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Re: [Amps] SB-200 repaired

To: "'N1MIW'" <n1miw@cox.net>, <Amps@contesting.com>
Subject: Re: [Amps] SB-200 repaired
From: "Joe Subich, W4TV" <w4tv@subich.com>
Date: Fri, 19 Dec 2008 19:52:30 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>
> ... I may be missing something else, I've done so much! I 
> believe most of my problems to be the choice of resistors. I 
> couldn't find 2 watt 47 ohm and 2 watt 33 ohm resistors at 
> the popular distributors, so I chose metal film instead - 
> WRONG!!! I don't have sophisticated test gear here to see 
> what was happening, but there must be some inductance in the 
> resistors I tried to use which threw off the whole amp.

This is very true ... metal film resistors have significant 
inductance.  They are a very bad choice for both the parasitic 
suppressor and the grid circuits!  For a lesson on the difference 
in resistor types take a look at W8JI's information on VHF 
stability: http://www.w8ji.com/vhf_stability.htm  The resistor 
data is about half way down the page. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: amps-bounces@contesting.com 
> [mailto:amps-bounces@contesting.com] On Behalf Of N1MIW
> Sent: Thursday, December 18, 2008 8:58 PM
> To: Amps@contesting.com
> Subject: [Amps] SB-200 repaired
> 
> 
>    Well, I've finally got this amplifier working back to 
> spec. The parasitic oscillations are gone, output is 500+ on 
> 80 through 15, and just about 500 on 10. All levels are with 
> 70 to 80 watts in. There were a whole bunch of things wrong with it:
> 
> - Parasitic suppressors were around 85 ohms
> 
> - 33 ohm grid resistors were fried
> 
> - 10 meter input coil ferrite slug was broken (overheated?)
> 
> - 1 tube socket was oriented 90 degrees, which left the 
> filament in a horizontal position (I thought these tubes 
> could be oriented in any position?).
> 
> - Grid caps were out of range
> 
> - I had ZERO knowledge when I started this
> 
> ... I may be missing something else, I've done so much! I 
> believe most of my problems to be the choice of resistors. I 
> couldn't find 2 watt 47 ohm and 2 watt 33 ohm resistors at 
> the popular distributors, so I chose metal film instead - 
> WRONG!!! I don't have sophisticated test gear here to see 
> what was happening, but there must be some inductance in the 
> resistors I tried to use which threw off the whole amp. I 
> even tried "newly designed" suppressors for the amp, but they 
> wouldn't work. If there's a carbon comp resistor you want to 
> replace, use the same thing to keep the amp happy.
> 
>    I've learned that if you are having problems with your 
> amp, replace components with EXACT replacements unless you 
> can adjust component values for anything that may change. 
> This amp was designed by intelligent people - no need to 
> change anything since it's a proven design (unless you're 
> making a conversion).
> 
>    I want to thank all of you who responded to the questions 
> I had - this has been quite a learning experience for me, and 
> I'm glad I'm through with it! Now it's time to just use it! 
> It was almost too much for a beginner like myself. I wish 
> everyone here a pleasant & safe holiday season. 73's... Larry N1MIW
> 
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