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Post subject: Signal and Coupling Cap Polarity
Posted: Fri Apr 12, 2013 9:02 am
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Roadie
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I'm going to install two .1/600 coupling caps in a 74' Bandmaster Reverb. I know positive goes to ground and some caps are in parallel and some in series and there's usually a mark (line) on the positive end of the cap. However, no two silverface Fender amps seem to be same. Looking at the inside of the Bandmaster Reverb chassis with speaker outputs facing me, can someone go through the polarity of the caps from left to right (two .1's to the channel caps .047 and .1 in series) I have great respect for the old Fender amp builders, but did they just wing it?


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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Fri Apr 12, 2013 9:21 am
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Krankster, coupling caps are non-polar. And are used in sections of the amp where polarity is not an issue. Such as coupling the anode stage of one section to the grid stage of the next. They pass AC signal voltage (mV) and block high DC voltage (hundred of VDC). So polarity is not an issue.

The mfd values of the cap should give you a hint as to the function. 0.01mfd to 0.47mfd rated at 400-600VDC are usually coupling caps. 8 mfd to 40mfd and more are electrolytic caps --- and have polarity.

One terminal does attach to an inner foil and the other to the outer foil. And some ppl CLAIM there is a diference in tone between the two --- and how the cap is sodlered into the circuit You can try reversing the cap and see if you hear any difference. The DC blocking character should NOT be affected by reversing the cap. This is the most important function of a coupling cap.

http://www.jt30.com/usenet-articles/cap ... -polarity/



HTH!


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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Fri Apr 12, 2013 10:00 am
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Roadie
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Thanks for the reply BMW2002Ti. I thought that was the case, but I spoke to a guy who used to work at Fender-he's selling me a couple .1/600 Paktron coupling caps-who said to make sure they were wired in parallel. Thought I'd ge some feedback.


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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Fri Apr 12, 2013 12:29 pm
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Krankster wrote:
Thanks for the reply BMW2002Ti. I thought that was the case, but I spoke to a guy who used to work at Fender-he's selling me a couple .1/600 Paktron coupling caps-who said to make sure they were wired in parallel. Thought I'd ge some feedback.


Why wire them in parallel? Two 0.1 mfd caps in parallel equal 0.2 mfd. Are there any 0.2 mfd caps in your amp now?

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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Fri Apr 12, 2013 12:31 pm
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BMW2002Ti wrote:
Krankster, coupling caps are non-polar. And are used in sections of the amp where polarity is not an issue. Such as coupling the anode stage of one section to the grid stage of the next. They pass AC signal voltage (mV) and block high DC voltage (hundred of VDC). So polarity is not an issue.

The mfd values of the cap should give you a hint as to the function. 0.01mfd to 0.47mfd rated at 400-600VDC are usually coupling caps. 8 mfd to 40mfd and more are electrolytic caps --- and have polarity.

One terminal does attach to an inner foil and the other to the outer foil. And some ppl CLAIM there is a diference in tone between the two --- and how the cap is sodlered into the circuit You can try reversing the cap and see if you hear any difference. The DC blocking character should NOT be affected by reversing the cap. This is the most important function of a coupling cap.

http://www.jt30.com/usenet-articles/cap ... -polarity/



HTH!


I orient all my coupling caps in a pseudorandom manner to keep the electrons guessing. :lol: :lol: :lol:

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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Sat Apr 13, 2013 5:00 am
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Krankster,

You know, I don't see any 0.2mfd/600VDC coupling caps in an AA270 SF BMR.

Are you sure he wasn't talking about the 0.1 mfd/600VDC coupling caps from the 12AT7 phase inverter anode plates to the signal grids of the 6L6GCs? These are not really in parallel, but couple the positive and negative portions of the signal sine wave to the output stage.

Sorry, if this all sounds Greek. Point is these caps are not in parallel.

http://vintagefenderamprepair.com/wp-co ... ematic.pdf


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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Sat Apr 13, 2013 6:35 am
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BMW2002Ti wrote:
Krankster,

You know, I don't see any 0.2mfd/600VDC coupling caps in an AA270 SF BMR.

