[Heathkit] Re: parasitic suppression - Better Title

[jeremy-ca]

The low frequency parasitic in vacuum tube ham equipment went out with 6V 
electrical systems in cars. Handbooks like to repeat themselves for several 
generations before some one catches on and edits. Some of those antique 
tubes even took off at audio.

And yes, some SS stages can have LF parasitics.

Since this IS a Heathkit reflector I am sticking to the tubes they use; they 
do not lend themselves to LF parasitics.

If you wish to discuss how to tame a 250KW SWBC amp that is a different 
story.

Carl
KM1H



----- Original Message ----- 
From: "rbethman" <rbethman at comcast.net>
To: <heathkit at mailman.qth.net>
Sent: Sunday, August 26, 2007 1:55 PM
Subject: parasitic suppression - Better Title


> Carl,
>
> Pick up one of the Radio Handbooks - look at the equipment design section.
> *Parasitic Resonance*
>
> There are *TWO* types!  A Low frequency type, I.E., your circuit is 
> designed to resonate at 7Mc, and you *also* have a low freq. parasite at 
> 2Mc.
>
> The other is *indeed* the VHF type.
>
> *MY* error was using the term *to ground*.
>
> Neither YOU or I *know* where Jorge is looking, doing, or going!
>
> Since a parasitic suppressor was explained to him once before *and* he has 
> come back again, I CANNOT assume I *know* what he *really* is asking *NOR* 
> can YOU!
>
> The first type has *TWO* possible solutions.
>
> 1. Neutralization
>
> or
>
> 2. Grounding of tube elements, because bypassing will not work in the 
> design.
>
> The second type has ultimately *ONE* solution.  That indeed *is* the 
> coil/inductor with a resistor in parallel with it.  We *normally* see it 
> in the manner that the resistor is INSIDE the coil/inductor
>
> The low frequency parasitic resonance, which *IS* dealt with by the two 
> possible solutions is *still* in existence today.  The question comes down 
> to whether He, You, or I, are building a transmitter *AND* dealing with 
> the final amplifier - Be it an 807, a 1625, a 6146, or even a 6JSC6, (And 
> there *are* others!). *OR* He, You, or I, are building an *external* 
> amplifier, where the *second* type is *MOST* prevalent.
>
> Therefore, your statement * Completely wrong information. * is *NOT* 
> necessarily true!
>
> As I said, the phrase "to ground", *ONLY* applies in the FIRST type - 
> *NEVER* in the second type!
>
> Bob - N0DGN
>
> > jeremy-ca wrote:
>> Completely wrong information.
>>
>> A parasitic is always near the upper VHF frequency limit of the tube and 
>> self oscillation is at or near the HF frequency the amplifier is tuned 
>> to.
>>
>> Neutralization is required when interelectrode capacitances and the tank 
>> circuit are of the correct value to cause self oscillation at or near the 
>> operating frequency. A small portion of the signal is fed back to the 
>> input 180* out of phase.
>>
>> Many without sufficient knowledge of amplifier theory often confuse or 
>> combine the two very different items.
>>
>> A good amplifier tube makes a great oscillator. In the 20's and 30's 
>> tubes were run that way deliberately and were called TNT or TPTG 
>> circuits.This was before crystal control was developed. You can still 
>> hear these rigs on the air during special events, 20's or 30's night, 
>> etc; they can really sound great. Im still looking for a pair of 211's to 
>> build my own. The regenative detector as used in receivers is a form of 
>> TPTG.
>>
>> Grounded grid amplifiers usually have sufficiently low gain that 
>> neutralization is not required but certain tubes still require attention 
>> to detail and short lead lengths. The 811A and 572B (same design going 
>> back to the 211 of 1928 and the original 811 of 1938) are marginally 
>> stable in some amps. The SB-200 is borderline at 10M and the FL-2100 
>> series is less stable and very prone to oscillation due to the incorrect 
>> neutralizing scheme (feedback is in phase!!) used.
>>
>> Modern amps such as the MFJ-Ameritron 811 and 572 series are neutralized.
>>
>> Most ceramic/metal tubes do not require suppressors or neutralizing; the 
>> 8877 is an excellent example of an extremely stable tube.
>>
>> High gain grid driven tubes as used in exciters almost always require 
>> neutralization. This includes all sweep tube and 6146 rigs.
>>
>> Carl
>> KM1H
>
>