Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Help I need a Rocket Scientist!!!!
#1

Hi Guys I found a little Kadette Jr to play with. It's a 1933 boy's radio, two tube job. First tube is a 6F7 which is the RF amp and Det stages. The second tube is a 12A7 it's the rectifier and audio output. Now for the RS part to achieve the heater voltage/current it use a resistor line cord. I need to lose 112vac @ 300ma what value condenser do I need to replace the resistor in the cord???
http://www.nostalgiaair.org/PagesByModel...009608.pdf
Here's a pic of one, not mine it hasn't gotten here yet. http://www.radiolaguy.com/Showcase/Plast...tte-Jr.htm
TNX Much!!
Terry
#2

Terry

What is the summary voltage of the heaters, as it dpends whether 12A6 has the halves of filament paralleled or not. Is it 18V or 12V?
#3

Meantime consideing you are dopping a lot, roughly your resistor should be just that, R=112/0.3 Ohms.

Then do C = 1/(6.28*60*R) in Farads (7 uF roughly).
#4

No tap on the 12A7 heater so it's 18v @300ma.
Terry
#5

The following link has the formula and near the end another link to a spreadsheet that will calculate it for you:

http://www.vintage-radio.com/repair-rest...calcs.html

John KK4ZLF
Lexington, KY
"illegitimis non carborundum"
#6

6.7 uF.
#7

As a true rocket sciantest I agree with the replies above. Icon_mrgreenIcon_mrgreen

Times I have been electrocuted in 2021
As of 1/01/2021
AC: 4 DC: 1
Last year: 6
#8

Terry

Here's how you do it.

You have 120V.
In order to drop what you need you do not care how much you drop, you care to keep the current 300mA, this way you keep your voltage 18V and drop the rest.

So

1. your current is your V over Z, where Z is total impedance.
I=V/Z=120/Z
I= 300mA= 0.3A
Z=V/I=120/0.3=400 Ohm


2. Express Z via Rc (capacitive impedance) and the R active (filament).
Z=SQRT(R*R + Rc*Rc) =400

3. Your R active
R= Vf/0.3= 18V/0.3A = 60 Ohm

4. Find cap impedance
R*R + Rc*Rc= Z*Z= 400*400= 160000
Rc*Rc=160000-3600=156400
Rc=395

5. Find Capacitance
Rc=1/(6.28*f*C)
C=1/(Rc*60*6.28 )=1/(395*60*6.28 )=6.71*e-6 F= 6.71 uF
#9

That little Kadette doesn't have very much spare room inside for a dropping capacitor so you might want to consider running it off a variac or set-up a transformer for it. If you do that I'd put something other than a standard AC plug on the cord so someone doesn't plug it into an outlet by mistake.

John KK4ZLF
Lexington, KY
"illegitimis non carborundum"
#10

I was doing Kirk's little PT something Philo, really small one, I got 6.2uF dropping cap inside.
#11

It might be able to be squeezed in there somewhere, but take a look at how small this little guy is:

http://www.radiolaguy.com/Showcase/Plast...tte-Jr.htm

They're really neat radios, especially for 1933, but there isn't much free space inside one of these guys.

John KK4ZLF
Lexington, KY
"illegitimis non carborundum"
#12

You are correct, it is tight. If there is such thing as 250V cap of such a value, it might be smaller than the one I was thinking off.
Terry will have to look into it.
#13

Probably not a good idea to use two electrolytic cap back to back? It is very cramped in there.
Terry
#14

No...not a good idea. The thing you are trying to do is to create a non-pol lytic.
This is possible ONLY if you pull the centepoint to a potential that is ALWAYS above (if it is the two plusses) the voltage you are passing through. But, if you have a transformer the two voltages are not referenced to each other, and if it is an ACDC set, then such potential may or may not be present in the schematic.


See if it is even remotely possible to cram the cap inside without touching something hot.
Or if you are willing to sacrifice some aesthetics and dissing by purists, if you could put a film cap in a box and attach it to the cord somehow to make the third wire for filaments; a 3-wire cord could be used.
#15

Here's a source for suitable caps. 7.5 uf shown, go to links to see other options. Small and relatively inexpensive.
http://www.digikey.com/product-search/en...itor+7.5uf

These are true AC rated caps....not just DC rated in some derated configuration. Dielectric differences, etc, etc




Users browsing this thread: 2 Guest(s)
[-]
Recent Posts
Jackson 715 not working
Mrfixr55, its working now. I cleaned the switches and sockets with spray cleaner. Thanks for helping on this one.daveone23 — 06:26 PM
Jackson 715 not working
Now that you posted the schematic, I don't know why that Sprague electrolytic cap is across the meter, as it is not indi...MrFixr55 — 05:51 PM
HiFi (Chifi) tube amp build - but my own design.
What may be lacking in the PP Tube amps may be the 2nd harmonics, which some, especially RCA back in the day called &quo...MrFixr55 — 05:32 PM
Jackson 715 not working
Usually in an emission tester, the tube under test is measured as if it were a diode. So, some testers connect all the g...RodB — 04:17 PM
Restoring Philco 37-604C
Yep. F5 is green, D5 is Red. Red is Bad. Green is Clean.morzh — 01:30 PM
Jackson 715 not working
I did start to do that but I stalled out because I could not figure out how the grid and plate get voltage. In this diag...daveone23 — 11:52 AM
Restoring Philco 37-604C
(Insert Homer Simpson "DOPF" Here.) When all fails, look at the can. Took the Ron Ramirez advice, red Caig D...MrFixr55 — 09:23 AM
Philco 91 Speaker Replacement
From your text I am not sure if you intend to use the existing speaker with a resistor instead of the field coil. It wo...morzh — 08:44 AM
Philco 91 Speaker Replacement
My field coil is bad. I am still hoping to find an original, but if I can't I will go with a fitting Philco speaker, 125...dconant — 08:34 AM
Philco 91 Speaker Replacement
As Rod said, it is OK to use a fitting speaker, and then look for an original one. If you buy a Hammond 125 output tr...morzh — 08:15 AM

[-]
Who's Online
There are currently 3853 online users. [Complete List]
» 1 Member(s) | 3852 Guest(s)
Avatar

>