1962 Fender Twin Caps

[Tom Phillips]

And, as you see in the photo, your 71 Super Reverb's caps have been replaced. The original 70uF first stage series caps have been replaced with 100uF parts producing the equivalent of 50uF first stage filtering. That "upgrade" is common practice since 70uF is not a common value these days.

If you decide to make mods to your old twin, be aware of the existing under board wiring.

[Justin Thomas]

If you decide to make mods to your old twin, be aware of the existing under board wiring.

I learned the hard way - the cap "doghouse" wiring is NOT shown on the layout diagrams!

Which is probably why so many guys here scream "SCHEMATIC! SCHEMATIC! SCHEMATIC!" instead of "layout... yippee."

Justin

[keithb7]

Thanks Tom. I replaced the caps in my 71 Super back in January this year. I just pulled them out and made sure I re-installed them the same way. Until this thread, I was unaware of the series, versus parallel wiring in the first stage of the caps. I am always learning, its great.

Are the little eyelet solder holes in the board, beside the caps, there to mount the 220k resistors, there for mounting convenience? A mounting place to secure the reistor only? There are no wires from these connections, under the board? Thinking I don't really want to punch a hole in the original board so it looks like my 71 Super. Could I silicone the resistor to the board, solder a wire on each end and run it to each end if the cap. Sound ok?

[Tom Phillips]

...Are the little eyelet solder holes in the board, beside the caps, there to mount the 220k resistors, there for mounting convenience? A mounting place to secure the reistor only? There are no wires from these connections, under the board?

Yes. Those eyelets are there specifically to hold the resistors. Around 1970 Fender started putting the wires connecting the resistors to the caps on top of the board. Before then, the wires were under the board. There may still be other connections under your board.

...Thinking I don't really want to punch a hole in the original board so it looks like my 71 Super. Could I silicone the resistor to the board, solder a wire on each end and run it to each end if the cap. Sound ok?

You could do that but I don't care for that kind of construction practice. I would just piggy back the resistors on the cap. You could use sleeving on the resistor wires and add add some extra bus wire to the resistor if the leads are too short.

When you compare your two amps you will also see that you will need to re-route at least one of the wires to the caps. All this extra work will not be necessary if you go with the plan to buy the 600V caps.

[keithb7]

Caps arrived here is what i am planning, but could use some assistance:

Last 5 caps in parallel same as stock.



Planning to lay the series caps as shown here:



Here is how I have attached the 220K 1W resistors:



Soldered it up, plugged in and blew the fuse. I must have missed something. Tom phillips you mentioned I will have to move one wire. Which one, to where? Anything else?
I am assuming there is a jumper wire or two under the board I need to pull. I am not sure.
Thanks folks.

Here is how I wired it up:

[g1]

Unless someone has a layout drawing, you will probably have to check resistance to find the wire that needs to be removed.
Aside from that, those resistors look more like 270K than 27K, perhaps it's the light but check them.

[g1]

If you have a "before" picture it will be very easy to say what needs to be changed.

[g1]

This should do it, where the yellow dashed lines are, those wires need to be removed (top and bottom of pic).
The blue line is where you need to add a wire.
The red + marks show how the caps should be oriented.

[keithb7]

Before pics:

[Justin Thomas]

270K than 27K, perhaps it's the light but check them.

Those bleeders are 220K... the first dropper is 27K...

Justin

[Tom Phillips]

g1's illustration in post #23 will work fine. An alternate method that achieves the same result is shown in the attached annotated picture. My picture shows how Fender wired the first stage filter in the blackface amps. Both G1's and my diagram will achieve the same result so you can choose which ever one is easier to implement.

[52 Bill]

Both of these photos don't mention that the other 5 caps still need to have their negative ends connected to ground.

[Enzo]

Um, your old way photo shows ALL the caps lined up the same way. Now look at your large cap on the end in your new arrangement. Why is it reversed? That is likely the problem.

The black wire upper left is likely ground, and I'd wager there is a wire jumper under the board from the lower left eyelet up to the upper second eyelet, putting the two large caps in series.

[Tom Phillips]

Both of these photos don't mention that the other 5 caps still need to have their negative ends connected to ground.

Opps. You are correct of course. We were focused on the conversion of the first stage filter from a parallel pair of caps to a series string of two.

Edit: The original Fender filter board had two ground connections from the filter cap board in the doghouse to the chassis. One connected the first stage filter cap (this "Cap" is two physical parts in parallel or two in series depending on the amp) to the chassis near the power transformer and the other connected the remaining filter caps to the brass strip at the pre-amp end of the chassis. If you try to measure the continuity with your Ohm meter you cannot easily tell where they are physically connected because all the grounds have continuity to each other through the chassis. You need to follow the wires visually. Then you will find that the grounds are kept separate between the filter board and the chassis. There are many discussion threads about grounding methods and ground loops that explain why the grounding is done this way. The attached annotated photo (From Keith's before work post) points out the other ground wire.


Keith,
When doing this type of work one always needs to consider the whole picture and make sure that all the final connections are correct. If the original circuit used only one ground lead for all the caps then an additional connection needs to be added. All the final connections need to be traced out to verify that your final modified circuit matched the circuits you intend. Look at how it is done on your 71 Super.

Well...now you know why back in post #10 I said "I agree that simply buying and installing the 600V Caps would be the best approach given your experience level. Why complicate maters and taking the chance of taking a wrong turn?"

You can get this done with careful work and additional checking. It's not difficult once you understand the circuit. That's the rub. It actually takes more time to explain in this thread than it takes to do the work.

Tom

[keithb7]

Thanks for all your help. You have told me what to do, and I appreciate it. I will do this later and update the thread.

I am struggling a little with the theory behind it, I would also like to understand why certain wires go where.
I am going to expose my ignorance and ask a few questions. If you have time and feel like answering them for me I appreciate it immensely. If not, no problem. I'll dig in a few books
and try to find the answers.

The input red wire to the second filter cap in stage 2, is 477V from the rectifier circuit. The power sits in those two caps until the standby switch it closed and the voltage flows through the amp
as needed. Correct? The high voltage is stored there as a reserve as needed. For example when you hit a loud power chord on the guitar, the voltage is pulled out by the tubes as needed
to amplify the line signal. The other smaller caps, also store voltage, close or slightly lower values to supply standby power as needed in tubes as well. These smaller caps need not be so big,
as only when the amp is in standby mode, there is extra voltage and only the first two caps see this voltage? My question is, do all the caps store and share this voltage
to be used as needed? Do the caps all share this same voltage. To illustrate a basic example,In basic simple terms, so I can grasp the idea: Do the caps sort of act together like a large pool of water,
with several hoses and pumps pulling water from the the one big reservoir pool of water?
The several hoses being the various circuits with tubes in need of supply.The rectifier circuit acts like a big water pump and keeps the pool full so hoses can pull more or less water as needed.
I understand how the rectifier circuit works, converting AC to DC.

Thanks again. Much appreciated. - Keith