I installed the Measures nichrome kit in a friends Henry 3k/8877. This was
after he did it and blew a tube. The solder connections he made with the
supplied materials was horrible.
I followed the instructions provided and it worked for a while. Blew up
again on 12M when the plate current got high.
Nichrome is a very difficult to solder. We used the solder and flux with the
kit.
To get a good connection, I had to supplement it with some highly activated
Organic Acid flux which I use for lead free soldering. That is why they use
a mechanical crimp in most nichrome to copper connections in consumer
products. Even with the solder flowing around the connections, can you be
assured you have a good connection? I rather doubt it. Then you have a
rather severe thermal cycling at this point as I have seen the nichrome get
red hot in his amp. I suppose the inter-metallic connection in the solder
joints fail. The nichrome wire supplied seems somewhat small in diameter for
a 1 A plate current.
73 N8DX Jack
On Mon, Aug 24, 2009 at 1:28 PM, Bill, W6WRT <dezrat1242@yahoo.com> wrote:
> ORIGINAL MESSAGE:
>
> On Mon, 24 Aug 2009 12:44:13 -0400, "Carl" <km1h@jeremy.mv.com> wrote:
>
> >And also please explain why the natural parasitic frequency of a tube as
> >determined by math and in a test jig doesnt change, except minimally if at
> >all, when in an amplifier circuit.
>
> REPLY:
>
> I had never heard of "the natural parasitic frequency" of a tube until you
> mentioned it in a previous post. I am assuming you mean the combination of
> anode-to-grounded-element capacitance together with the internal lead
> inductance?
>
> Of course that characteristic of a tube is a SERIES resonant circuit and
> will
> not sustain oscillation by itself. You must connect the tube to the
> operating
> tank circuit and by doing so, you inadvertently create a PARALLEL resonant
> circuit which is the actual source of the high VHF impedance necessary to
> sustain VHF oscillation.
>
> The length of those connecting leads is highly important in establishing
> the VHF
> parasitic frequency and shorter is always better. Shorter leads moves the
> VHF
> resonance higher where tube gain is less and parasitic suppression can be
> done
> with a smaller inductance, which reduces the tendency to smoke the
> suppressor
> resistors when operating in the high HF region.
>
> This should not be confused with the natural self-neutralized frequency of
> a
> tube. That frequency applies to the actual operating frequency, the
> frequency
> being amplified, which is entirely different from the VHF parasitic
> frequency.
>
> 73, Bill W6WRT
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