65 Deluxe Reverb RI rebuild

[Randall]

I have decided to gut my made in 2010 '65 Deluxe Reverb re-issue and wire it up as a stock AB763. As I am sure I will have multiple questions, this thread is where I will try to ask them as I progress. Today I pulled the PCBs, snipped all the remaining push-on connectors, and wired up what I could until the parts come in. I will use the original filter cap board, as it is wired pretty much the same as the original, but which leads me to my first query. What is with the extra 220/100v cap and two extra resistors? The 47uF/500v cap I get in place of the original two 16uFs, but can someone explain what the extra components do in that circuit, and make a good argument either way whether to keep or not? What business does a 100v cap have in a 500v rated circuit?

http://ampwares.com/schematics/65_Deluxe_Reverb_RI.pdf

[Jazz P Bass]

Come on man!
How about some component designators.
(What is with the extra 220/100v cap and three extra resistors?)

[Randall]

Sorry. It is the usual Fender filter circuit that I am used to seeing, straight off the rectifier tube. I didn't think I needed to spell it out for you guys. C32 is the 47/500 on the hot side of the stand by switch. The other ones that are there that I ask about are C31, R65 and R66

[g1]

^^ What he (JPB) said.
That is some crazy totem pole arrangement, never seen that done before (unequal value caps). The 220uf (C31) in series with the 47uf (C32) combine to make 40uf, the limit for a 5AR4.
R65 and R66 maintain the proper voltages on those caps. Not sure which 3 resistors you are talking about.

[Randall]

Sorry again, two extra resistors. R65 and R66.

[Bruce / Mission Amps]

^^ What he (JPB) said.
That is some crazy totem pole arrangement, never seen that done before (unequal value caps). The 220uf (C31) in series with the 47uf (C32) combine to make 40uf, the limit for a 5AR4.
R65 and R66 maintain the proper voltages on those caps. Not sure which 3 resistors you are talking about.

Didn't Peter Traynor used to do stuff like this or was it Ampeg?

[Randall]

So, if the original had two 16uF/450v caps on the hot side of the standby switch, equaling 32uF, and the re-issue has this 47uF/500v and 220uF/100v in series to equal 40uF in the same place, which is better and why? Were they aiming for 40uF specifically? Is 40 better than 32 in this case? And if so, why stack two unmatched caps, with one only having 100v rating? How does that not fail? Why not put a 40uF cap in that position?

And while we're at it, why did they increase the values of the other filter caps (C33, C34, C35) from the original 16uF to 22uF? Do the mew guys know something that Leo didn't?

[JoeM]

Not really much difference between 32 and 40uf, and considering cap tolerances a 32uf could easily measure 40uf or more. Same with the 16uf and 22uf.

The 100v cap doesn't fail because the voltage across it does not exceed it's rating, you can see the two resistors in parallel keep the voltages seen by the two series caps within their respective voltage ratings. They're not equal values (the typical 220k) seen on Fender amps that use the totem pole arrangement.

I have no idea why Fender used those particular values, maybe someone with more technical chops can shed some light.

[loudthud]

The voltage at TP1 can go quite high when the amp is in standby mode. Possibly near 500V. Is enough of the amp still together to measure it?

[Randall]

No, the amp is thoroughly gutted, and well on it's way to it's new life.

[g1]

So, if the original had two 16uF/450v caps on the hot side of the standby switch, equaling 32uF, and the re-issue has this 47uF/500v and 220uF/100v in series to equal 40uF in the same place, which is better and why? Were they aiming for 40uF specifically? Is 40 better than 32 in this case? And if so, why stack two unmatched caps, with one only having 100v rating? How does that not fail? Why not put a 40uF cap in that position?

And while we're at it, why did they increase the values of the other filter caps (C33, C34, C35) from the original 16uF to 22uF? Do the mew guys know something that Leo didn't?

It's mostly about what kind of caps are commonly available today. 40uf is the max value a 5AR4 is rated to run into. 47uf is a standard value but they don't want to exceed 40uf as tube vendors might not warranty such usage. They came up with a creative means of obtaining the 40uf by putting 220uf and 47uf in series. The resistors R65 and R66 form a voltage divider and ensure the voltages stay within the caps ratings.
16uf is no longer a standard value, 22uf is.

[Randall]

OK, I think I get it. Can someone check me? If I understand what is going on here, using 400v as a round B+ value, there would be 330v across the 47uF/470K ohm 1 watt for 0.231 watts, and 70v across the 220uF/100K ohm 1/2 watt for 0.049 watts. This results in 38.7uF total at P13 and everything is within rated values. Is this right?

And if it is correct, is there any reason to pull this arrangement and put in two 16uF's as per original? ie does the amp react any differently? Or are microfarads just microfarads?

[g1]

Yes, you are correct with the math. Consider also that the voltage ratings of the caps in series adds, where in parallel they do not. So you have the equivalent of a 38uf/600volt cap. I'm not sure you would find 16uf/600volt caps, if so they would be expensive. And you would need 2 in parallel to make 32uf.
Cap. prices increase with voltage, this is part of the reason why they use the series arrangements.
There is no reason to change over to the old values. The difference between 32 and 38uf will not be noticeable, and the new style arrangement can handle much higher voltages.

[Randall]

Thank you very much g-one for helping me with my latest breakthrough. It all makes sense now. I very much appreciate the help from the more experienced minds on this forum. What a great resource. I hope I can help someone some day.

[Randall]

Both pre- amp circuits are essentially the same except that the vibrato channel has a 47pF on the vol pot, and the other does not. ???

And would anyone care to opine on the best way to approach the grounding scheme on a AB763? There was nothing fancy about soldering to chassis points and pot shells on the originals, and no magic on the re-issue pcbs I'm pretty sure. This one was actually pretty quiet as a re-issue. If I did run a copper ground bus for the pre-amp side, where to ground it is my biggest concern. Some say input side of chassis, some say star ground it with everything else. I don't know.