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 Post subject: Purpose of 10 mfd dry electrolytic positive ground ?
PostPosted: Aug Fri 28, 2015 9:47 pm 
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Joined: Jan Thu 01, 1970 12:00 am
Posts: 1293
Location: Southwestern,Ontario Canada
Hello,
I am finishing up re-capping this newly acquired 1938 Rogers Majestic 12 tube console. I have the schematic and alignment info ,but no parts list. I've managed to cross reference all of the Auto-Lite Tiger brand capacitors and have a list (at least for this radio). About the only capacitor that is left seems to be a DRY auto-lite tiger brand that has the POSITIVE end connected to chassis ground. Can someone explain the purpose of the way this cap is installed ? (I'm assuming it has something to do with grid bias on the dual 6F6's). The schematic is in pdf and the cap in question is marked C67 between R30 and R32. It is just to the right of the 4 2X3 rectifiers.
I hope the link will work.
http://radio.codegods.ca/node/6475

Auto-Lite Tiger brand (house numbered parts) seems to be a Canadian thing.

38102 = .05 @ 200V
38105 = .05 @ 400V
38106 = .1 @ 200V
38107 = .25 @ 400V
38108 = .5 @ 400V
38121 = .1 @ 400V
38128 = .003 @200V
38133 = .005 @ 200V
13533 = 10 ufd dry electrolytic @ ??

Thanks in advance..
Tony


Last edited by Tony F on Aug Sat 29, 2015 12:42 am, edited 1 time in total.

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 Post subject: Re: Purpose of 10 ufd dry electrolytic positive ground ?
PostPosted: Aug Fri 28, 2015 10:23 pm 
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Joined: Jan Thu 01, 1970 12:00 am
Posts: 3082
Location: Cortez, Colorado
Well the voltage on the capacitor is negative. It's a bias voltage for the 6F6s, because the 6F6s need their grids to be more negative than their cathodes, about 18 volts more negative. R29 to 33 are voltage dividers to get the -65 volts down (or actually up) to -18 for the 6F6s, and the capacitor filters it and keeps audio from crossing R22 and 23. So the positive side of the capacitor goes to ground, and the side that usually goes to the chassis, goes to the negative voltage.


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 Post subject: Re: Purpose of 10 ufd dry electrolytic positive ground ?
PostPosted: Aug Fri 28, 2015 10:38 pm 
Silent Key
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Location: Annapolis, MD
When looking at the polarity of filter caps, forget about "ground"......any point in the B+ circuit could be connected to the chassis. The B+ circuit goes from the most positive point (cathode of the rectifier), thru all the tubes, and eventually back to the most negative (center tap of the HV secondary). The voltage and polarity for a filter cap depends on where it is connected in the loop.

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 Post subject: Re: Purpose of 10 ufd dry electrolytic positive ground ?
PostPosted: Aug Sat 29, 2015 12:02 am 
Silent Key
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Joined: Jan Thu 01, 1970 12:00 am
Posts: 31285
Location: Maryland 20709, USA
Pix is correct. The polarity can always be determined by tracing back to the transformer's HV winding.

A common-positive capacitor was used where there was an impedance between the HV center tap and ground.
That impedance could be a resistor string, a filter choke, a speaker field coil, or any combination thereof.

The capacitor's positive lead connected to the rectifier cathode.

One negative capacitor lead connected to the HV winding center tap, that being the input filter cap.

Another negative lead connected to ground, that being the output filter cap.

Some configurations had a third cap with its negative going to some point in the impedance string to ground.

- Leigh

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 Post subject: Re: Purpose of 10 ufd dry electrolytic positive ground ?
PostPosted: Aug Sat 29, 2015 12:27 am 
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Joined: Jan Tue 16, 2007 6:02 am
Posts: 5360
Location: Lexington, KY USA
Tony,

Thanks for the link to a schematic!

The advice about the polarity of C67 is correct. + goes to "ground", or the chassis.

Looks to me as if the draftsman drew C67 backwards, or upside down. The symbol shows the positive side as the outside can part, and the negative as the inside or guts of the cap. And no other indication of polarity is on the schematic. Bet more than a few sets had replacements put in there backwards. Oh, well...

Ted


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 Post subject: Re: Purpose of 10 ufd dry electrolytic positive ground ?
PostPosted: Aug Sat 29, 2015 12:41 am 
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Joined: Jan Thu 01, 1970 12:00 am
Posts: 1293
Location: Southwestern,Ontario Canada
Thanks for clearing that up guys. :) It kind of threw me because the cap just looks like a large paper cap (no values on it) just POSITIVE printed on the ground (chassis) end. The schematic to me anyway shows negative to ground. All the other filter caps are negative ground.
C68, C69 both 30 mfd, C70 8 mfd and then a cardboard "box" with (2)- 4 mfd C64, C65, and a 2 mfd C66. The only oddball seemed to be the C67. I guess I wasn't quite sure what I was looking at.
Thanks again !!
Tony


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 Post subject: Re: Purpose of 10 mfd dry electrolytic positive ground ?
PostPosted: Aug Sat 29, 2015 1:53 am 
Silent Key
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Posts: 31285
Location: Maryland 20709, USA
Just so everybody can see what we're talking about:
Image

I believe the double capacitor with common positive is C68 C69, outlined in red on the drawing.
That common positive goes to the cathodes of all four 2X3 rectifiers.

The negative of C70 (outlined in blue) and of C68 both go to the transformer HV center tap, which is -65v to ground.
Tony F wrote:
The only oddball seemed to be the C67. I guess I wasn't quite sure what I was looking at.
C67 (at upper-right) appears as a normal negative-ground electrolytic.

This schematic symbol for an electrolytic capacitor, with an electrode in a "bottle", derives from the original Mershon wet electrolytic, which really was built that way, with a liquid electrolyte in a glass bottle.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Purpose of 10 mfd dry electrolytic positive ground ?
PostPosted: Aug Sat 29, 2015 3:12 am 
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Joined: Jan Tue 16, 2007 6:02 am
Posts: 5360
Location: Lexington, KY USA
Tony,

The circuit designers had a good time with this one. Makes it a bit difficult to figure out. No shame in wanting to ask to make sure. I realize that you asked only about C67.

The entire resistor network around C67 operates negative with respect to chassis ground. This is R29 ... R33. Grid bias for the 6F7 phase inverter triode, and for the 6F6 output tubes comes from this network. Also, a negative voltage is supplied to the jack labeled "P.J.". This puts a negative voltage on the AVC bus when something is plugged into the jack. All this is in addition to at least one bias battery. This is a highly biased radio!

The way C67 is connected makes it pretty clear that the symbol is reversed. When you trace the wiring on the part of the schematic not included in the above post, you will see that the small center part of C67 connects to the 6F6 grids, through the grid resistors, while the large outside electrode connects directly to the cathodes of these tubes.

C67 sees a low operating voltage, and is fed through a pretty high resistance, so it is unlikely go bang if connected backwards. However, if it is backwards, the bias voltages will be low, at least until it forms-up in the reverse direction.

To put your mind at ease: the radio will work without C67; you can put your meter across the cap's connecting points and verify the polarity there, before installing it.

Ted


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