Re: [Amps] Source of Mica Caps?

["Carl" <km1h@jeremy.mv.com>]

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@telus.net>
To: <amps@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@jeremy.mv.com>
> To: "Jim W7RY" <w7ry@centurytel.net>, "Randall K Martin"
> <rkmassoc@comcast.net>, <amps@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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