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[AMPS] impedance of nichrome lower

To: <amps@contesting.com>
Subject: [AMPS] impedance of nichrome lower
From: w8jitom@postoffice.worldnet.att.net (w8jitom@postoffice.worldnet.att.net)
Date: Mon, 15 Sep 1997 10:35:47 +0000
> From:          km1h@juno.com (km1h @ juno.com)
> Subject:       Re: [AMPS] impedance of nichrome lower
To: <amps@contesting.com>
> Date:          Fri, 12 Sep 97 15:46:04 +0000

Hi Carl,

> The way I read Wes' report is that he wound the nichrome suppressor
> himself, possibly from raw material supplied by Rich.....its not clear.

That was my understanding of what he did.
 
> How many turns of wire are on the AL-80 suppressor?

About four turns over a 1/2 watt carbon composition type resistor.
 
> Is the AL-82 the  same?

Yes.

>The reason I ask is that most 3-500Z suppressors I am familiar with
> are 2 to 3 turns since the parasitic is up in the 130MHz region.

That must have been a 220 or something with longer grid pin to 
chassis leads. If you tie the grid terminals to the chassis right 
at the socket, the oscillation frequency moves up to the 180-220 MHz 
range.  

>The 4-5
> turn suppressors are more in line with 572B and similar tubes with
> parasitics in the low VHF region. 

I had to use a bunch of inductance in the new 572B amp I just did. I 
don't have the data in front of me now, but I think it was almost 8 
turns. The 572B's oscillated around 90 MHz, and also on ten meters 
(the neutralizing system cured ten meters, but not VHF of course).
 
> Wes also measured just the wire alone. Was this in coil configuration or
> just straight wire?

In a coil, the same size as the 80B parasitic suppressor inductor.

> Anyone who has worked at VHF/UHF knows that you do not use nichrome,
> stainless or other materials with a high RF resistance....at any power
> level. It kills the Q, and dissipates power.  So why would it not be a
> good material to resist VHF energy in a HF amp?

It is a lossy material Carl, all by itself. You can plainly see it is 
much lossier than the copper wire coil in Wes' measurements, by 
itself.

I think the point everyone misses is if the suppressor coil is LOADED 
with a parallel resistance, it doesn't matter what material you use. 
The Q and impedance of the suppressor is mainly established by 
the inductance in the coil and the value of the parallel resistance, 
NOT by the small series resistance in the wire itself.

Picture this circuit. We have a long anode lead that goes from the 
tube to the tuning cap, and through the cap to the chassis and back 
to the tube. That path almost always looks very inductive at VHF, and 
the stray C of the tube "tunes" it like a parallel circuit.

The suppressor is in series with that lead, not in parallel with it. 

Ask yourself this question. What value of resistance in SERIES 
with that inductance would lower the Q the most?

Of course the answer is the highest resistance we can get!

What we want the coil of the suppressor to do is look like an open 
circuit at VHF, so ALL of the RF path is through the resistance that 
dampens the anode system. That's why the AL80 suppressor coil has 
such a very high impedance at 200 MHz (that's about where the AL80A 
and B oscillate at, if the suppressor is jumpered).

The nichrome is just fine if the PA oscillates at HF, or lower VHF, 
because the current through the inductor is large. But on upper VHF, 
where the current through the suppressor's inductor is small (the Z 
of the AL80B suppressor is well over 10000 ohms at 200 MHz), 
it makes very little difference at all if the coil material is silver 
or carbon!

Wes measured 43000 ohms of suppressor coil impedance on 200 MHz, 
because the stray C of the coil parallel resonates it on 200 MHz. It 
was intentionally DESIGNED that way.

With such a high coil Z, who cares what material it is! All the amp 
ever  "sees"  is the low value carbon resistor in parallel with the 
coil, and that resistor de-Q's and LOADS the anode system.

> content as well as Ni and Ch. If this is standard resistance wire at DC,
> as used in meter shunts,  then why is it not performing the same function
> at RF?   

It does, it's just in a meaningless part of the system at VHF. It is 
in the LF signal and DC signals main path, NOT the VHF signal path!
 
> The tests were run at the microwatt level, what happens at the KW level
> with RF heating? 

Nothing, unless the component is destroyed.

> Why was the ESR not listed for the AL-80 suppressor when it was for the
> NiCh60? The omission is quite glaring to say the least.

You'd have to e-mail Wes and ask him. I have no idea.

> The AG6K kit also includes 4 1 Ohm anode fuse resistors and an additional
> pair of NiCh60 wires  for connection to the plate blocking cap. In all
> fairness, since this is all in the kit, it should have been tested that
> way.   

One ohm is nothing compared to the 100 ohms plus of the suppressor 
resistance.

> I appreciate your offer of amps for independent testing. But would it
> really be a fair test?  As you say the AL-80 is extremely stable as
> should be any decent 3CX800 amp.  How about testing in a less than
> perfect amp....SB-220, TL-922, LK-500, etc ?

I can't offer any amp I don't have.

> After all Rich is selling retrofits to known problem amps...not good
> ones. What about an early design AL-1200 since we all seem to agree that
> was a hard one to tame? 

Nichrome would certainly work, since that amp oscillates BELOW the FM 
band. What also works very well is a resonant suppressor. Ameritron 
uses a resonant in that amp, rather than a traditional suppressor. 

I'm not saying the nichrome is not useful for every application. In 
some cases it can work, in other cases it makes things worse. But 
there is ALWAYS a combination LCR systems that would do the same job 
with less loss on other frequencies.

All things considered, the nichrome is not much different than the 
stock suppressor at upper VHF. 

> If you really want to push your luck we can try SB-200 and 
FL-2100's !  

They oscillate at HF, not VHF. They really need neutralized, not 
nichromed. Another thing to consider is that tube quality, especially 
voltage breakdown, is nowhere as good as it was before Richardsons 
got their dirty mitts on Cetron.

73, Tom W8JI

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