[R-390] Ballast Tubes

Bob Camp ham at cq.nu
Sat Jul 3 15:03:53 EDT 2004 


> From: Bob Camp <ham at cq.nu>
> Date: July 3, 2004 10:03:42 AM EDT
> To: Schluensen <schluensen at freenet.de>
> Cc: Forrest Myers <femyers01 at bellsouth.net>,
> Subject: [GreenKeys] Re: [R-390] Ballast Tubes
>
> Hi
>
> The resistor value does not change for 230 volt versus 115 volt 
> operation. The transformer strapping on the primary takes care of this 
> variation.
>
> If you have line voltage that is significantly different than the US 
> normal 120 / 240 volts AC then there may be differences in the 
> resistor value. The same might be true for a 50 Hz system compared to 
> a 60 Hz system. Most of us over here have no direct  with these radios 
> operated off of 50 Hz.
>
> I have several radios that came to me with resistors instead of 
> ballast tubes. No two radios have the same resistor in them. They all 
> seem to work just fine. As far as I can tell people just grabbed what 
> ever was in their junk box when the ballast tube went out.
>
> Here's the basic math on the resistor:
>
> The transformer winding is set up for 25.2 VAC with 115 volts input 
> *and* with the ovens turned on. We don't use the ovens any more. The 
> net result is that the 25.2 VAC is more like 26.9 VAC.
>
> Two tubes are in series with the ballast, V505 and V701. They are both 
> 6BA6W tubes with 6 volt filaments. Most data books show the correct 
> voltage and current for the filament as 6.3 volts and 300 ma.
>
> If we just take the center values then we need 26.9 - ( 6.3 + 6.3) = 
> 14.3 volts on the tube. If we want 300 ma at that point then 14.3 / 
> 0.3 = 47.667 ohms. That's not a real common value.
>
> Obviously a 47 ohm resistor is a standard value and it should work 
> just fine. A 51 ohm resistor would drop the voltage a bit and will 
> give you a bit more tube life. A 56 ohm resistor is probably pushing 
> things a little, but should work with normal tubes. If you want to 
> experiment a little then you can probably go up to 62 or 68 ohms and 
> still have things work pretty well. I would not recommend going below 
> the 47 ohm number for experimentation.  Going that way will work well  
> but it will shorten the tube life significantly.
>
> The radios I have are set up with a 47 ohm, a 56 ohm, and one with an 
> unknown value. At least that's what I remember from the last time I 
> looked at them .... I also seem to remember a radio with a 39 ohm 
> resistor in it at a hamfest. Not my radio so I have no idea if it 
> worked at all.
>
> The 47 ohm resistor will dissipate 5 watts when the tubes are warmed 
> up. It will run quite a bit more than this for the brief period that 
> the tubes are warming up. Normal practice on a resistor is to use one 
> that will dissipate 2X the running power. This gets you up to the 10 
> watt region.
>
> On page 921 of the US DIgikey catalog they have ALSR-10 power 
> resistors listed. A reasonable part number seems to be ALSR10-51-ND 
> for the 51 ohms or ALSR10-47-ND for the 47 ohms. Both are $1.69 making 
> them a lot less expensive than a ballast tube.  Both are rated to 
> handle the turn on power in the tubes. They should be a part that will 
> last forever. They are $0.13 more expensive than the 5 watt parts that 
> would be running at maximum power.
>
> A lot of people make resistors and a lot of people sell resistors. My 
> only connection with Huntingtion resistors is that I had dinner in the 
> town they are in once. I use Digikey for small parts since they are 
> willing to sell you small quantities via credit card. They also seem 
> to have a catalog in German  ....
>
> Mounting the resistors is the next challenge. SInce the resistor 
> replaces the ballast the modules will still be interchangeable between 
> radios. The only thing to be careful about is plugging a ballast tube 
> into a radio with a resistor.  No matter how you do it the resistor 
> needs to go between pins 2 and 7 on the tube socket.  There are a few 
> other possibilities like tying into pin 4 on V505 but I would 
> recommend sticking with the ballast tube socket.
