Thanks Carl,
The suppressors in my 78 are pretty high in value, one measured 88 ohms and
the other 2 around 70 ohms. All three look just fine. This amp was a US
government employee in a past life and came to me in a pretty rough state
and minus the tube skirts and chimneys. Can only assume that if they ran
them like that they must have eaten tubes though the ones that were in it
worked okay. As an aside we found 8 of these Alpha 78's in a pile at a
renderers due to be melted down. They were rescued at what was about US$60
per amp!
Ref the inductance of MOF. G3SEK quotes looking at some 3W MOF resistors at
what looks to me to be 100 ohms and finding 1.5 spiral turns about 4mm
diameter and 10mm in length. He calculates this to be about 3 nanohenries.
Assuming the increase in inductance with resistance is not excessive then a
pair in parallel might be around 2 nanohenries. How is this going to affect
the approximate 50nH coil across them?
I'm not familiar with these sorts of resistors being other than spiral in
manufacture. Can you clarify? Maybe I need to be a bit specific when looking
at them.
If the 3W Matsushita resistors glow then I'm assuming they're dissipating
power. Is it one resistor!? How many ohms is it supposed to be?
Thanks again for the useful feedback, all good stuff.
Martin, HS0ZED
-----Original Message-----
From: Carl [mailto:km1h@jeremy.mv.com]
Sent: 28 July 2010 19:00
To: Martin Sole; amps@contesting.com
Subject: Re: [Amps] More parasitic choke questions
I have yet to hear of an Alpha 8874 suppressor go up in smoke. I get plenty
of the old ones in here for other problems but suppressors R look as new.
Not all 2W carbon composition are the same. Allen Bradley is at the very top
of the list and some of the others arent worth using.
Metal film and MOX vary in inductance depending upon the manufacturing
method with the spiral type having considerably more inductance.
The little 3W Matsushita that Measures likes have minimal L but they glow on
10M when using a 3-500Z or similar power tube. So does the #22 ?? nichrome
wire.
Carl
KM1H
----- Original Message -----
From: "Martin Sole" <hs0zed@csloxinfo.com>
To: <amps@contesting.com>
Sent: Tuesday, July 27, 2010 9:50 PM
Subject: Re: [Amps] More parasitic choke questions
> This was somewhat germane to my original question. Really more about how
> much preferred a purely carbon resistor would be to one of the more modern
> equivalents. Having done a little digging, RADCOM July 2005 and August
> 2009,
> In Practice column, Ian writes that paralleled MF resistors are the
> components of choice for parasitic suppressors of HF power amplifiers. The
> paralleled bit being more to minimise the associated inductance than
> specifically increase power handling. This suggests to me that using MF is
> not likely to be akin to using WW, rather any impact on suppressor
> performance should be minimal and unnoticed in normal use with what has
> otherwise been a very stable amplifier.
>
> I'm thinking 4 resistors in a boxlike arrangement for use in the Henry
> where
> these will fit inside the original coils. For the Alpha 78 I may reduce
> the
> coil length to account for any increase in coil diameter when wound to fit
> over two or three MF resistors.
>
> Steve, I couldn't find your 30 Aug 06 post, found several others around
> that
> time on similar topics but would still be interested to know what you
> measured.
>
> Cheers
> Martin, HS0ZED
>
>
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Steve Thompson
> Sent: 27 July 2010 13:36
> To: amps@contesting.com
> Subject: Re: [Amps] More parasitic choke questions
>
>> It does seem though as
>> if the preferred carbon composition resistors are no longer easily found.
>
> What is preferred about carbon comp? Given the heat and the need for long
> term stability in value they're the least suitable component you could
> choose.
>
> Some years ago I measured some different suppressors(results in a posting
> on
> 30 Aug 06), subsequently I swapped the carbon comp resistors for MOX on a
> couple of them, and couldn't measure the difference. Whenever I measure 2
> or
> 3W MOX resistors in the 20-100 ohms range, the inductance is in the
> regions
> of 2-3nH, which has minimal impact in a suppressor. Inductance is higher
> in
> the 5W ones I've tried.
>
> Steve
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