[Amps] GS35b FWB issue

[Han Higasa]

hi Dave

Are you using a series protection resistor in series of
HV capacitor and the cold end of plate RF choke?

30 - 50 ohm 50 - 100 W resistor is enough to protect
diodes, IP metering circuits and tubes etc.

Any arc from HV to the ground is the same state as a short.
With this resistor arc remain small and short and put no damage
on these circuits. I am using two EDHD15K HV diode packs
and a 50 ohm 100W resistor on high-side of HV line for many
years.

A kit from GM3SEK also has this type of protection resistor.

Also I am using a step-start timer on primary line (200V).
For 5 seconds a 30 ohm 100W resistor is connected in series
of primary line and then shorted by a power relay.

FB DX de Han JE1BMJ

----- Original Message ----- 
From: "David H Craig" <davidhcraig at verizon.net>
To: <amps at contesting.com>
Sent: Monday, July 11, 2011 1:10 PM
Subject: [Amps] GS35b FWB issue


>    I lost a diode string on an 8x4 string 1N5408 FWB on my single GS35b 6m 
> HB amp.  To whit, 8 5408s per leg of a FWB for a total of 32 diodes. 
> Presumably a lightening strike / blown lightening arrestor, but now I 
> wonder.
>    I replaced the diodes on the blown string & tested again- worked fine 
> with just HV trans running to the bridge.  Then, put back together w/ cap 
> bank- blown fuse again & no solid HV.  Checked again, another leg of the 
> FWB blown.  Replaced those 8 5408s & tried again with the cap bank inline. 
> Ergo, another string was blown.  Replaced them, and then tested with just 
> HV & the FWB.  No problem.  Then connected the cap bacn (overkill, 12 x 
> 450V in series, nominal 800 MFD Mallories w/ 6 x 27k ohm Rs across each 
> cap) WITHOUT the RF deck connected, and another audible glitch.  Tested 
> all the 5408s in the bank, and 3 of the (just) replaced diodes show x Meg 
> ohm in the WRONG direction (yes, I tested all before & after installation 
> before poweup).  All the rest were fine.  Then, tested each cap in the 12 
> x 800 MFD bank, and discovered nothing (none shorted), but on a hunch, 
> tried to tighten each screw, and low and behold, after 2+ years in 
> service, EVERY screw needed 1/8 to 1/4 turn
> to get tight.  Question:  Could the combined losening of the screws create 
> enough resistance to draw enough current to blow a diode string in the 
> FWB?  I have checked everything else in the PS, and the common denominator 
> seems to be the cap bank.  Am I missing something?
>
>    Without load at least, the high current relay works 100% & the HV 
> soft-start relay is working 100% as is the 24 volt ps that works the 
> relay.  The 2 second TDR that controls the soft-start is similarly fine.
>
>    The 250ohm 50w B- float resistor is also fine (tested).
>
>    Having isolated the problem to the HV supply, I am baffled.  I plan to 
> replace the now-bad diodes, again, but am not sanguine it will work, and I 
> am running out of 5408s...  I don't want to waste bigger diode blocks on 
> this if they'll short too.
>
>    Any advice or suggestions would be appreciated.
>
>    73 Dave N3DB
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