>
> I have two radios that have the resistor soldered into top side of the 
> tube socket (uggg ...). A second radio has the resistor on a metal 
> plug that fits in the tube socket. I have not investigated the room 
> under the chassis for mounting the resistor in a more rational 
> fashion. If you mount the resistor under the chassis you probably 
> should do something on the top side to make it plain that a ballast 
> tube no longer belongs in the socket. Strange things can happen when 
> you are putting your radio back together at 4AM ...
>
> So now we'll see if I got any of that right ....
>
> 	Take Care!
> 		
> 		Bob Camp
> 		KB8TQ
>
>
>
>
>
> On Jul 2, 2004, at 8:33 PM, Schluensen wrote:
>
>> Hi Bob,
>>
>> do you know where I can find more Info about the "3TF7 to resistor" -
>> modification???
>> (socket pins, resistor value for 230Volts AC ...)
>>
>> 73,
>> Frank, DK1LX
>>
>>
>>
>> ----- Original Message -----
>> From: "Bob Camp" <ham at cq.nu>
>> To: "Forrest Myers" <femyers01 at bellsouth.net>
>> Cc: "r-390" <r-390 at mailman.qth.net>
>> Sent: Saturday, July 03, 2004 1:24 AM
>> Subject: Re: [R-390] Ballast Tubes
>>
>>
>>> Hi
>>>
>>>  From what's been said here they are still available for something in
>>> the $30 to $60 range. Not quite a price that would encourage me to 
>>> grab
>>> a couple dozen.
>>>
>>> One modification that has not been mentioned as part of this thread 
>>> on
>>> ballast tubes is probably the oldest of the batch. Grab a plug that
>>> looks like a tube base and wire a resistor to it. The value needs to 
>>> be
>>> right to get the filaments to run right but that's about all there is
>>> to it. It pulls no more power than the ballast tube and it's a 
>>> totally
>>> reversible mod. When the bottom drops out of the ballast tube market
>>> you can plug one of those two dollar ballast tubes in there and 
>>> nobody
>>> will ever know what you did.
>>>
>>> Somehow I doubt that ballast tubes will get cheap again unless there 
>>> is
>>> a Chinese factory we know nothing about making them by the ton. They
>>> are not terribly high tech so it is a possibility. There may be a 
>>> long
>>> forgotten warehouse in South East Asia with a few hundred thousand of
>>> them sitting on the shelves - stranger things have happened.
>>>
>>> If it was my radio I think I would do one of the re-wire mods to
>>> eliminate the beast. The filaments would be un-regulated but there
>>> would be less heat and no additional stuff inside the cabinet.
>>>
>>> The function of the ballast tube in the radio is questionable. With
>>> modern wall voltages the original ballast is running at best on the
>>> edge and at worst over the edge of it's ratings. It's not doing a 
>>> real
>>> good job of stabilizing the filament voltages on a radio plugged into
>>> 120 to 125 VAC. Fortunately for all of us the filament voltage has a
>>> pretty small impact on the tubes in the radio. It's my belief that 
>>> the
>>> problem comes in on the low voltage end of the equation. If you try a
>>> radio on 100 VAC then the ballast is probably a good idea.  Don't see
>>> much of that coming out of the wall outlets around here ....
>>>
>>> Take Care!
>>>
>>> Bob Camp
>>> KB8TQ
>>>
>>>
>>>
>>>
>>>
>>> On Jul 2, 2004, at 4:17 PM, Forrest Myers wrote:
>>>
>>>> Hello All,
>>>>  Came into the shack and found my r-390a dead. A quick check found 
>>>> the
>>>> ballast tube was shot. I've seen a lot going by on ballast tubes 
>>>> these
>>>> past
>>>> few days but am interested in getting a real ballast tube, if I can
>>>> afford
>>>> it, back into the radio.
>>>>  Anyone have a source for a 3TF7?
>>>>  If I must, I'll put in a mod to get around the 3TF7 but would 
>>>> rather
>>>> not.
>>>>
>>>> Thanks,
>>>> Forrest Myers
>>>> AG4ND
>>>>
>>>> -------------------------------------------------------------------
>>>> "Security is mostly a superstition.  It does not exist in nature...
>>>> Life is either a daring adventure or nothing."  -- Helen Keller
>>>> -------------------------------------------------------------------
>>>>
>>>>
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>>
>
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