[NJARC] Current limiting Devices

[TrainBee]

Hi Jim,

    This opened up quite the conversation yesterday.  Al Klase made what seemed to be a big issue rather simple.  Sure I could go and get the varistor (mail order for me) or I could just drop in a 50 ohm 5W resistor.  The 50 Ohm resistor is the value for the radio under normal operation.  The schematic called for 880 ohms for the start-up, then to drop to 50 ohms.  If I had the room, I could have put a monster (for this size radio) 10W resistor in there.  It didn't take long to make the choice.  Then I got to thinking, all I needed to do was to bleed off some of the unused voltage and have something to absorb the inrush current.  This little tike uses 4 tubes, 50C5, 35W4, 12AU6 and 12AV6.  It's no wonder the original part blew up.  It is about the diameter of a pencil eraser and maybe 3/8" thick.  It couldn't take years of heat.  

    Here in Maine, AM during the day is a pipe dream.  What I couldn't believe was that I was picking up a station, in the basement, at 10:30 AM, from not too far from here on it.  You can't do too much to tweak it to hear better.  This was made for local station use at best.  I'll be curious to see what I can hear with it tonight. 

    So, I hope some of us learned a little bit about varistors and alternate solutions.  Just in case you missed what this radio is, look for a Philco C-579 of the 1954 vintage.  Curiously, someone actually sold one of these a few years ago for $95.00.  Who would have guessed?  I picked this up at a local flea market for $10 and it's in pretty good cosmetic condition.  I attached the Beitmans page if you care to take a look.

Regards and thanks.

Joe Devonshire

 

 

 

-----Original Message-----
From: JIM HEARNEY via NJARC <njarc at mailman.qth.net>
To: njarc <njarc at mailman.qth.net>
Sent: Tue, Nov 29, 2016 6:37 am
Subject: [NJARC] Current limiting Devices

Just remember
Reply = Poster
Reply All = Everyone

_________________________________________________________


I believe the varistor should work.  This reminds me of a problem that I have not been able to solve:
 
"how to you use voltage measuring equipment, aka analog oscilloscope, to measure a very short transient such as a current surge? "
 
The first part I think is solved using a current probe, bench made or store bought, but how do you record the high speed transient.....the old fashioned way....poloroid camera on scope.. and single triggered trace?  What do you use for the trigger?  A scope with memory option makes second part a lot easier...I imagine!
 
Jim

______________________________________________________________
NJARC mailing list
Home: http://mailman.qth.net/mailman/listinfo/njarc
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:NJARC at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/mailman/private/njarc/attachments/20161129/89f95a1e/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: C-579.djvu
Type: application/octet-stream
Size: 396638 bytes
Desc: not available
URL: <http://mailman.qth.net/mailman/private/njarc/attachments/20161129/89f95a1e/attachment-0001.obj>