Fender 1969 Twin Reverb bias pot caps?

[MickG]

Hi folks! First time poster here....Recently I inspected the innards of my Fender Twin Reverb, (aluminum trim grill, most likely AC568 circuit) amp and discovered these 2 black, new looking caps near the bias pot. I'm wondering, are these 2 black caps stock or a mod? What do they do? I cannot find a layout for the AC568 circuit....I have seen a resistor in the same place where one of the caps is on other schematics such as the AA769....Pics attached... I have no desire at this point to Blackface it or mod it in any way....I love the headroom. Thanks in advance for anyone's help!

[stokes]

They are "filter caps" for your bias voltage.They perform the same function as the filter caps in your B+,smoothing out the DC ripple,but in this case they smooth out your bias voltage.They arent stock,but I wouldnt call them a "mod".Somebody probably did a cap job.I would bet if you take a look under the cap pan on the top side of your chassis,you will see newer caps as well.

[Dave Curtis, dB AudioTech]

They are "filter caps" for your bias voltage.They perform the same function as the filter caps in your B+,smoothing out the DC ripple,but in this case they smooth out your bias voltage.They arent stock,but I wouldnt call them a "mod".Somebody probably did a cap job.I would bet if you take a look under the cap pan on the top side of your chassis,you will see newer caps as well.

Maybe, but you'd think the tech would have done the cathode caps as well.

[Enzo]

We all like to think we do things right, and that others ought to as well, but I find there is often no good to come from trying to figure out what was on the mind of that mysterious "someone."

[stokes]

Maybe he did do the cathode caps,cant see them in those pics.But in any case like Enzo said,whats the point in speculating on what "someone" may or may not have done and why?

[bnwitt]

Maybe he did do the cathode caps,cant see them in those pics.But in any case like Enzo said,whats the point in speculating on what "someone" may or may not have done and why?

Stokes,
you can see three old white mallory electrolytics in the two pics so the early post is correct.

[Dave Curtis, dB AudioTech]

Maybe he did do the cathode caps,cant see them in those pics.But in any case like Enzo said,whats the point in speculating on what "someone" may or may not have done and why?

Which is why I posted that. Could be just as likely the old caps are still under the ashtray. Guesses are best left confirmed.

[stokes]

Stokes,
you can see three old white mallory electrolytics in the two pics so the early post is correct.

One appears to be across a 100k resistor,not likely a cathode cap,and the others are at the wrong end of the chassis to be cathode caps.But like dave says "Guesses are..."

[bnwitt]

Stokes,
the one across the 100K resistor (sitting next to the opto roach) is indeed the cathode bypass capacitor for the V5B tremolo triode. See the attached schematic.

[stokes]

Okay then.

[MickG]

Blackfacing info for this circuit AC568

The caps in the doghouse have been replaced....If I wanted to Blackface this amp I need to clip out the 2 10k resistors forking off of the bias pot....correct? and then I am to replace a 15k load resistor with a 24 or 27k? Where is this? Can someone help me with Blackfacing instructions for this circuit....I think I should bite the bullet and go for it. Thanks again---mick

[bnwitt]

http://www.unclespot.com/SF2BF.html

http://www.blueguitar.org/new/schem/...-sf_bf_mod.pdf

http://music-electronics-forum.com/s...ead.php?t=9956

Do a search on this forum and on google

[MickG]

They are "filter caps" for your bias voltage.They perform the same function as the filter caps in your B+,smoothing out the DC ripple,but in this case they smooth out your bias voltage.They arent stock,but I wouldnt call them a "mod".Somebody probably did a cap job.I would bet if you take a look under the cap pan on the top side of your chassis,you will see newer caps as well.

OK....my problem is
1. that when I go in to rewire the bias pot I am following the wire soldered to the wiper of the pot to 2 resistors which are supposed to be there next to each other, I am using Gerald Weber's DVD example here, but instead of seeing them nicely next to one another in a V pattern, I see one where it should be (68k in value) and next to it where the other should be is a big black cap instead. Now there is a resistor underneath of this cap, also 68k....is this the other one? Am I to replace these resistors with 220k 1/2 watt resistors-BF stock...is this right? And what about the 2 big black caps....do I leave them there? All this refers to pic 2925 from my first post...
2. As for the Phase inverter I've got 3 resistors soldered together....mine as is now are 1m, 270ohm, and 1m. Are all 3 supposed to be 1m? Thanks for any help....

[bnwitt]

If you are converting to blackface read this link:

http://www.el34world.com/charts/bias_conversions.htm

and this one:

http://www.el34world.com/charts/Biascircuits.htm

On the first link in the upper left you will see a picture of what you have but there are 100k resistors instead of 68k resistors. The two black electrolytic caps in your amp are someone's modification. I have attached a close up of the Blackface AB763 bias section. The one caveat would be to use a 100 volt 47uf capacitor on the bias board rather than the 50/50 shown. You need to change your resistor values to 220k where the 68k resistors are now. Bottom line, you'll need to change all component values to what is shown on the attached layout. Once done, if your bias voltage range is not right, you may need to alter the value of the 27k resistor on the pot case.

[MickG]

2 black caps....gone.

Thanks for confirming that the 2 caps were someone's mod.....I cut em out and did most of the blackface conversion....The bias pot was already stock wired with the middle tab on the pot wired to the circuit board and the right tab going to the bias cap area. no tab on the back of the bias pot so I assume it was stock this way. I took out the cathode resistors, grounded pin 8 with some of the excess wire...and clipped out the 2 grid resistors that were on two of the power tubes. I have read another thread here about the bias pot's resistor having been changed to 27k and trouble resulted somehow....I am going to leave the 15k in there.
Also the 50-50 bias cap....what should that be changed to....and why? Mine's in good shape but if it helps to bias the amp I'll change it.
The phase inverter had 2 1m resistors on either side of a 270 ohm resistor. Stock BF is 470 instead of 270....should I change this to 470?

last but not least....the chocolate drops. I could, while I'm in there replace these.....should this be done in your opinion? Thanks again---Mick

;91280]If you are converting to blackface read this link:

http://www.el34world.com/charts/bias_conversions.htm

and this one:

http://www.el34world.com/charts/Biascircuits.htm

On the first link in the upper left you will see a picture of what you have but there are 100k resistors instead of 68k resistors. The two black electrolytic caps in your amp are someone's modification. I have attached a close up of the Blackface AB763 bias section. The one caveat would be to use a 100 volt 47uf capacitor on the bias board rather than the 50/50 shown. You need to change your resistor values to 220k where the 68k resistors are now. Bottom line, you'll need to change all component values to what is shown on the attached layout. Once done, if your bias voltage range is not right, you may need to alter the value of the 27k resistor on the pot case.[/QUOTE]