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