[AMPS] Re: Parasitics

[km1h@juno.com]

On Wed, 20 May 1998 07:13:04 +0000 Tom Rauch
<10eesfams2mi@mass1-pop.pmm.mci.net> writes:
>> 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.

I agree up to a point. The FL-2100 instability is partially due to a
design/layout problem. Long before the Yaesu Bulletin I found that some
simple rewiring in the relay area calmed that amp right down. The bias
change was incorporated in the later FL-2100Z version long before Chinese
or Russian tubes were available so Yaesu must have had problems even with
Cetron tubes but they were very slow in admitting it. 

>
>>  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.

Your first "linear" was grid driven Class C ?   Thats an interesting
statement.
My first amp was a 250TH with 811 modulators; driven by a TBS-50. 
My first SSB linear was a grid driven 4-250A  and a CE10A driver.


>
>> 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!

I dont remember reading that on this reflector.

>
>> 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.

Good point since MFJ/Ameritron has models with just about every tube that
is produced.....talk about marketing confusion....but I have used them as
a parts source for years.

73  Carl  KM1H

>
>73, Tom W8JI
>w8ji.tom@MCIONE.com
>

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