[GreenKeys] Re: [R-390] Ballast Tubes

Sat Jul 3 10:03:42 EDT 2004

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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