[Amps] Source of Mica Caps?
Carl
km1h at jeremy.mv.com
Mon Jun 3 14:26:45 EDT 2013
I have 2 TL-922's on the bench as I type. Both had a tube arc that opened a
grid choke and prevented any further damage with their original 1989 Eimacs.
Since those are among the troublesome date codes a replacement tube will get
them back to their owners while directly grounding could have cost a lot of
money.
I'll let amp owners decide which choice they want and hopefully not be
swayed by a lot of published nonsense from a tiny minority. If an amp
becomes unstable replace the parasitic suppressors with something resembling
the up to 40+ year originals and not voodoo nichrome crap. Learn how to
diagnose and fix a problem and not a symptom.
Ive been converting amps to 6M since the mid 60's and have gone thru several
suppressor designs; if an old one failed then replace the resistor.
Grounding the grids "may" work and they "may" also take off with new tubes;
my experience is it "will" happen at times especially with Chinese tubes. I
leaned to the side of stability and consider suppressor resistors as
consumables just as spark plugs and modern resistors last longer just as do
ignition system components.
Stop constantly whining about the SB-220 choke voltage drop. It affects
nothing and if it bothers you that much put an electrolytic across the
zener.
Carl
KM1H
----- Original Message -----
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Monday, June 03, 2013 12:42 PM
Subject: [Amps] Source of Mica Caps?
> Date: Sun, 2 Jun 2013 19:58:58 -0400
> From: "Carl" <km1h at jeremy.mv.com>
> To: "Jim W7RY" <w7ry at centurytel.net>, "Randall K Martin"
> <rkmassoc at comcast.net>, <amps at contesting.com>
> Subject: Re: [Amps] Source of Mica Caps?
>
>
> I still dont understand the claim made by a few that increasing gain and
> power output increases stability when hard grounding the grids.
>
> I would take various web sites with a grain of salt and try to separate
> the
> good from bad.
>
> The 115pf caps were chosen by Heath at the end of the SB-220 run and a
> bulletin suggested changing them. The SB-221 and HL-2200 use the 115pf but
> Ive replaced with whatever 110 and 120pf I come across in bulk. Ive yet to
> have instability in a well built and updated SB-220, even on 6M, with the
> Heath design.
>
> Carl
> KM1H
>
> ## TL-922 are rock solid when oem suppressors used and grids directly
> grounded, and chokes tossed.
> Ditto with drake L4B..and just about every other amp that floats the grids
> with caps... including SB-220.
>
> ## Sb-220 with grids directly grounded to chassis works superb on 6m.
> I have seen KM1H SB-220
> 6m conversions, where the grids were grounded to chassis...after the fact
> by the owner..works great.
>
> ## The 25 ohm DC resistance of the chokes used in the SB-220 isnt doing
> you nay good..just more
> unwanted V drop. The DC resistance of the chokes used in the Drake
> L4B is less than 1 ohm.
> I still tossed the chokes and caps in all 4 of my L4B amps.
>
> Jim VE7RF
>
> .
>
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