[R-390] R-390A ballast replacement

[Llgpt at aol.com]

This has to be the "deadest horse that has ever been beaten,"
 
Just put a resistor in there and be done with a useless tube.
 
Les Locklear
Monitoring from the Gulf of  Mexico
Ten Tec  RX-340
Ten Tec RX-350D
Alpha Delta Sloper
Quantum QX Loop
Various  Longwires
CU-2279/BRC  Multicoupler
http://www.hammarlund.info/homepage.html

PS: Yep, no more  R-390xx's, so my 2 cents won't count anymore. No tubes  
either..........



In a message dated 2/13/2005 1:30:23 PM Central Standard Time,  N4BUQ at aol.com 
writes:

Sounds  like a good reason to add four more 26Z5W's  :~P

Barry(III) -  N4BUQ

----- Original Message -----
From: "Bob Camp"  <ham at cq.nu>
To: "charles bolland" <ka4prf at peoplepc.com>; "R-390  HF Receiver List"
<r-390 at mailman.qth.net>
Sent: Saturday, February  12, 2005 10:19 AM
Subject: Re: [R-390] R-390A ballast  replacement


> Hi
>
> The only issue with the solid  state ballast tube replacements is that
> the ones that are easy to  build all rectify the filament voltage. With
> modern diodes this  generates RFI on the filament circuit.
>
> Depending on how your  particular radio is wired and bypassed this may
> be more or less of a  problem to you. There are several postings in the
> archives about hum  modulation on CW signals that tracked back to
> various mods that  rectify the filament voltage. Simply put you are
> doing something that  the original designers of the radio did not
> expect. Since they did not  expect it the bypassing was not set up
> specifically to handle  it.
>
> If you want to get into the technical details here's more  or less what
> is going on:
>
> If you put in a full wave  rectifier bridge ( 4 diodes) and then attach
> a resistor to the output  of the bridge current will flow as long as the
> diodes in the bridge  are forward biased. With normal diodes this
> happens somewhere in  the  1 to 1.5 volt range. When you are below the
> turn on voltage  no current is flowing. Turning the current on and off,
> even at a 1  volt level generates noise.
>
> If you put a capacitor across the  resistor then current only flows when
> the AC voltage is greater than  the DC voltage on the capacitor plus the
> turn on voltage of the  diodes. If the capacitor is charged to say 70%
> or the peak AC voltage  then the current is flowing less than half the
> time. This generates  even more switching noise since the current it
> turning on and off at a  higher voltage.
>
> Now if you put a solid state gizmo on the  capacitor you *may* even
> increase the turn on voltage a bit more. More  is not a good thing in
> this case.
>
> Bypassing and  grounding and filtering is a possibility. Since the
> bypassing has to  go to the ballast tube socket you will only be able to
> do just so  well.
>
> The question is weather it's all worth it. A fixed  resistor soldered to
> a tube base works pretty darn well with normal  line voltage variations.
> They also are very reliable. I have never  heard of a wire wound
> resistor melting and taking out the wiring  harness of an R-390. Of
> course I have not heard of any of the solid  state mods doing that
> either ....
>
>      Take Care!
>
>         Bob  Camp
>          KB8TQ
>