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]

Originally Posted by 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.

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]

Originally Posted by 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?

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

[Dave Curtis, dB AudioTech]

Originally Posted by 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?

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]

Originally Posted by bnwitt

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]

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

Fender silverface / blackface bias question

Do a search on this forum and on google

[MickG]

Originally Posted by 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.

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]

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]

[bnwitt]

Mick,
the reason I recommend the replacement of the 50uf/50V bias cap is twofold. One, age, two the 50 volt rating is below the actual voltage present in that part of the circuit. The commonly available cap today is a
47uf/100Volt. Don't worry about the choclate drops. I would rather see you change all of the white Mallory electrolytic caps first.

[darrellcheng]

Originally Posted by MickG

I have seen a resistor in the same place where one of the caps is on other schematics such as the AA769

That is because it was a bias adjust circuit, instead of your bias balance. You can easily change to bias adjust if you want, to correctly bias your output valves.

[Dave Curtis, dB AudioTech]

Originally Posted by bnwitt

Mick,
the reason I recommend the replacement of the 50uf/50V bias cap is twofold. One, age, two the 50 volt rating is below the actual voltage present in that part of the circuit. The commonly available cap today is a
47uf/100Volt. Don't worry about the choclate drops. I would rather see you change all of the white Mallory electrolytic caps first.

The AC568 TR used a 50uF/70V cap here. There is nothing wrong with reusing one of the Sprague Atom 100/100s; they're not very old, and are date-coded so you can check. It's the same cap I use for bias circuits in Fenders. It'll stiffen up the bias supply, which is a good thing.

[Dave Curtis, dB AudioTech]

Originally Posted by darrellcheng

That is because it was a bias adjust circuit, instead of your bias balance. You can easily change to bias adjust if you want, to correctly bias your output valves.

Or even better, add a pot and have bias balance and bias level. I did that on my '76 Pro Reverb. The AC568 has a wonky bias balance anyway; it only adjusts the bias on one pair of tubes. Overall bias is set by changing the 15K dropping resistor.

[MickG]

Originally Posted by Dave Curtis, dB AudioTech

The AC568 TR used a 50uF/70V cap here. There is nothing wrong with reusing one of the Sprague Atom 100/100s; they're not very old, and are date-coded so you can check. It's the same cap I use for bias circuits in Fenders. It'll stiffen up the bias supply, which is a good thing.

Good I like reusing....it just feels right. I'll replace the 50/50 with one of the 100/100s, and as for the white Mallory 25/25s, how are the Sprague Atom blue 25/25 axials or are the Sprague premiums a lot better as replacements? Any opinions?---Mick

[bnwitt]

I would use 50 volt caps for the white mallories. The sprague's are fine so are Illinois caps

[MickG]

Originally Posted by bnwitt

I would use 50 volt caps for the white mallories. The sprague's are fine so are Illinois caps

I am learning a lot and want to understand it all so....why? Why the 50 volt instead of stock 25?

[bnwitt]

Modern electrolytic capacitors are smaller than the ones from that time period and todays 50 volt units are a little smaller than those old Mallory 25 volters. With a 50 volt rating, you've got more headroom and insurance so to speak on those caps. It's pretty standard with many techs to go to the 50 volt units for that application. And they fit right in there.

[MickG]

Well I have done most of the mod but upon trying a test to hear what it is sounding like I got way too much power somehow into the tubes....they were glowing pretty strong so I turned her off and prayed....anyway....Here's what I have done so far to bring me to this point:
1. clipped out the cathode resistors and grounded all pin 8s..
2. removed the resistors on the tube grids (2 of them)
3. removed the 10k resistors on the bias pot
4. Removed 2 100uf caps off the bias pot
5. Changed the bias supply cap from the stock 50/70? to 100/100.
Now I have not replaced the 68k resistors with 220k yet and I have not yet replaced the middle resistor in the phase inverter to stock 470ohm....mine is still 270ohm.
I have not adjusted the bias at all....I have no bias rite or sockets to use wioth a multimeter....yet. Can I turn it down just to be safe by just using the bias pot?
I may have some cold joints on the new bias cap.....will check and/or redo that, also the ground on one of the power tubes pin 8 looks like a cold joint too. So I will check and report back.....all you comments really are helping a great deal so thanks again----Mick

[bnwitt]

If you put a 1 0hm resistor on each cathode to ground you can use them to bias the amp with your multimeter. I wouldn't put the power tubes in until you prove the range of negative voltage out of the bias circuit to pin 5 of the power tubes. I also wouldn't crank it up until I finish all of the component replacements.

[Dave Curtis, dB AudioTech]

Originally Posted by bnwitt

Modern electrolytic capacitors are smaller than the ones from that time period and todays 50 volt units are a little smaller than those old Mallory 25 volters. With a 50 volt rating, you've got more headroom and insurance so to speak on those caps. It's pretty standard with many techs to go to the 50 volt units for that application. And they fit right in there.

Since the voltage on those caps in this amp are well under 5V, the 25v parts are fine, and are plenty overrated. (IMHO)

BTW, if you use the Spragues, don't tweak the leads very much, I've had one break off.

[bnwitt]

Originally Posted by Dave Curtis, dB AudioTech

Since the voltage on those caps in this amp are well under 5V, the 25v parts are fine, and are plenty overrated. (IMHO)

BTW, if you use the Spragues, don't tweak the leads very much, I've had one break off.

You're probably right but The voltage on many 12AT7's in the fender amps is at 10 volts and it is a personal preference of mine to use the 50 volters through out the amp. Saves buying two values. I admit, it's overkill.