[AMPS] Re: Parasitics

[Tom Rauch]

> Date:          Tue, 19 May 1998 10:43:06 -0400 (EDT)
> From:          km1h@juno.com
>
> >Back then, tubes were pretty crummy. Even the best state of the 
> >art tubes had VERY long thin leads.
> 
> The 811 is a 30's era design and the 572B is a takeoff on it. Several
> other popular ham tubes go back to the early 40's and are still produced
> today.

Right, that's why I gave them as examples. A pair of 572B's, such as 
used in the Yaesu amp, will oscillate at the operating frequency if 
load is removed from the input and output and the tube biased into 
conduction!

That's why the FL-2100 on standby oscillates and arcs the bandswitch 
on some bands when used with certain tubes requiring more bias for 
cut-off! 

Image the poor technology that allows a tube to have so much feedback 
it is unstable at HF.

>  External circuits weren't much 
> >better, well shielded enclosures with excellent ground plane 
> >characteristics were virtually unheard of,
> 
> Not in most home brew equipment but the quality commercial gear made
> effective  use of shielding, etc. Military transmitters were very stable,
> the old TBK, TBM and TBL  series that I used aboard ship were mid-30's
> designs; failures were almost unheard of and they were still operational
> 30+ years later.

Handbook articles are for hams. 

> Why would anyone want to use the 304TL for an amp? The 304TH was the
> popular tube for early day SSB amps, the TL was used mostly as
> modulators.
> I've used home brew 100TH, 250TH, 450TH and 750TH amps; they all worked
> fine, never did nasty things in linear or Class C.

My first linear used 812's, a lower mu triode than the 811 because it 
was grid driven class C. Most rigs were NOT cathode driven back then.

> But those suppressors appear to help in amps that were marginal to start
> with, either by design or aging of the original suppressor. Not everyone
> has the knowledge, time, patience, parts or test equipment to develop a
> one-off design for their amp.

No argument there. If you are working in the blind, shotgunning 
might be the only viable approach. Suggesting a shotgun approach 
for use on commercial amp might not be a good idea, especially when 
nearly all problems have nothing to do with VHF parasitics.  

> If Rich can make a few $$ by selling an aftermarket replacement so what?

No one cares about that. What I care about is misleading information 
that all problems are rooted in nearly one cause and there is only 
ONE cure. My way or the highway science.

Not only that, many of the other suggestions are harmful...and can 
actually cause amplifier damage. One that is particularry bad is the 
suggestion that a resistor in the grid makes a good "fuse" that 
protects the grid from RF drive damage!

> In most cases the original manufacturer is out of business and/or the amp
> owner cant find new 2W carbons without a $25 minimum order. 

Call Ameritron, ETO, or someone who uses many thousands of correct 
parts each year. The price is less and the part is correct for the 
application.

73, Tom W8JI
w8ji.tom@MCIONE.com

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