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Post subject: White ground wire from tremolo to bias pot?
Posted: Sun Jan 08, 2017 9:13 pm
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Roadie
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I took this amp in to have the vibrato looked at, the idot blackfaced my amp without telling me and changed the wire. I'm trying to put it back to stock. I'm almost done, however there is a white wire from the 3.3 meg resistor to ground I'm not sure about. The schematic show it grounded after the 80uf cap anlong with the brown wire. This wire was too short, not sure if the guy snipped it, bit can I connect it to the fourth lug of the bias balance pot along with the 33K resistor and in front of the 80uf cap that goes to ground? Or should I splice some with and extend it to ground after the 80uf cap. I included a picture. Thanks and Happy New Year, Krank
[img][IMG]http://i1335.photobucket.com/albums/w669/krankster6/Ground_zpso3axqyst.jpg[/img][/img]


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Sun Jan 08, 2017 9:15 pm
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Roadie
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I'll try the image again
Image


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 3:04 am
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It will helpful to know what amp are you talking about ?

It look a Silverface with pull-push master volume ! Better to keep it Blackfaced.

There is no layout available ?

I have the Twin Reverb amp 100 watts pull push master volume layout looking like your picture


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 3:17 am
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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 3:17 am
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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 7:10 am
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Krankster, from your photo & description, this appears to be a balance pot setup (not bias pot). As described by layout which Stratele supplied.

It's a 3.3K-ohm, 10% tolerance (red-red-orange-silver) resistor. And should be a dropping resistor within the bias supply, from bias supply diode to first filter cap. The line goes through the cap, then is is grounded. It's not a 3.3 meg ohm resistor.

In any event, the usual giveaway are the 68k-ohm, 10% (blue-gray-orange-silver) resistors. Each one coming from opposite legs of the balance pot. One is soldered to the pot and the other is on the board, behind the phase inverter coupling caps. Your photo shows one 68k-ohm resistor leg.

These resistors are not in bias pot type Fender amps.

HTH!


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 8:45 am
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Roadie
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Yes it is a 72' 100 watt twin with master volume. The white wire attached to the fourth pin of the bias balance pot, the 33k resistor and 80uf cap, goes to the 3.3 meg resistor in the vibrato circuit. A tech had blackfaced the amp without my knowledge. I restored it to bias balance. Is the white wire on the fourth pin connected correctly? Thanks


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 9:05 am
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Bias adjustment pot is more useful than balance, best it is have both.

Layout show no ( white ) wire at this fourth pin.


Last edited by stratele52 on Mon Jan 09, 2017 9:08 am, edited 1 time in total.

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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 9:06 am
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Krankster, a bit confused, here.

1.) The MV Twin Reverb (100 watt) has a 2.2 meg-ohm resistor, 10% (red-red-green-silver) tying the bias circuit to the vibrato/tremolo circuit. Not a 3.3 meg-ohm.

2.) It's 3.3k-ohms (not 33k-ohms). This resistor is voltage dropping for the bias supply. From diode to first filter cap.

3.) Do you have a photos of the "blackfaced" edition (what amp tech did)?

4.) Yes, the solder point of the 3.3k-ohm resistor and 80mfd/75VDC is correct. This is simply being used as a solder point with no influence on the pot's function

Thanks! :)

http://ampwares.com/schematics/100W_Mas ... in_Rev.pdf


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 9:09 am
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BMW2002Ti wrote:
Krankster, a bit confused, here.

1.) The MV Twin Reverb (100 watt) has a 2.2 meg-ohm resistor, 10% (red-red-green-silver) tying the bias circuit to the vibrato/tremolo circuit. Not a 3.3 meg-ohm.

2.) It's 3.3k-ohms (not 33k-ohms). This resistor is voltage dropping for the bias supply. From diode to first filter cap.

3.) Do you have a photos of the "blackfaced" edition (what amp tech did)?

Thanks! :)

http://ampwares.com/schematics/100W_Mas ... in_Rev.pdf


The tech may only blackfaced the balance pot ?


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 9:31 am
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I don't think so. You need to redo the entire G2G resistor setup. From asymmetric 68k-ohm resistors, back to the symmetric 220k-ohm ones, used in all Fender 6L6GC amps.

Would need to look more like this, to be bias pot circuit:

Image


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 12:42 pm
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Roadie
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Yes, he only changed the wiring to bias adjust, not balance. My question is about the white wire going from the vibrato circuit to ground. Is the fourth pin on the bias balance pot the correct place to solder it? Sorry if I my question was confusing.


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 12:43 pm
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Roadie
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I'm restoring it to bias balance (stock).


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Mon Jan 09, 2017 12:45 pm
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Krankster wrote:
I'm restoring it to bias balance (stock).


Bad choice IMO

I wish to know why.


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Post subject: Re: White ground wire from tremolo to bias pot?
Posted: Wed Jan 11, 2017 10:11 pm
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Roadie
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This amp belonged a good friend who bought it new. He has the original paper work and tags. I took it to a tech who is on this forum for an unrelated problem. Without calling me, he took rewired the amp to bias adjust, but he left the 68k resistor of the variable bias side and installed a 220k on the fixed side. It sounded horrible. Instead of going around with this person, I returned it to the original specs before returning it to my friend who is an elderly gentleman. It sounds good and biases great now. Being a gentleman, I won't mention his name, nut I'm sure he has or will read this post. Shame on you.


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