Vic,
Most of those "caddock" style thick film jobs are rated for several
hundred volts standoff. Maybe you torqued the mounting screw too tight?
Or maybe it was the series resistor that died. That *might* be caused by
the RF current flowing through the dielectric in the capacitor formed by
the insulation between the thick film substrate and the flange. Just
guessing.
I don't like to use prepackaged attenuators for these applications
because I always want a non-standard value. As in the 1.65 dB attenuator
for the 3-500's.
Larry
On 15-Nov-10 1:30 PM, Vic K2VCO wrote:
> I see what you did -- simple idea. I used some flat resistors that bolt to a
> heatsink,
> each one rated at 20 watts. I don't recall, but I think they are called
> 'thick film'
> resistors. Not the same as the metal film ones that you used.
>
> Very nice work on your amp. I enjoyed the statement on your site that
>
>> I always start with good intentions but after all the "mods" and
>> "improvements", my
>> projects usually look like the junk they were made from.
>
> because I just determined that my 160-m tank needed a few more turns and,
> lacking a new
> piece of identical coil stock big enough for the whole thing, I just grafted
> a chunk of
> coil onto it! There goes the 'factory' appearance.
>
>
> On 11/15/2010 1:09 PM, Larry wrote:
>> Vic,
>>
>> I think you just got a bad resistor. I've been using similar resistors (from
>> digikey)
>> for my attenuators up to 100 watts and 144 MHz with no issues at all. You
>> can see
>> photos of one of them on the 3-500 amp topic on my web page.
>>
>> 73,
>>
>> Larry - W7IUV central WA - DN07dg http://w7iuv.com
>>
>>
>>
>> On 15-Nov-10 12:38 PM, Vic K2VCO wrote:
>>> I made a 6 db attenuator for the input of my grid-driven amplifier in order
>>> to bring
>>> the drive requirement up a bit and to provide a better SWR for the exciter
>>> during
>>> tuning. I used some film resistors from Mouser. I suspect that these have a
>>> low
>>> voltage rating, because one of them shorted after a few minutes of 10-meter
>>> operation. They didn't show any sign of heating, so I think it was a
>>> voltage/frequency issue and not simply overheating.
>>>
>>> Anyway, I need a 50-ohm attenuator that can dissipate about 40 watts and
>>> can be used
>>> at HF. Anything from 3 to 6 db will work. I had a Henry solid-state amp
>>> that had a
>>> couple of these, which bolted to the heat sink. Does anyone know where I
>>> can buy such
>>> a thing, or how to make one from reasonably priced parts?
>
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