[Radioroslyn]

At 250ma the 5Y3 would act like a fuse as it's only good for half that. Wouldn't be a bad idea to measure the hv current. I did a rough calculation on what the current should be and came up w/ abt 100ma. What the actual current in the circuit is now I don't know.

[gregb]

WAAAAAY to high for that radio, you have an issue. Without looking up tube currents I would think 50ma would be more in line.

Gregb

[jcassity]

jumper wire bypass of this 300ohm resistor yielded about 500v on pin 3 of both 6k6's
they ramped up then came back down to somewhere around 325v on each 6k6.. way too much.

with a 500ohm resistor installed, there is an 11volt drop.
with 500 ohms in the ckt, v7 pin 3 was at 206vdc, pin 3 of v6 was at 217vdc

[Mondial]

I would agree with Terry, normal DC current draw should be about 100 mA. You have:

2 6K6 @ 35 mA each = 70 mA
2 7H7 @ 12 mA       = 24 mA
1 7F8   @10 mA        = 10 mA

   Total                     = 104 mA

The dissipation in the 300 ohm resistor is not the straightforward DC  V  x I, because the current through it is not pure DC. Rather, it is a high peak to average ratio pulsating current, since the rectifier only conducts for a portion of the AC cycle at the peak of the waveform. So the DC voltage drop across the 300 ohms does not accurately predict the power dissipated.

The 500 ohm resistor now being used is getting extra hot since it is dissipating 66% more power than a 300 ohm resistor would under the same conditions. If you had a 300 ohm of the same size and power rating as the present 500 ohm it would not get nearly as hot. Using this series resistor was never really a great idea to replace the field coil speaker. It was just something Philco did to avoid using the right power transformer and get sets out the door. The original and replacement resistors did not last long in service as was mentioned.

If you want to measure the actual DC current draw, just measure the DC voltage across R30, the 170 ohm Candohm bias resistor. Current = Vmeasured/170 ohms.

In any case, no matter what type 300 ohm resistor you use it has to dissipate appreciable power, so why not just use the 50 watt chassis mount and be done with it?

[morzh]

>>>So the DC voltage drop across the 300 ohms does not accurately predict the power dissipated.

If your meter is True RMS, which most of today's meters are, yes, it does.

[jcassity]

, so why not just use the 50 watt chassis mount and be done with it?

I just built the 500ohm out for temporary sturdy means of allowing the radio to be able to be powered up while i continure troubleshooting the Shortwave problem.
i have ordered several adjustable resistors and loktal sockets and a few 7AF7's from play thing so of the past.

I also ordered two 300ohm 50watt chassis mount resistors from mouser. 

what i am doing is working around problems while replacement parts come in.

[Mondial]

Yes, if you have a true RMS meter and are measuring in AC mode then you will get an accurate measurement. Calculating E squared over R with the RMS AC voltage will give the true power dissipated in the resistor

If you are using a DC voltmeter (or the RMS capable meter in DC mode),  it will only measure the average DC value of the complex pulsed waveform, not the RMS value and the power calculation will be inaccurate.

[jcassity]

yup,, big difference between .707 & .637 ,, & Peak to peak

Funny, now when i power up my R300 gets hot. ,, its a tiny one under my tuner.
the white paint marke went brown and some tad bits of smoke got loose.
this is directly off my red wire T2 ! LOL. adding that to my list .

so i just measured the voltage on each side of the 500 ohm resistor. I was skeptical my original resistor would hold up casue i felt like there was a reason why it was big in the first place but considered it to be a bandaid to poor existing ckts and it compensating.

so i got 236vdc on one side of the resistor R102 (call out is 300 ohms for a pm speaker) but i have a 500ohm installed temp. i have 236vdc on one side and 306vdc on the other after i hear the speaker cthatoming on line.

thats about 70VDC ,, 70^/500= bout 10watts.. or about 15 true "ish".
if we use 236vdc , thats 111watts,, or 150w true "ish".

so why am i installing a 50watt soon?
i want this to say cool enough so that when i touch it , it is warm,, not boiling hot

to be honest i almost bought a 120watt chassis mount today but decided to go with what i was told.

something tells me this is a very dirty dc signal and has characteristics that lead me to think i should treat it as a AC circuit for math and component sizing reasons.

