Ive stayed out of this until now and have been enjoying many of the replies.
The best are from those who never got past reading books or drivel from some
web site and wouldnt know which end of a SA is which. "Documented tests" ?
What does that mean?
As far as an IMD test, Ive done it many times and Heathkit had it right from
the beginning. The difference is minimal but the gain reduction improves IMD
and stability.
Considering the thousands upon thousands of SB-220/221/HL-2200's that are
running fine without the benefit of this and other forums Id say its a waste
of bandwidth.
If amp owners would spend as much time on basic PM such as replacing bad
parasitic suppressor resistors, leaky grid caps, adding a HV surge resistor
in place of RFC-2, and cleaning out the dirt and nicotine from variable caps
and bandswitch, as they do discussing non issues (or creating new ones) they
would not be having problems.
Heath took a few shortcuts but thats not part of this thread.
Carl
KM1H
----- Original Message -----
From: "Bill W5WVO" <w5wvo@cybermesa.net>
To: <amps@contesting.com>
Sent: Thursday, July 30, 2009 1:53 PM
Subject: Re: [Amps] grounding grids
> So there seems to be some difference of opinion as to whether the Heath
> SB-220
> grid-grounding implementation actually suipplies negative feedback or not,
> actually reduces IMD components in the output or not.
>
> We all want to transmit the cleanest possible signal. At the end of the
> day,
> those of us who have SB-220s need to decide whether to remove the Heath
> circuitry and ground the grids directly with short heavy conductors, or
> leave it
> alone. In the absence of incontrovertible engineering evidence (i.e.,
> showing
> the math), perhaps somebody with an SB-220 and a spectrum analyzer needs
> to do
> an IMD profile using each grid circuit and SEE which one produces the
> cleaner
> output.
>
> Bill W5WVO
>
>
> Bill, W6WRT wrote:
>> ORIGINAL MESSAGE:
>>
>> On Thu, 30 Jul 2009 11:28:38 -0400, Bill Fuqua <wlfuqu00@uky.edu>
>> wrote:
>>
>>> vious message applied to the negative feedback in a grounded grid
>>> amplifier due to the cathode and drive impedances.Similar to the
>>> negative feedback due to an un-bypassed emitter resistor in a common
>>> emitter amplifier. I was not referring to a un-bypassed grid
>>> resistor at all.
>>
>> REPLY:
>>
>> An un-bypassed emitter resistor is not negative feedback, it is just
>> gain reduction. For true NFB you have to take a sample of the output
>> and feed it back to the input, out of phase with the input.
>>
>> True NFB always reduces the input impedance because the fed-back
>> signal opposes the input signal and therefore requires more drive. In
>> the emitter resistor case, the input impedance is actually increased.
>>
>> For a full discussion of feedback amplifiers, both negative and
>> positive, see:
>>
>> http://en.wikipedia.org/wiki/Negative_feedback_amplifier
>>
>> Caution: This may be more information than you wanted. :-)
>>
>> 73, Bill W6WRT
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>
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