Fender AB763 Filter Cap Voltage Ratings

mistersparkle

TDPRI Member
Joined
Jan 5, 2011
Posts
16
Location
southeast PA
Edit: I'm embarrassed that I accidentally attached the only AB763 schematic that doesn't reflect the query in my original post below (the Vibroverb schematic). I'm calling my build a "Vibroverb" clone only because I'm planning to use a single 15" speaker. Please refer to the now-attached Vibrolux schematic for reference to my cap voltage rating question stated below.

I'm in the process of building what basically amounts to a Fender Vibroverb clone, and I note that in all of the AB763-based schematics the five 16uF filter caps are all rated at 450VDC. Meanwhile, I also note that all the amps using 2x6L6 output tubes in this circuit have power transformers with 350-0-350 secondaries. The rectifier tube is a 5AR4/GZ34 with relatively low voltage drop, and two of the five filter caps are placed in front of the standby switch (in parallel, not series as is done in some later Fender circuits).

Everything I know about electronics tells me that, if the standby switch is opened, the filter caps in front of the switch will charge up very close to the peak of the AC voltage being applied, which in this case would be about 500V, give-or-take. Am I wrong, and if not, how did Fender get away with 450VDC rated caps? And if I'm right, should I build my amp with a pair of, let's say, F&T 80uF/450V caps in series (with balancing resistors, of course) for the reservoir? Btw, I already have the 350-0-350 PT (Mercury FDP-20) and I don't really want to lower the B+. And no, I'll probably never use the standby switch, but still....
 

Attachments

  • vibrolux_ab763_schem.pdf
    47.3 KB · Views: 53
Last edited:

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,809
Location
Cincinnati, OH, USA
I'm in the process of building what basically amounts to a Fender Vibroverb clone, and I note that in all of the AB763-based schematics the five 16uF filter caps are all rated at 450VDC. Meanwhile, I also note that all the amps using 2x6L6 output tubes in this circuit have power transformers with 350-0-350 secondaries. The rectifier tube is a 5AR4/GZ34 with relatively low voltage drop, and two of the five filter caps are placed in front of the standby switch (in parallel, not series as is done in some later Fender circuits).

Everything I know about electronics tells me that, if the standby switch is opened, the filter caps in front of the switch will charge up very close to the peak of the AC voltage being applied, which in this case would be about 500V, give-or-take. Am I wrong, and if not, how did Fender get away with 450VDC rated caps? And if I'm right, should I build my amp with a pair of, let's say, F&T 80uF/450V caps in series (with balancing resistors, of course) for the reservoir? Btw, I already have the 350-0-350 PT (Mercury FDP-20) and I don't really want to lower the B+. And no, I'll probably never use the standby switch, but still....

You've got me confused. The schematic you linked to shows (2) 350V caps in series before the standby switch and then 525V (???) caps for the rest of the power supply.
 

mistersparkle

TDPRI Member
Joined
Jan 5, 2011
Posts
16
Location
southeast PA
You've got me confused. The schematic you linked to shows (2) 350V caps in series before the standby switch and then 525V (???) caps for the rest of the power supply.
I'm sorry! You are correct that the Vibroverb schematic I attached shows two 350V caps in series before the standby switch and 525V caps elsewhere. But have a look at this Vibrolux schematic. This schematic, and many other AB763 schematics, show the parallel 450V caps I'm referring to. I can't quite believe I never noticed the difference in the Vibroverb schematic. D'Oh!!
 

Attachments

  • vibrolux_ab763_schem.pdf
    47.3 KB · Views: 37

Lowerleftcoast

Poster Extraordinaire
Joined
Dec 30, 2019
Posts
5,510
Location
california
IIRC the higher B+ amps had the reservoir cap in series. The rest of the caps were rated 525v (or higher).
Caps have a voltage rating but they also have a surge voltage rating. They can withstand higher voltages than the *voltage rating* when the amp charges up the caps.
should I build my amp with a pair of, let's say, F&T 80uF/450V caps in series (with balancing resistors, of course) for the reservoir?
Short answer... Yes. You do what ya gotta do.

