gov>
>>
>> I got the RFPP HFS1000G converted to 5 MHz a week back, at work. It
>> was at ISM freq 13.56 MHz. Now it is driving a dedicated experiment,
>> easily providing a kW output. Efficiency is about 69% not counting
>> drive feedthru. The 8877/3CX1500A7 takes plenty of abuse without even
>> flinching. It self biases via cathode resistor only, at about 200 mA
>> idling. Dumping about 600 watts in the anode in this case. With drive
>> it is better.
>>
>> Changed the input to a simple pi, low Q, with no tuning needed.
>> Output L was replaced with 3/16 refrig tubing coil, 12 turns, about 7
>> uH. Adde
>amic doorknob caps, one NPO
>> and one N750, for the C-tune. C-load padded with a stack of heavy
>> duty surplus Erie RF chip caps at 670 pF. Used Hp impedance meter
>> looking at the anode, with a 50 ohm load on the output. Set the
>> controls for 2500 ohm loadline at the tube, and then it came on first
>> time, hardly needed any tuning. By the way, the 8877 doesn't have
>> parasitic suppressor. Its very stable - no surprises. Using an ENI
>> 240L 40 watt solid state broadband amp as the driver for the 8877.
>>
>> 73
>> John
>> K5PRO
>
>
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- Rich..., 805.386.3734, www.vcnet.com/measures.
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