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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Mon May 14, 2012 6:55 am
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There have been some electrolytic capacitors replaced and that's a normal procedure when servicing an amp,there's a 10uf/100v capacitor added to the bias pot,I'm not sure why,and the capacitor in the bias section has been replaced with a 47uf/160v,there's no way of telling what was there before....I'm not sure about the resistors in the phase inverter section that were mentioned in the change to the AC568 circuit.
Show a couple of pics of the power tube sockets if you can.
From what I've seen,and I surely could be wrong,I can't say for sure that it was the AB763 or the AC568 when it came from the factory....because of the components that have been changed.


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Mon May 14, 2012 7:58 am
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#1 of 2
Best photos of the power tubes' circuitry
1968 Fender Twin Reveb Amp, 2x12


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Mon May 14, 2012 8:03 am
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#2 of 2
Best photos of the power tubes' circuitry
1968 Fender Twin Reveb Amp, 2x12


These pictures are for the discovery process to see if this Twin Reverb is:
1) naturally blackfaced
2) unnaturally backfaced
3) not blackfaced at all

Anybody can chime in, but I want to thank Arjay, so far.
Finding out will allow me to sleep longer and
maybe save me a trip to the local amp repair artist :?


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Mon May 14, 2012 9:22 am
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I'm no expert on the Twin Reverb components,or any kind of expert,but from what I can tell from the pic of the power tube sockets,on the grounds from pin 8,those long grounds,they have replaced another component because they look to be tied and soldered into the existing ground on the chassis.
Also the resistors look to be replaced in the phase inverter section,which according to the schematic were changed from the AB763 when the AC568 circuit came about.
My guess....it was the AC568 that has been 'blackfaced'....Arjay and BMW know way more about this than I do,maybe they will take a look.


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Tue May 15, 2012 6:04 am
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Rebelsoul wrote:
I'm no expert on the Twin Reverb components,or any kind of expert,but from what I can tell from the pic of the power tube sockets,on the grounds from pin 8,those long grounds,they have replaced another component because they look to be tied and soldered into the existing ground on the chassis.
Also the resistors look to be replaced in the phase inverter section,which according to the schematic were changed from the AB763 when the AC568 circuit came about.
My guess....it was the AC568 that has been 'blackfaced'....Arjay and BMW know way more about this than I do,maybe they will take a look.


Thank you for taking time, Rebel. If BMW or Arjay do chip in, that'd be great.
For now, I will plan on keeping my mind open until I get it to a knowledeable
vintage Fender amp expert. Unless anyone can see more precisely.

Thanks again, Rebel. Toppscore 8)

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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Tue May 15, 2012 6:19 am
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You're welcome Topp....sometimes the only way to be sure is to examine the chassis closely in person....pics only show so much.... :D


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Tue May 15, 2012 9:41 am
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Rebelsoul wrote:
You're welcome Topp....sometimes the only way to be sure is to examine the chassis closely in person....pics only show so much.... :D


+1

Frankly, my aging eyes have a problem discerning anything really useful from the pics but one thing does jump out as remarkable -- the advent of the AC568 circuit is generally regarded as the switch-over point at which cloth push-back wire was abandoned in favor of the neoprene-insulated type. However, the amp in question seems to be factory-wired with cloth. It's possible that this amp was at least partially wired prior to May of '68 then for some unfathomable reason, shelved for a time and then assembled some months later. Thus, a bonafide enigma among the myiad others that comprise the legend of Fender.

Arjay

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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Tue May 15, 2012 9:46 am
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Topp,

What would be nice is a photo of the 3-4 inches of the rightside of that breadboard. The AC568 had noticeable changes from the AB763.

Those are long ground straps, even for a AC568. Could be previous owner or past tech put a 1-ohm resistor inline, for bias readings.

Thanks!


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Wed May 16, 2012 5:15 am
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BMW2002Ti wrote:
Topp, What would be nice is a photo of the 3-4 inches of the rightside of that breadboard. The AC568 had noticeable changes from the AB763.
Those are long ground straps, even for a AC568. Could be previous owner or past tech put a 1-ohm resistor inline, for bias readings. Thanks!

Not sure of terminology "breadboard".
I will try to post other pictures, and please point out the breadboard. Thank you.

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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Wed May 16, 2012 8:50 am
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Topp, the very first photo is what is nice. It would be nice to have a clearer photo to see the values of the resistors & wiring changes. But, DEFINITELY some modifications and subbed in resistors can be seen. From the third photo the grid-to-ground resistors have been replaced. New ones = 220K. With the 100K and 82K networks, like the AB763 amp. If all the rest of the amp was converted back, that 10-k pot could be bias control and not the original balance pot.

If you look at the layout diagram of the AC568 versus the AB763, you'll will the difference between the two amps. Scroll to bottom of links to see layout.

I used the AA270 schemo/layout, as I couldn't find the AC568 layout. But the change in resistors and topology of the bias/balance section seems the same as the AC568.

Note: the 3-4 inch edge is on the left side of the layout. But, on the right side of your photo (amp is 180 degrees flipped from layout).

HTH. And wasn't too confusing.

http://www.webphix.com/schematic%20heav ... _schem.pdf

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Wed May 16, 2012 9:27 am
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Topp, here is a photo of same area of the breadboard (cardboard holding the components) of my 1969 SF Super Reverb. You can see the 220K-ohm Dales and two 100K-ohm (I reverted it to earlier balanced PI, AA763 circuit) . Plus added resistor to the now, idle bias pot. Most Fenders of this era had similar layouts specific to the circuit used (AB763 versus AC568)--- when it came to how the bias/balance issue was handled.

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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Wed May 16, 2012 9:45 am
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BMW....what is that 10uf/100v cap for,on the bias pot of Toppscore's amp?


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Wed May 16, 2012 9:58 am
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Rebel, as far as I can tell it's adding a tad of filtering to the bias power supply. A better component would be a resistor from the pot to ground. To insure negative voltage in case of pot failure. Maybe a 27k-ohm, 1/2 watt would be nice.

This is just a WAG, as I haven't seen the modified schematic to this amp.


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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Thu May 17, 2012 4:31 am
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If you could let me know which exact area of the chassis to photo shoot,
because I do not know all of the parts.
"Breadboard" is still throwing me for a Fruit-Loop - hahaha

If BMW's 1969 picture is the one, PLMK.

Then I will look into my files.

Or, I may take out the chassis to take more pictures.
Lot of heavy "Arnold Mr USA Iron Weightlifting", though :shock:


*** What I see as breadboards searching the Internet are nothing like the tag boards.
So, I am open to learning. Toppscore :)

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Post subject: Re: Silverface Grill Cloth & Drip Edge Trim * Source Buy Rep
Posted: Thu May 17, 2012 5:56 am
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Topp, you are correct. I call it a breadboard (more for DIY ppl), but it is a tagboard. It's the board that holds most of the resistors, capacitors, and other components of the amp.

Here's a photo of my 1962 Pro amp, showing the tagboard, bottom bell of the PT. And the position of the control pots: HTH!


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