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Re: [Amps] Source of Mica Caps?

To: "Jim Thomson" <>, <>
Subject: Re: [Amps] Source of Mica Caps?
From: "Carl" <>
Date: Mon, 3 Jun 2013 14:26:45 -0400
List-post: <">>
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.


----- Original Message ----- From: "Jim Thomson" <>
To: <>
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" <>
To: "Jim W7RY" <>, "Randall K Martin"
<>, <>
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.


## 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 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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