Are you sure he wasn't talking about the 0.1 mfd/600VDC coupling caps from the 12AT7 phase inverter anode plates to the signal grids of the 6L6GCs? These are not really in parallel, but couple the positive and negative portions of the signal sine wave to the output stage.

Sorry, if this all sounds Greek. Point is these caps are not in parallel.

http://vintagefenderamprepair.com/wp-co ... ematic.pdf


That's what I thought and posted earlier. I just wasn't sure which schematic the OP was working with.

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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Sat Apr 13, 2013 12:29 pm
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I am speaking about the .1/600 coupling caps from the at7 to the tubes. The first two on the left. Bob Rissi, an old Fender amp builder, made that parallel comment. One lead is connected to the inner foil (marked on some caps) and the other to the outer foil. Coupling caps are non-polar, but the inner foil should be hooked to the low impedance side. This provides shielding and can affect HF roll-off. Fender didn't worry about it too much, but if it makes a difference in the sound, why not do it right.


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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Sat Apr 13, 2013 1:30 pm
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Krankster wrote:
I am speaking about the .1/600 coupling caps from the at7 to the tubes. The first two on the left. Bob Rissi, an old Fender amp builder, made that parallel comment. One lead is connected to the inner foil (marked on some caps) and the other to the outer foil. Coupling caps are non-polar, but the inner foil should be hooked to the low impedance side. This provides shielding and can affect HF roll-off. Fender didn't worry about it too much, but if it makes a difference in the sound, why not do it right.


If you are referring to the two 0.1 mfd caps in the inverting and non-inverting legs between the PI and power tubes, those are separate circuits. They are not in parallel. As far as how the foils are connected, I follow no fixed pattern in my amp builds and they sound just fine. Not all caps are marked with the foil end.

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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Sat Apr 13, 2013 4:00 pm
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Bob Rossi is known to some of you on this forum. He worked at Fender for decades-the good ones-building amp and lives a mile from the old Fullerton plant. Leo bought or built houses near the plant so his employees could live close. I was surprised when he said, " make sure to install them in parallel. I'll speak to him again in the near future and get some more info. Thanks for the replies. Where is Arjay. Thought he'd chime in on this one. You guys are the best.


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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Sat Apr 13, 2013 7:11 pm
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I know of no Bob "Rossi" associated with Leo Fender or the company that bears his name.

I am aware of a Bob Rissi though......he helped design and engineer Fender's first-generation solid-state amps, among many other products he helped bring to market. I've conversed with him on numerous occasions and bought a few vintage Fender parts from him in the past.

Arjay

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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Sat Apr 13, 2013 8:05 pm
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Roadie
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Bob Rissi-typo. Real nice guy with lots of Fender stories and amp info. He knows his stuph.


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Post subject: Re: Signal and Coupling Cap Polarity
Posted: Sat Apr 13, 2013 10:22 pm
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Yeah, I started orienting the pio caps in the amps, some years ago. The outer foil towards the lowest impedance. Outer foil towards the shortest AC path to ground or the plate of the previous stage. Even some poly caps, especially the phase inverter feed cap. Benefits are questionable!!! But, some caps are noisier in one direction. An easy test for noise, outer foil end, or microphonic caps: 1)guitar cable plugged into tube amp(such as; a Champ or any low watt amp with white noise) 2) amp on 3) hold cap between fingers 4) contact one cap lead to plug tip 5) the other lead to the sleeve of the plug 6) listen for the difference in the white noise. The cap lead contacting the sleeve, with the least noise is the outer foil. If you tap the cap with a finger, you can determine microphonics. An easy tester can be made, using a guitar cable and connecting alligator clips to one end. BTW, Krank, I think Mr Rizzi was referring to the fact that those two caps should be oriented in the same direction. Both outer foils in the same direction. JMHO, Art

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