Silvertone 100.385 schematic/info

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Re: Silvertone 100.385 schematic/info

Post by Chas on Sun May 07, 2017 6:28 pm

Best reflector for IR (heat) is gold, failing that $$$, polished brass panel mounted to the transformer bolts should be effective. A sandwich of two or three brass sheets should be most effective, far enough apart to allow free flow of air. I worked with first surface mirrors trying to get the max. from Xenon strobe to CCD camera, gold plated aluminum mirror greatly enhanced IR reflection.

There is data on tubes about operating them at below specification, has to do with the ion cloud damaging the oxide coating on the cathode, shortens the life of the coating... Simple, if the tube(s) are above filament voltage limits then reduce, never go below the limit, it saves nothing, reduces sensitivity and volume. Exception, thoriated tubes designed to have adjustable filament voltage.

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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Sun May 07, 2017 6:58 pm

Thinking for a very quick experiment I could use heavy duty aluminum foil, two sheets with an air gap. Nothing permanent at first, just to see if any effect.
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Re: Silvertone 100.385 schematic/info

Post by Chas on Sun May 07, 2017 10:30 pm

Aluminum will work, sheet brass much better, it has to do with the longer wavelength of IR reflecting rather than being absorbed... Crinkling up the surfaces works even better,. I found the reflectance of brass, but copper is the same as gold. Surface has to remain unoxidized. There are ceramic fiber mats available, but the gimmick is keeping air space on both the tube and transformer side.

I would not recommend covering the rectifier tube with a shield. However, there are tube shields that not only protect against magnetic and capacitive coupling but also provide cooling properties.

IERC tube shield:

http://www.qsl.net/kh6grt/page4/shields/shields.htm

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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Sun May 07, 2017 10:45 pm

Hi, Chas - I wasn't going to cover the tubes. Just making a temporary shield. Flat pieces, shiny sides toward the tubes, hopefully to reflect their heat and to shield the corner of the xfmr. Pretty flimsy, but no matter at this point. If that makes a noticeable difference then I'll go to something more elaborate.
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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Mon May 08, 2017 8:22 pm

Well, the insulator idea didn't help. Folded sheet of heavy-duty aluminum foil was placed between the very hot tubes and that corner of the xfmr. Gave it 1hr 10 min run time, ambient was 71 deg. Final temp was 108 rear corner, 133 front corner. Heat shield wasn't the greatest, but it didn't seem to make any difference. Ugh. Still, probably less than the UL max allowable. I just don't like it to be that hot.

Gonna pull the chassis out and check things previously suggested, and likely lower the AC input voltage for good measure.
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Re: Silvertone 100.385 schematic/info

Post by Bill Cahill on Tue May 09, 2017 11:53 am

Could be something is pulling too much current load, and, causing transformer to overheat.

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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Thu May 11, 2017 3:32 pm

Bill, I'm still hoping to find another perhaps easier solution, than replacing this replacement xfmr. Although I know you may be entirely correct. The transformer is a pull, apparently tested good, and a little better current ratings than the original. Also physically a little larger.

Reading other posts about hot running transformers gave me lots more to think about.  Solid state 5Y3GT replacements weren't recommended; what I'd save in filament current would be offset by higher B+ to the radio. If the xfmr is at or close to saturation, the heating goes way up. According to other posts, a small decrease of input primary voltage decreases heating much more, as it's proportional to the square of the voltage. So I ordered a small 12v xfmr to buck the input, still within the original specs for this set. A dropping resistor is initially cheaper but the postage makes if more expensive than the small xfmr! LOL.
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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Fri May 19, 2017 11:01 pm

Found the info about the replacement pwr xfmr, Crest #6416, new old stock. Company was in Chicago, but Google turns up nothing current about Crest.  Rated 350-0-350@100mA, 5V@3A, 6.3V@4A. Seems like it should be able to handle a chassis rated as drawing 60 watts. Dropping the input to 110VAC resulted in a drop of 2-3 degrees after 1hr 45min. I can lay my hand on the transformer for 30 seconds or so, it gets uncomfortable, but not blistering. Not what I had hoped for. I guess I'll pull the chassis again and see if there's something I overlooked. Appreciate all the input!
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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Tue May 23, 2017 1:07 am