Be mindful of the B+ voltage at each filter cap so you know the cap has the correct voltage rating to handle that part of the circuit. You may have to use the F&T 22uF 500V instead of their 16uF 475V for the first stage or two in your high B+ version.
 

Phrygian77

Poster Extraordinaire
Gold Supporter
Joined
Apr 30, 2016
Posts
5,480
Location
Crawfordville, FL
Actually, the original AB763 Vibroverb schematic that I found shows parallel 20uF 525uF.

What are you building this in? I would assume a Super Reverb chassis. If so, that's the large cap board.
 

mistersparkle

TDPRI Member
Joined
Jan 5, 2011
Posts
16
Location
southeast PA
Actually, the original AB763 Vibroverb schematic that I found shows parallel 20uF 525uF.

What are you building this in? I would assume a Super Reverb chassis. If so, that's the large cap board.
I'm using a Weber 6A20HP chassis and fabricating the eyelet boards myself. I can make any cap board I want, but before I do I want to know if I need to accommodate a series pair of reservoir caps or if I can trust the "iffy" 16uF/450VDC caps shown on the attached Vibrolux schematic.
 

Attachments

  • vibrolux_ab763_schem.pdf
    47.3 KB · Views: 18

Phrygian77

Poster Extraordinaire
Gold Supporter
Joined
Apr 30, 2016
Posts
5,480
Location
Crawfordville, FL
Okay, let's clear this up. Are you building a Vibroverb or a Vibrolux??

Edit: nevermind, I see you edited your initial post.
 
Last edited:

Phrygian77

Poster Extraordinaire
Gold Supporter
Joined
Apr 30, 2016
Posts
5,480
Location
Crawfordville, FL
Forget what's on the the schematic. Nobody recaps those amps with 450V caps, so why are you hung up on that? Fender used under spec bias caps too in some amps. There's absolutely no good reason to replicate that.

Just use series caps for the first filter. Use 475V 16uF for the rest if you really want. I still recommend what I said in my first post.

Does that DR sized FDP-20 power transformer really have the current capacity to run 6L6GCs? I'm sceptical.
 

timfred

Tele-Meister
Joined
Mar 14, 2016
Posts
183
Location
Northern CA
Remember that the difference between 450 and 500V is only 10% so they are basically the same voltage rating. High quality name-brand caps of either rating have higher surge ratings and will work at 500V, even sustained, without catastrophic failure.

For a while at least. Higher voltage and temperature rated caps will last longer before needing replacement, but both 450 and 500 are at their spec limits will in a 450V+ B+ amp. Ideally you would spec 2X over B+ to maximize life. If you want the caps to go 20 years without replacement definitely put them in series, preferably with bleed resistors across them.
 

Arfage

Tele-Meister
Joined
Apr 18, 2019
Posts
471
Age
61
Location
Alameda Ca
Edit: I'm embarrassed that I accidentally attached the only AB763 schematic that doesn't reflect the query in my original post below (the Vibroverb schematic). I'm calling my build a "Vibroverb" clone only because I'm planning to use a single 15" speaker. Please refer to the now-attached Vibrolux schematic for reference to my cap voltage rating question stated below.

I'm in the process of building what basically amounts to a Fender Vibroverb clone, and I note that in all of the AB763-based schematics the five 16uF filter caps are all rated at 450VDC. Meanwhile, I also note that all the amps using 2x6L6 output tubes in this circuit have power transformers with 350-0-350 secondaries. The rectifier tube is a 5AR4/GZ34 with relatively low voltage drop, and two of the five filter caps are placed in front of the standby switch (in parallel, not series as is done in some later Fender circuits).

Everything I know about electronics tells me that, if the standby switch is opened, the filter caps in front of the switch will charge up very close to the peak of the AC voltage being applied, which in this case would be about 500V, give-or-take. Am I wrong, and if not, how did Fender get away with 450VDC rated caps? And if I'm right, should I build my amp with a pair of, let's say, F&T 80uF/450V caps in series (with balancing resistors, of course) for the reservoir? Btw, I already have the 350-0-350 PT (Mercury FDP-20) and I don't really want to lower the B+. And no, I'll probably never use the standby switch, but still....
Whatever's going on there hasn't been a "common problem" Vibroverb owners have become familiar with over the years, so I'd just go with it. However if it does bother you, it's easy just to copy a blackface showman setup with the series caps at the beginning.
 