[jcassity]

my last tube is actually a 7AF7,, its a code 122 update

[morzh]

Still....if you have 70V across 500 ohm (do I understand that you only saw 70V across 500 ohm and not through 300 ohm?) then if we are to allow for constant current draw (first approximation) then acrisx 300 ohm you will have 42V which will ( if we forget of AC component) yield about 6W of dissip.

And this also tells that the current is 140mA. Still high but not 250mA.

[Mondial]

But you can't ignore the AC component in the power calculation  because it is very significant. The current waveform though the resistor looks nothing like "pure DC". Look at the diode current waveform (b) below.

[Image: http://www.industrial-electronics.com/im..._1-6-3.jpg]


The current through the 300 ohm series resistor looks like the waveform in (b) above. Remember, it is directly in series with the rectifier cathode before the first filter cap. The waveform illustrates the peak pulse of current which flows through the cathode of the rectifier tube each half cycle. It consists of an average DC component plus significant AC components which cannot be ignored in the power calculation.

[jcassity]

Still....if you have 70V across 500 ohm (do I understand that you only saw 70V across 500 ohm and not through 300 ohm?)

yes, thats right,, i am working with a very large 500ohm resistor for R102., and that is where i took my voltage readings for you last night.

 i let "some" smoke out my single small in size original replacement 300ohm resistor, it was about the size of a small tootsie roll.
to answer this problem, I put 3) 1k in parallel and those started to discolor the white paint indicator marker. these 1k types were a tad longer than 1 inch.

picture below shows my original replacement 300 ohm R102 .. and for the wattage pure dc , I believe i met or exceeded the calcs by what i installed.

I was leaning towards the same thing that im in the middle of the filter ckt so i am acutally dealing with pulsating DC ,, one half the AC peak to peak for the ball park guestimate.  if so, the ac input to the rect tube pin 6 is 400v.or 
200v pulsating DC.
either way it seems if if E^/R=P, then if i do lower my resistance to 300 ohms, in this area in among the filtration circuit, my wattage dispation will need to go "UP !".

Terry mentioned using a filter coil instead,, i dont have any tools to measure henry's but he mentioend a 5henry / 100ma rendition should soak up all the heat well enough.

one pic below shows an arrow at my blue 300ohm resistor which started to smoke a little, the white paint indicator marker allowed me to watch it start happening..
the next pic shows my 500ohm answer to addressing heat..

[jcassity]

now you can see i went from what would be considred a good original replacement choice by the "numbers" but in practice it failed.
my blue 300 ohm was porcelain design.
my green 500 ohm is porcelain and hollow in design with my fabricated heat sync & inserted copper tubuing in the center.
I believe my 500ohm solution is probably 5 times larger in size and should suit the application but it does not.
I suspect that :
A- i still have a problem somewhere like someone said earlier or.....
B- the circuit is talking to me, telling me i need to actually be somwhere in the 120w range.

[jcassity]

I remember back in the navy sometimes we has circuit cards that we had to dig into clear silicone to get to the bad part.
the entire board was installed in a small box, then we used a crap load of clear silicone overtop the whole board out to the metal box perimeter. the silicone would wick the heat out to the metal case. without the silicone , the smoke would get out.

[jcassity]

here is the stuff im getting from play things of the past...............
Qty 2
tube “7AF7”,, not seeing this in your list.. do you have any? ************ new $10.25

Need 1 Ea of the below:
4540-012 RESISTOR 50W 400, 50W, ADJ, 4 1/2" LONG... 5 3.25
4540-014 RESISTOR 50W 500, 50W, ADJ, 4" LONG, USED. 10 2.25
4540-044 RESISTOR 50W 250, 50W, ADJ, OHMITE, 4"L, U 1 2.25
4540-062 RESISTOR 50W 250, OHMITE 0568,50W,ADJ,4",U 1 2.75
4540-065 RESISTOR 50W 300, 50W, ADJ, 4"L, USED..... 1 2.00
4540-136 RESISTOR 50W 400, 50W, ADJ., 4" LONG, NEW. 1 4.25
4540-136 RESISTOR 50W 300, 50W, ADJ., 4" LONG, NEW. 1 4.25
Need 4)
completeLoktal sockets - #1030-844 is 1 5/16" spacing - $2.00 each


Here is the item im getting from Mouser
RH050300R0FE02 -Desc.: Wirewound Resistors - Chassis Mount 50watt 300ohm 1% qty2 5.57 ea.