Problem identified. Schematics didn't give any voltage measurements, so I was going by a bit of guesswork, and what was available. The replacement transformer delivers too much HV. Measured 360 V on the 6K6 plate, specs are 315 V Maximum. Oh, well. Gonna try dropping resistors between HV winding and 5Y3 plates. Maybe drop 75 volts or so. It's been an interesting journey so far...
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Re: Silvertone 100.385 schematic/info

Post by Bill Cahill on Wed May 24, 2017 1:42 am

Wrong place. Put a resistor from 5Y3 filament to first, or, second filter, depending how it is wired. Not sure what value, but, a 15 Watt may be what you need. Doing what you want to do is not correct, and, you would damage things. The plates on the 5Y3 have AC voltage, Not DC.

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Re: Silvertone 100.385 schematic/info

Post by Bill Cahill on Wed May 24, 2017 1:45 am

Oh. I see what you are trying to do, but, that's still the wrong place. Resistor should be in series with the 5Y3 filament, and, first filter.

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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Wed May 24, 2017 4:17 pm

Trying single resistor as you posted, Bill. 950 ohms gets the B+ about right, 290 or so. Dissipating about 5 watts, not impossible to deal with. But there's still 460 volts that the filter caps see before the other tubes warm up (they are 450V rated so I don't like that).

Some guitar forums have interesting ideas. They play around with B+ on their tube amps to change the sound, and some of theirs are running well over 400V of B+. I'm looking at a mosfet/zener combo that will drop 75 volts placed between HV CT and chassis ground. I shot an email to the company that sells it with my questions. If it brings down that 460V the caps see initially, I'm game to try it!
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Re: Silvertone 100.385 schematic/info

Post by Bill Cahill on Thu May 25, 2017 3:08 am

Most radios will have a surge voltage until the set warms up. Voltage should slowly drop..
I'm not sure the resistance you gave is correct and, I don't know how you wired it in....
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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Thu May 25, 2017 10:32 am

Bill, I wired the resistor per your suggestion, between rectifier filament and first filter cap. I measured voltage at the capacitor as the set warmed up. After it warms up and the other tubes are conducting, the voltage dropped across the 950 ohm resistor is about 70 volts, so I=E/R and so I=.0734A, close enough to my rough calculation of 85mA (what it might be considering the tube complement). Probably explains why running all the tubes at 360 volts would put the transformer over its 100 ma rating enough to cause it to heat up.

The initial 460 volts bothered me. The filter caps are new, but only rated 450 volts. Maybe that's not too much for them, but even the rest of the old chassis, for a few seconds, is getting at least 100 volts more than it did when new. Just as a precaution I would rather have something that regulated from power-on, eliminating the 460 volts surge, so I was looking around at zeners and mosfets.
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Re: Silvertone 100.385 schematic/info

Post by Bill Cahill on Fri May 26, 2017 3:40 am

Part of the voltage reading IS due to several things.
First, Line voltage is about twenty volts higher, and, often differs higher, or, lower.

Another indeed is from current draw of radio.
If replacement transformer is less than the set draws, It could cause over heating, and, transformer failure.
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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Fri May 26, 2017 3:46 pm

Thanks Bill. The transformer turned out to be overkill on the 350-0-350 HV side, had plenty of current available (rated 100 mA) but the set was getting 360 volts of B+ when its original design probably called for closer to 260-280 volts. I measured both with 120 VAC input and also dropped to 110 VAC, and there was still way too much B+. Look at the tube manual, the max plate voltage listed for the 6K6GT, was 315. Also the screen grid was getting 350, with a listed maximum being 285 volts, so nothing good could come of that, and probably explained why the whole chassis was drawing more than the 100 mA that the transformer was rated for.

