[Amps] LK500 Amp Problem

[R.Measures]

On Jul 19, 2005, at 1:53 AM, Adrian Rees (M1LCR) wrote:

> Hi all
> We have an LK 500 Amplifier (the pair of 3-500Z's model) that started 
> to
> spark and arc quite badly last weekend when we tested it ready for 
> IOTA.
>
> We've had a look at the amp, and it appears that the varnish on the 
> anode
> choke has come off, and there are signs of tracking.
>
> Given that it'll need re-winding, what you boys suggest I coat the 
> choke in,
> to stop it from unravelling ?

Hello, Adrian -- I use clear, gloss. polyurethane varnish that I apply 
with my finger.  A suitable type of Cu  wire for HV RFC service is sold 
by electric-motor rewinding shops.   It is sold by weight.  Bring your 
own spool.  Details on using such is covered in 'Amplifiers', Part 4 on 
my Web site.

>
> Any other comments ?

The arcing you describe in the LK-500 amplifier is typically caused by 
the combination of the anode's self-resonance c. 100MHz and the 0.3pF 
feedback-C in the 3-500Zs.  The result is intermittent regeneration c. 
100MHz.  Since the HF tank is a low-pass configuration, VHF energy can 
not pass through, so it runs amuck and arcs the Tune-C and / or the 
bandswitch.  Decreasing the Q of the VHF suppressors will decrease the 
amplification at VHF -- which may stop the oscillation problem.  The 
tradeoff is an additional power loss of c. 2% at 29MHz.
>
> Adrian (M1LCR)
>
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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org