What's this? Blurry schematic.

mightymike · Post by **mightymike** » Thu Jan 06, 2005 1:47 pm

I got this off a schematic of a Super Reverb. No Matter how much I enlarge this, it is still blurry. Look just below point X and Y. (You might have to rotate the pick so X and Y are facing up)

See where the 1M and the 22K resistor meet? There's some blurry writing, (Does anyone
know what that writing says?) and it looks like a jumper wire (maybe) going from that point to a capacitor; and while we're at the capacitor:

It says .1-

I asume that means .1 uf and that it is the negative side of the cap.
That point is jumped to pin 7 on the 12AT7

On the other side of the cap it says 200. What the 200 for? Is that some kind of rating for the cap or the 820 ohm resistor abobe it?

mightymike · Post by **mightymike** » Thu Jan 06, 2005 2:21 pm

Correction:
The attchment is from a Super Reverb AB763 layout (not schematic).
I'm looking over the schematic right now to see if I can figure this out.

mightymike · Post by **mightymike** » Thu Jan 06, 2005 2:50 pm

Maybe this is it. From the scamatic I figured out the 200 was just a rating on the .01 cap.
I think the blurry writing might be a reference point and might be 104 volts, or 104.5.

Here's a pic of that section

Billy Batz · Post by **Billy Batz** » Thu Jan 06, 2005 2:55 pm

It says +104.5V

Lemme put up the schematic for you. You have the chassis layout. You can see it clearly on the schematic.

Billy Batz · Post by **Billy Batz** » Thu Jan 06, 2005 2:57 pm

You beat me to it you SOB.....

mightymike · Post by **mightymike** » Thu Jan 06, 2005 3:04 pm

LMAO Hey one thing I would like to know, since you have the schematic and the layout , is which cap is the Cathode bypass cap?

Billy Batz · Post by **Billy Batz** » Thu Jan 06, 2005 3:16 pm

Im not sure what you mean. You know what a cathode is right? A simple explanation is its pins 3 and 8 of each preamp tube and the cathode bypass cap most people are talking about modding are the ones on the first preamp stages. So looking at your chassis layout of an AB763 circuit your cathode bypass caps (or Ck in shorthand, Rk for cathode resistor) for V1 and V2 are all 25u-25v electrolytic caps. V1pin8 and V2pin8 share an Rk/Ck pair. The vibrato and phase inverter have their own circuits. When people talk about modding the Cks in Fenders their talking about lowering them to improve the bass responce. IME you improve bass responce by dialing out the bass early on. Cutting the bass later in the chain thins out the entire amp. So if you have trouble with your bass responce you can try lowering the first 3 25/25 caps. The second value the 25v isnt important. You can use a larger value if you want. It doesnt effect anything as long as it can handle the voltage.

Most people feel 5u is a good value. Others like 1u (so do I) and others go even lower. So if that is why your asking try changing each one starting with 5 and see if that gets you the results you want.

Fender also used a multi cap for those Cks. So looking in youd see the 2 cathode resistors making an inverted V shape and the Cks are combined in one cap with one negative axial lead and 2 positive axial leads.

Billy Batz · Post by **Billy Batz** » Thu Jan 06, 2005 3:31 pm

Some Fenders have that multicap so if you want to replace the one Ck on V2 you have to take the multi cap out and put a cap in both spots. Fender also uses the lead from the negative side of the multicap to run through the eyelet and all the way to the brass ground plate. Youll have to keep that in mind as well.

What is it your doing anyway?

mightymike · Post by **mightymike** » Thu Jan 06, 2005 5:21 pm

You ask what am I doing. Well I'll tell you.
Oh and BTW thanks for teaching me about this stuff. They never taught me me squat about Tubes in Electronic School, and that was 20 years ago.

1) First I'm trying to understand what all you guys were telling me the other day, and to better understand tube amps in general. (Not that I'm going to rush out and change caps or anything. When I get those parts in (should be today) I'm going to try that high pass filter input trick you were telling me bout in the Fender Flubby Thread.) But I want to know what people mean when they name these certain parts. Like Cathode bypass caps, and those coupling caps too. BTW, which are the coupling caps?

2) I've been enlarging the schematic and layout, and printing it to multiple pages. (The tag board alone takes 2) Eventually I want to have a sort of cheat sheet that says what all the tubes are for, and I will also name some of these parts just in case I forget. And make a parts list.

3) Then who knows maybe one day I can try making a clone of my Super Reverb to experiment on

Now I have a question for you. Are the tubes numbered V1, V2 ..ect from left to right on this layout?

Let me see I have this right: Any Cap comming off of a pin 3 or 8 would be considered a Cathode Bypass cap? Or just a Cathode Cap (Ck)? Likewise with any resistor comming off those pins(Rk)?

For example: V5 (counting left to right) pin 8 goes up to 25-25 cap in parrallel with a 100k resistor. Would those be considered the Ck and RK for V5?

"Fender also used a multi cap for those Cks. So looking in youd see the 2 cathode resistors making an inverted V shape and the Cks are combined in one cap with one negative axial lead and 2 positive axial leads."

I'm trying to look for an example of this on my schematic and layout.

mightymike · Post by **mightymike** » Thu Jan 06, 2005 7:40 pm

Is this a multicap/ck ?

Billy Batz · Post by **Billy Batz** » Thu Jan 06, 2005 9:23 pm

I have a number of files Ill send you right now that will explain alot to you. Tube pinouts, anatomy graphics, etc....

In general terms V1 starts with the first tube and moves onto the last output tube or the tube rectifier if the amp uses one. So V1 would start on the rightmost 9-pin preamp tube socket in the chassis layout you have and continue on to the left.

The multi cap is a convenience thing. Theres no symbol for it on a schematic. Look at the photo I put up of the bypass caps. They make an inverted V. Instead of using 2 caps for the V Fender used a multicap which is basically 2 caps in one casing and one negative ground lead between them, which is no different electroncally then the 2 seperate components on the schem. Its a convenience thing. The positiv side of the multicap has 2 leads. One goes to the left eyelet and one to the right.

The only reason I pointed it out to you was in case you were going to try lowering the Ck values. If its a stock older Fender you cant change just one cap because each cap in a "V" shares casings in the multi cap. And its only an "if" case. Ive only ever seen them used on my 68 Bandmaster. Ive had a few other 60s and 70s Fenders that didnt use them and just had 2 capsthe way it is on the chassis layout. So just in case you see them.

Ill look for a photo and post it.

Billy Batz · Post by **Billy Batz** » Thu Jan 06, 2005 9:29 pm

Heres one good site with an amp anatomy. Youll find an old fender board anatomy on the Fender tech page.

http://www.obsoleteelectronics.com/Mars ... l_tech.htm

http://marshall.redpt.com/clay/marshall ... s_101.html

Heres another to do with Marshall