By first using a dropping resistor, I dropped 70 volts to get into the right B+ voltage ballpark. The current draw for the entire chassis at that point was under 75 mA. So I researched and found the mosfet/zener device that drops 75 volts at all times, so even the turn-on surge will only be about 385 volts, and my new filter caps should not suffer. Then after warmup the device will drop 75 volts from the 360, making 285 volts of B+ available to the chassis. At the lower B+, the transformer should be supplying 75 mA, well within the transformer specs.
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Re: Silvertone 100.385 schematic/info

Post by Bill Cahill on Mon May 29, 2017 2:33 am

Good work. Sometimes, we don't have a lot of choices. Have you got the radio playing, yet??

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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Mon May 29, 2017 2:21 pm

The voltage dropping device is in transit now. Hope to have it installed this week. Thanks, Bill.
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Re: Silvertone 100.385 schematic/info

Post by Paulieb880G on Wed May 31, 2017 11:47 am

I believe you are over-thinking the situation. 460 volts surge should not hurt a 450 volt cap. They are built with a tolerance. Would it not be better to get the right transformer and install it rather than mess with a kluge with various doodads. Experimentation notwithstanding.

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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Wed May 31, 2017 6:38 pm

Thanks, that would indeed be nice, but time is a big factor here. The "right" transformer must supply the correct voltage, adequate current, and mount reasonably easily. And I must be able to find it. Not so much worried about initial surge to the caps, just what the chassis would have to live with. The kluge is just a mosfet and a zener diode (sort of a "black box" that drops 75 volts) connected between HV CT and chassis ground. I don't mind that these parts hidden under the chassis are not strictly original, LOL.
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Re: Silvertone 100.385 schematic/info

Post by Paulieb880G on Wed May 31, 2017 9:59 pm

You'll have to pardon me.  I'm an auto mechanic, not a radio guy.  If someone hangs extra parts on a car for whatever reason, I call that a kluge and toss that extra stuff into the round file.  I suppose haging extra parts on a radio is de riguer, then?


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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Thu Jun 01, 2017 1:20 am

Hi, Paulieb880G. No offense taken, I ran into many haywire rigs on customer vehicles in my earlier automotive career. LOL, try finding an exact replacement for a 75 year old transformer that you haven't got the original specs for. While I'd rather stay original, the main thing is to have it operating reliably when I haul it 900 miles back to a family reunion. Wish I had gotten closer on my first try, but it looks great and the modern parts will be out of sight.
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Re: Silvertone 100.385 schematic/info

Post by Paulieb880G on Fri Jun 02, 2017 11:15 am

I'm still a rookie at this radio stuff. I read a lot and get into trouble a lot messing with this stuff. I was not sure I wanted to mess with it, but the boys think I get in their way and want me to find something to occupy my time. My brilliant oldest boy rigged up a jack on the back of Becca's "new" Grunow to play some little contraption she has thru the radio. If he gets into radios, I might have a fighting chance. I have to change my thinking to ever be successful working on electronics. When you work on cars, voltage flows from positive to negative. When you work on radios, electrons flow toward the greater potential. Backwards. How's an old dog like me supposed to learn new tricks?

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Re: Silvertone 100.385 schematic/info

Post by tubesrgr8 on Fri Jun 02, 2017 5:45 pm

I suppose we all started as rookies, just some earlier than others LOL. I had the good fortune to be encouraged to build the "Cub Scout Crystal Radio" when I was about eight. The electronics bug never left me. I always got the electrical problems in the auto business, and when the cars started getting Digital Computers, I thought why not do Computers in a different environment. So I took the ET exam (USPS), got hired, and spent the next 20+ years working on computers, interfacing stuff, and even--gasp--bearings, gears, chains, and grease! Always liked the tube stuff, and have kept that as a hobby. I admit I'm not usually a stickler for total originality, but if it's a nice or semi-rare set, I do try harder.

To your original point, try to get a copy (check out eBay) of the RCA Receiving Tube Manual. There's a ton of stuff in the front explaining "Electrons, Electrodes, and Electron Tubes".  Best wishes!
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Re: Silvertone 100.385 schematic/info

Post by Paulieb880G on Sat Jun 03, 2017 2:34 am

I have an RCA tube manual from 1973. I get a lot of good out of it, just reading. Thanks.

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Re: Silvertone 100.385 schematic/info

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