[Amps] SB-200 Problem

[Richard]

>Hi there.  I have a problem with an Heathkit SB-200 that I'd like to
>throw at the list.  I did a cursory search of the archives and did not
>find anything specific to my problem.
>
>Background:
>
>Heathkit SB-200 Built circa 1977.  Currently in need of a minor rebuild
>for maximum life which I plan to undertake (Harbach power supply, new
>antenna relay, replace some ancient caps, etc...).  Currently has new
>572B's with < 5 hours operating time on them.
>
>Symptom:
>
>While transmitting a while back, I let the smoke out of a resistor that
>is part of the RF choke on the anode clip (one of the 80 5w wirewound
>guys with the coil wound around it).  Obtained new resistor and soldered
>it in.
>
>Proceeded to put amp back in service.  Upon hooking it up, found a clear
>frequency and got ready to transmit.  As soon as the antenna relay
>clicks shut, the plate and grid current peg and another resistor self
>destructs.  This one is off of lug 2 of V2 to ground (right most tube
>socket when looking at the back of it).
>
>I'm thinking that it might be that the antenna relay is not very happy
>with life right now.  I'm a bit of a novice here and wanted to get some
>insight from the group.

€  Newer 572Bs reportedly have more gain than those produced in days of 
yore,  More gain means more ability to regenerate/oscillate.  My 
un-expert guess is that your new tubes had an intermittent VHF parasitic. 
 Lower (than original) VHF-Q suppressors might be in order.  The tradeoff 
is c. 2% less power at 29MHz.  If you have a dipmeter, measuring the VHF 
resonance at the blocking cap. will tell you what the likely freq. of the 
parasite is.  If you want to try a low VHF-Q parasitic suppressor 
retrofit-kit, let me know and we will send one.  They are guaranteed to 
make the amplifier less squirrely or you don't owe us the $13.  
>
cheers, Chuck

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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