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Re: [Amps] Questions about Amps/Tubes/Design

To: "Gary Hinson" <>, <>
Subject: Re: [Amps] Questions about Amps/Tubes/Design
From: "jeremy-ca" <>
Date: Sat, 15 Mar 2008 20:14:21 -0400
List-post: <">>
Here is what I sent to another ZL Gary.

Replace the horrible nylon washers between the ceramic spacers and wafer
with a RF grade plastic or Teflon.

Build up the junction between the wafer and the spacer & insulators with Q
Dope or melted polystyrene. You want the switch solder lug to have
absolutely no path to the metal screw holding the sections together. All the
ones Ive had to repair or replace arc from the lug and thru the nylon.

Super glue or epoxy an insulating spacer between the 160M padder lug and the
ground next to it.

Complete new bandswitches are available thru Mouser for around $230 but
Electroswitch wont supply just a single wafer.

Its really not an "easy fix" as it takes time to remove and disassemble the
switch but even a partially eaten away wafer can be salvaged. You also have
the option of swapping sections.

Note that the Dentron MLA-2500 uses the identical switch and the single
wafer in the DTR-2000L is the same. Various 160-10M 7x series Alphas use a
similar switch. Ive mentioned those in case some "dead" ones are available
down there.

Of course any amp with a blown switch can be gutted and rebuilt for 6M! I
just finished an Alpha 76CA.


----- Original Message ----- 
From: "Gary Hinson" <>
To: <>
Sent: Saturday, March 15, 2008 7:59 PM
Subject: Re: [Amps] Questions about Amps/Tubes/Design

>> Amp Supply. The LK-500 had several models and every model had its own
>> production changes. Weak link is the bandswitch configuration
>> that allows it
>> to arc to the support hardware. Easy fix if you do it before
>> it happens.
> I'd be interested to know more about that.  More below.
> My LK550 is still going but not quite perfectly.  Maintenance/repairs so 
> far
> have involved:
> - New set of HV supply smoothing caps and load-sharing resistors, not long
> after I got it (about 1995?)
> - 2 or 3 replacement Jennings vacuum output relays (I guess the QSK 
> circuit
> doesn't stop mine hot-switching: I use semi-QSK now)
> - At least 3 new power switches (no slow-start installed so the PD HT
> transformer thuds at switch on - that's another "one day" project!)
> - Popped a padder cap in the output tuning on 160m (changed 5 paralleled
> 100pF HV RF ceramics, I think)
> - New valves, one due to a heater short caused by heavy-handed removal
> people, others to restore full output after a good decade of use
> - Recently popped a bypass cap in the anode compartment (swapped a bunch 
> of
> bypasses and couplers while I was in there - figured they are getting old)
> The LK550 has 3 x 3-500s and would need about 150W of drive, I guess, for
> full output.  My little old TS850s can't get close, and to be honest I 
> don't
> like to thrash the gear so that plus the 500W license limit here ensures I
> get long life out of the valves  :-)
> Currently I'm having trouble tuning the amp on 80m (plate and tune 
> controls
> both max out - I've changed the padder caps for 80 but that didn't help, 
> so
> now I suspect the bandswitch) and a weird problem on topband (output 
> starts
> at 100W and takes about 5 secs to 'warm up' - no idea what's happening
> there!).
> Any clues on solving the current problems would be much appreciated.
> 73
> Gary  ZL2iFB
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