TCW
Full Member
Indianapolis, Indiana USA
Posts: 222
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Post by TCW on Jun 15, 2022 4:35:06 GMT
Hello,
Beginning to restore a 1446 with what I believe has a 18640-A amp. I have a three wire, sandwiched resistor assembly riveted to the box that looks like it has leaked some how. It has very low resistance when tested (450 Ohms). They call it Item 10 "Filter Resistor".
I am trying to determine the wattage of the 6800 Ohm and 1540 Ohm resistors.
The drawing shows it as two resistors wired in series, 6800 Ohm and 1540 Ohm. One end of the series resistors runs to the plates of a 6SN7-GT and the other end to a 5U4-G. It also shows three wires going to it from three 20 MFD 450V caps. It's around a 24 gage wire.
So any idea as to the wattage of the 6800 Ohm and 1540 Ohm resistors?
Tom
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Post by Ron Rich on Jun 15, 2022 11:50:00 GMT
Hi Tom, Going from my great--but not too long memory, some "leakage", is "normal" from those-- How are you measuring it -- with what, are you measuring it ? I am GUESSING, 25 watts  Ron Rich |
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Post by jukenorman on Jun 15, 2022 12:52:25 GMT
Ohms Law, Tom! The schematic I have for a 1446 shows a 17795-A amp but it does have 1540 and 6800 ohm resistors in the power supply. According to the schematic, 85V is being dropped across 1540 ohms so you can work out the current and then the power dissipated - which is just under 5W so a 7W resistor would do the job. 60V is being dropped across 6800 ohms equating to just over 0.5W so a 1W resistor should be fine there.
Bear in mind that is a 17795-A amplifier although I cannot see another amp using the same resistor values being much different but you can always resort to Ohms Law!
Norman.
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TCW
Full Member
Indianapolis, Indiana USA
Posts: 222
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Post by TCW on Jun 15, 2022 13:45:48 GMT
Bear in mind that is a 17795-A amplifier although I cannot see another amp using the same resistor values being much different but you can always resort to Ohms Law! Norman. Here is what I have Norman, would this change anything?  |
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Post by jukenorman on Jun 15, 2022 14:10:54 GMT
Hi Tom,
I would be surprised if it changed anything, but I cannot read the diagram! I can see 385V at the 5U4. After the 1540 ohm resistor, there is an arrow with a voltage, what is it? If you follow the the line from the 6800 ohm resistor up to the 6SN7, there's a voltage marked (at pin 2), what is it?
Norman.
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TCW
Full Member
Indianapolis, Indiana USA
Posts: 222
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Post by TCW on Jun 15, 2022 14:46:34 GMT
Norman, Hi Tom, After the 1540 ohm resistor, there is an arrow with a voltage, what is it? [The arrow points at the junction of the 6800Ω and 1540Ω, it reads +285V. The one shown after the 1540Ω is (+305V or) 385V.] If you follow the the line from the 6800 ohm resistor up to the 6SN7, there's a voltage marked (at pin 2), what is it? [+240V]Norman. I don't know if this will help but if you right click on on the image and select Open Image in a New Tab it will show the full sized image. Thank you for your help. Tom |
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TCW
Full Member
Indianapolis, Indiana USA
Posts: 222
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Post by TCW on Jun 15, 2022 14:57:03 GMT
Hi Tom, Going from my great--but not too long memory, some "leakage", is "normal" from those-- How are you measuring it -- with what, are you measuring it ? I am GUESSING, 25 watts  Ron Rich Hi Ron, Reading individually with a DVM "in circuit". The 1540Ω reads 70Ω and the 6800Ω reads 157Ω and 226Ω across both in series. Tom |
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Post by Ron Rich on Jun 15, 2022 15:48:25 GMT
Tom,
I dono--but I don't think they are in "series"-- I assume the center is common, and the two "ends" are the other ends of two resistors ? Ron Rich
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Post by jukenorman on Jun 15, 2022 15:53:23 GMT
OK Tom, you appear to have 100V across the 1540 ohm which puts 6.5W there - so 7W may be be a bit borderline so up it to 10W. You will then have a lower volt drop (45V) across the 6800 ohms so 1W is OK there.
I didn't realise you could enlarge the image by right clicking, but I still couldn't read it lol! I also realised that I am following the circuit in the direction of current flow where you are going L to R. So the 385V you are seeing after the 1540 ohms, I consider that to be before it!
Norman.
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Post by jukenorman on Jun 15, 2022 16:06:59 GMT
That 1540 ohm resistor looks a bit of a strange arrangement. It's tapped with an electrolytic on the tap. That electrolytic would augment the 285V for sure and would theoretically augment the 385V if it dropped below the voltage of the tapped electrolytic (maybe the tap is at something like 10%?) by reverse current flow. Maybe it's just a cheap alternative to increasing the values of the electrolytics either side? Might electrolytics have been very expensive in the 50s' so worthwhile?
Norman.
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TCW
Full Member
Indianapolis, Indiana USA
Posts: 222
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Post by TCW on Jun 15, 2022 22:07:04 GMT
That 1540 ohm resistor looks a bit of a strange arrangement. It's tapped with an electrolytic on the tap. Norman. Norman, Finding a 10W, Center Taped 1540Ω resistor could be a tough order to find. Could I use two 10W 750Ω resistors in series and tap the series junction for the center tap? Tom |
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Post by jukenorman on Jun 16, 2022 8:33:25 GMT
Hi Tom,
I don't think we know that the tap is in the centre? Actually I see CT on the resistor tap, probably means centre tap so what you suggest is fine - if you can find 750 ohm resistors; they only need to be 5W. You might be better going for 820 and 680, I can't see centre tapping being critical.
Norman.
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Post by Ron Rich on Jun 23, 2022 5:02:11 GMT
Tom,
These are "paper caps"-- MUST be replaced ! Ron Rich
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TCW
Full Member
Indianapolis, Indiana USA
Posts: 222
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Post by TCW on Jun 23, 2022 14:50:13 GMT
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Ron Rich 
Ron Rich 
