Slightly off top amp question

[Vk2NU]

Apologies in advance for the off topic question!

I've just bought an old FL-2100B amp to do up, and run with my K2 (I have
the bits and pieces on order for the external keying circuit!!).

When trying the amp out I found that I am getting a pretty high input SWR,
and thought I should Align the input coils on the amp.

This seemed to work OK on 40m, and 15 and 10m are not too bad (~2:1), but I
have a very high input SWR on 80 and 20 irrespective of where I tune the
slugs.

It looks like the valves in the Amp are the original Centron 572B/T160's,
which are lighting up, but are obviously very old (>20 years I would
imagine).

If the valves were on the way out, could this account for my inability to
tune the input coils, or should I look elsewhere for the problem ?

"See I managed to get a reference in there for the K2"!!

Thanks

David, Vk2NU

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[Vic K2VCO]

David Douglass wrote:

> Apologies in advance for the off topic question!
>
> I've just bought an old FL-2100B amp to do up, and run with my K2 (I have
> the bits and pieces on order for the external keying circuit!!).
>
> When trying the amp out I found that I am getting a pretty high input SWR,
> and thought I should Align the input coils on the amp.
>
> This seemed to work OK on 40m, and 15 and 10m are not too bad (~2:1), but I
> have a very high input SWR on 80 and 20 irrespective of where I tune the
> slugs.
>
> It looks like the valves in the Amp are the original Centron 572B/T160's,
> which are lighting up, but are obviously very old (>20 years I would
> imagine).
>
> If the valves were on the way out, could this account for my inability to
> tune the input coils, or should I look elsewhere for the problem ?

Are you getting reasonable output on the bands where the SWR is OK?  If
so, then the problem is not the tubes (valves).  Check that the
bandswitch is indexing properly.  Also, a problem in the output tank
circuit can cause a high input SWR -- look very carefully at the
bandswitch contacts to verify that they are not burned and are indexing
correctly.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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[peter gerba]

Hi David and Vic;

David in addition to Vic's suggestion you might want to check the input
circuit on each band with a grid dip meter to confirm the settings. If bad,
look at the slugs in the input coils (if used) for cracks or breaks or check
the compression Caps (if used) for proper operation.

Is the grid current the same on all bands, proper output or not?

Vic is on the money with input switch problems.. check the contacts. Clean
them and check them again.

gl, pete

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]]On Behalf Of Vic K2VCO
Sent: Tuesday, June 27, 2006 7:52 AM
To: David Douglass
Cc: [hidden email]
Subject: Re: [Elecraft] Slightly off top amp question


David Douglass wrote:
> Apologies in advance for the off topic question!
>
> I've just bought an old FL-2100B amp to do up, and run with my K2 (I have
> the bits and pieces on order for the external keying circuit!!).
>
> When trying the amp out I found that I am getting a pretty high input SWR,
> and thought I should Align the input coils on the amp.
>
> This seemed to work OK on 40m, and 15 and 10m are not too bad (~2:1), but
I
> have a very high input SWR on 80 and 20 irrespective of where I tune the
> slugs.
>
> It looks like the valves in the Amp are the original Centron 572B/T160's,
> which are lighting up, but are obviously very old (>20 years I would
> imagine).
>
> If the valves were on the way out, could this account for my inability to
> tune the input coils, or should I look elsewhere for the problem ?

Are you getting reasonable output on the bands where the SWR is OK?  If
so, then the problem is not the tubes (valves).  Check that the
bandswitch is indexing properly.  Also, a problem in the output tank
circuit can cause a high input SWR -- look very carefully at the
bandswitch contacts to verify that they are not burned and are indexing
correctly.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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[Geoffrey Mackenzie-Kennedy-2]

David,

In addition to Vic's and Pete's suggestions, take a close look at the plate
choke for any signs of localized heating, ancient or modern.

I'm flying blind without the schematic and am assuming that the amp has a pi
output network with a RFC at the high Z end for DC feed.

73,
Geoff
GM4ESD

----- Original Message -----
From: "David Douglass" <[hidden email]>
To: <[hidden email]>
Sent: Tuesday, June 27, 2006 2:11 PM
Subject: [Elecraft] Slightly off top amp question


> Apologies in advance for the off topic question!
>
> I've just bought an old FL-2100B amp to do up, and run with my K2 (I have
> the bits and pieces on order for the external keying circuit!!).
>
> When trying the amp out I found that I am getting a pretty high input SWR,
> and thought I should Align the input coils on the amp.



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[Mike Harris-9]

G'day,

| > I've just bought an old FL-2100B amp to do up, and run with my K2 (I
have
| > the bits and pieces on order for the external keying circuit!!).

| Are you getting reasonable output on the bands where the SWR is OK?  If
| so, then the problem is not the tubes (valves).  Check that the
| bandswitch is indexing properly.  Also, a problem in the output tank
| circuit can cause a high input SWR -- look very carefully at the
| bandswitch contacts to verify that they are not burned and are indexing
| correctly.

I had one of these 30+ years ago in Antarctica ('74 through '76).  Twice
it fried fixed contacts on the output band switch.  Can't remember which
two bands it happened on.  The fix was easy.  I had some spare ceramic
switches with silver plated contacts.  I drilled the rivet out, removed
the contacts, did the same with the melted contacts on the band switch,
fitted the replacements, held in place with a fine screw and nut.  No more
problems.

Gone quiet again on the KPA800/1500 front.  Bought an Acom1000 instead.
So looks like my next Elecraft project will be a 50MHz transverter.

Regards,

Mike VP8NO

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