> ... 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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