[electricroo]

I finished the 41-221C chassis video. The chassis took some time but it works real nice. It's a keeper!
https://youtu.be/UkeScO9xHY8?si=VbdQ0fXEcs5ysJQL

[radiorich]

Hello electricroo,
Sounds great! I’ll check it out this evening.
Sincerely Richard

[morzh]

Ron,

Great job in the radio. Lots of work, rebuilding from scratch.

About that choke replacement regulator: what is the power dissipation on that transistor?

[electricroo]

The transistor can handle up to 100ma without a heatsink. So the radio should draw arond 70ma at m9st. I was concerned about temperature due to the warm tube chassis because there was not enough room to add a heatsink in the location. After operating for 2 hours I checked the temperature on the tab with my temperature probe and it was 60c. According the TIP50 data sheet maximum temperature is 150c. Though I have used the circuit in two amplifiers that I built as output tube screen voltage regulators, I had room to include heatsinks on the transistor.

[klondike98]

Great restoration and video. Thanks for posting it.

[morzh]

Ron

I put the calculation in another thread here just a week ago for that very circuit.
The 150F is the op temp, but that without a heatsink will be quickly exceeded even at 25C ambient at 62.5C/W with 2W dissipation.
The current could be way more as long as the voltage drop is way less.

At 70mA you will dissipate 1W at 14V dropout and 2W at 28V dropout.

So, I understand, the dropout is really small, like 10V or so?

[electricroo]

Morzh;
The differential needs to be at least 15v. So I'm running it at 90. Voltage here is between 120 and 125. So at minimum 120 my differential is 30v. So it could go as low 115v and work. At 120v with a 30 volt differential and at 100ma dissapation would be 3 watts, so dissipation is somewhat less in this radio. If you didn't see it in the video description here is a link to the complete article. Though the chart does not show resistance value for 90v output it was easy to figure out.
https://w4npn.org/wp-content/uploads/202...-Choke.pdf

[morzh]

Ron

I know this circuit; we had a discussion where Rod pointed to it about a week ago. There are the calculations also.

https://philcoradio.com/phorum/showthrea...445&page=2

If by resistance you mean the U/I across the TIP50 transistor, you do not need it, it is U*I, and you should know both.
I mean, if you know for a fact that it feels warm and not hot, then you are good. I just like measuring stuff to know exactly whaty the operation is like. And, to remind, as the ambient temp rises, so does the operating temp of the transistor.

[electricroo]

Na, I was mentioning the resistance of R102 to set the output at 90v. Later today I'll hit it with my IR probe instead of the other temp probe that I used after a few hours and see what that reads. I have it nestled in the opening behind the speaker to let the heat rise a bit. I realize that it will heat up more when it is in the cabinet. Have to wait and see. If I need to move it onto a heatsink at a different I can. I noticed the loctal metal bases retain quit a bit of heat.

[morzh]

It is easy, should be around 144K.
This will give you roughly 90V. Or 89.6V.

[electricroo]

I rearranged things and was able to fit a heatsink coming up behind the back of the speaker magnet. It's probably pushing that no heatsink 100ma limit that was specified. Don't know why I was thinking 70ma. The tag on the radio specifies 30w at 115v. So ballpark figure is 260ma total at 115v. 150ma for the heaters leaves 100 - 110 ma putting it at it's limit without a heatsink. That 30w spec I'm sure is not exact, probably more. And as you said with the chassis in the cabinet temps will increase. I didn't realize that Antique Wireless sells a PC board to make an adjustable HV supply of that circuit. Their video mentions up to 1/2 amp with a large heatsink. So out of curiosity I'll take a heatsink temp, then compare it with the chassis in the cabinet when I get the cabinet finished.

[morzh]

What was the voltage dropout across that transistor? 25V? (115V-90V)
If this is the case, and the worst I=110mA, then yes, , 2.75W without heatsink is not just pushing it, that is very well exceeding it.
115V is AC though. You need to know DC before and after the transistor.

I do recommend you measure exact voltages so you do not guess.
Here's how I would do it.

1. Take 1 Ohm resistor (could be small as 100mA will dissipate 10mW) and put it in series with the transistor (either right before the Collector or right after the Emitter. And it has to be right before the pin, NOT before the input capacitor or after the output capacitor: it has to be Between the input cap and Collector or Between the Emitter and the Output cap).
2. Put a Voltmeter across that resistor. The volts will be your current in Amps. 100mV will mean 100mA.
3. Put the Voltmeter between E and C of the transistor. Measure the drop-out. Or. Put the Negative lead to the Minusses of the two electrolytics (they are connected together) and measure the E and then C voltage; then record them and the difference.
Whatever is easier.

Then multiply the drop-out by the Current.

Of course you have to wait until the radio is warmed up and the Voltages have stabilized.

[electricroo]

Current is 56ma. Dropout is 36v, so 2.016 watts. I measured the dropout previously and thought I had 30v. Today my mains voltage is higher, 125. It can vary out here 120 to 125. Anyway I looked back at my previous notes (if I don't write it down it goes somewhere else!) and I did measure 56ma with ma meter awhile ago according to my notes when testing. But that was a month or so ago and I forget things. I don't know why I thought 70ma. I miss understood Leigh's article as he mentioned "The TIP50, the combination can supply up to 100 mA continuously. In most applications, the pass transistor will not require a heat sink. For example, in a circuit with 50 volts differential across the TIP50 and 100 mA load, the pass transistor is only dissipating 5 watts. The TIP50 is rated 40 watts dissipation." I didn't think about it hard enough as usual. What was I thinking, 5w with no heatsink Lol! And we can't ask him anymore, I heard he passed away some years ago, too bad. So the heatsink I attached will do the job, even though I did run it for a few hours without a heatsink but I'm sure the temp would raise in the cabinet. And thanks for the help!

[morzh]

The R(theta-j)A is 62.5C/W.
So you temp rise vs the Ambient is 126C.
At 25C ambient (and this is when your radio starts) this is already 151C which is 1C above the 150C limit.
And if the ambient rises just 10 degrees (which should be easy with tubes around, plus the transistor itself, being very hot, helps bring it up), well, that is then 161C.

So, yes, get thee a heatsink that lowers the The R(theta-j)A to half of its no-sink value, and you just might be OK.

Here's your calcs.

Say, again, your TIP50 has R(Theta-J)C (to case) as 3.125C/W.
Let's use a heatsink with Thermal resistance "R(theta)SA (sink-ambient)" of 24C/W.
https://www.digikey.com/en/products/deta...0228387720

You resulting R(theta-j)A is therefore reduced to R(theta-j)C+R(theta)SAZ= 24+3.125= 27.125 C/W

At 2.016 watts your temp rise over Ambient is 54.7 C. So you could potentially have your ambient air at 90C and still not exceed the limit. And if you have space for a bit better heatsink, this will bring it down even more.
Like this one, with resistance of just 12C/W.

https://www.digikey.com/en/products/deta...0228387720

[electricroo]

This is what I ended up doing. I've got a stash of different heatsinks. The first heat sink I though was going to fit behind the speaker was almost perfect. I didn't want to modify the chassis to make it fit because if I happen to find another speaker assembly down the road I would want to be able to convert it back without a hacked up chassis. So I'm mounting a larger heat sink beside the chassis. Two holes in the bottom of the cabinet and one in the side of the chassis. Monted the Tip50 leads and wires on a small piece of single sided copper perf board then coated connectors with urethane conformal coating then heat shrink. This way i     t would be easy to remove and install an original working speaker if I came across one. And thank you for the help!