Toroid

TDPRI Member
Joined
May 18, 2021
Posts
11
Age
69
Location
Netherlands
Edit: I'm embarrassed that I accidentally attached the only AB763 schematic that doesn't reflect the query in my original post below (the Vibroverb schematic). I'm calling my build a "Vibroverb" clone only because I'm planning to use a single 15" speaker. Please refer to the now-attached Vibrolux schematic for reference to my cap voltage rating question stated below.

I'm in the process of building what basically amounts to a Fender Vibroverb clone, and I note that in all of the AB763-based schematics the five 16uF filter caps are all rated at 450VDC. Meanwhile, I also note that all the amps using 2x6L6 output tubes in this circuit have power transformers with 350-0-350 secondaries. The rectifier tube is a 5AR4/GZ34 with relatively low voltage drop, and two of the five filter caps are placed in front of the standby switch (in parallel, not series as is done in some later Fender circuits).

Everything I know about electronics tells me that, if the standby switch is opened, the filter caps in front of the switch will charge up very close to the peak of the AC voltage being applied, which in this case would be about 500V, give-or-take. Am I wrong, and if not, how did Fender get away with 450VDC rated caps? And if I'm right, should I build my amp with a pair of, let's say, F&T 80uF/450V caps in series (with balancing resistors, of course) for the reservoir? Btw, I already have the 350-0-350 PT (Mercury FDP-20) and I don't really want to lower the B+. And no, I'll probably never use the standby switch, bu
 

pfarrell

TDPRI Member
Gold Supporter
Joined
Jan 26, 2021
Posts
99
Location
Upstate, NY
I built a Vibroverb from scratch recently. It's a great amp. You can see it here. @robrob is always worth a look. I cherry picked a bunch of mods based on my other amps and the notion that a lot of people seem to not spend much time in the normal channel I took the opportunity to change it—and that was great for me. Also played with my 2x10 cab and now have a 1x15 and it's awesome, per original setup. In that build thread is also a Marshall I built—I got distracted—It's awesome too!
 

mistersparkle

TDPRI Member
Joined
Jan 5, 2011
Posts
16
Location
southeast PA
I built a Vibroverb from scratch recently. It's a great amp. You can see it here. @robrob is always worth a look. I cherry picked a bunch of mods based on my other amps and the notion that a lot of people seem to not spend much time in the normal channel I took the opportunity to change it—and that was great for me. Also played with my 2x10 cab and now have a 1x15 and it's awesome, per original setup. In that build thread is also a Marshall I built—I got distracted—It's awesome too!
Rob Robinette is indeed a wealth of info on tube amps and I am incorporating several of his mods into my build. This amp is intended for my Fender D8 Stringmaster steel guitar, hence the single 15" speaker.
 

Toroid

TDPRI Member
Joined
May 18, 2021
Posts
11
Age
69
Location
Netherlands
According to the AB763 schematic in The Tube Amp book by Aspen Pittman 4.1 Edition the high voltage secundary on the power transformer reads 325Volt. This would result in 450 Volt DC on the first reservoir caps, 2x 16 uF in parallel, when there is zero current. On closer inspection, your schematic also reads 325V, although this is not very well legible. The voltage of the big filter ELcaps is usually chosen at 20 percent over the rated DC voltage when the amp is engaged.
You might consider using the longlife, high temperature caps by Kemet, like the series PEG 124, though these too only reach up to 450 VDC. And Kemet are very firm about not exceeding the rated max voltage (DC plus AC ripple).
 

cowasaki

TDPRI Member
Joined
Mar 28, 2020
Posts
3
Location
UK, Leyland.
Edit: I'm embarrassed that I accidentally attached the only AB763 schematic that doesn't reflect the query in my original post below (the Vibroverb schematic). I'm calling my build a "Vibroverb" clone only because I'm planning to use a single 15" speaker. Please refer to the now-attached Vibrolux schematic for reference to my cap voltage rating question stated below.

I'm in the process of building what basically amounts to a Fender Vibroverb clone, and I note that in all of the AB763-based schematics the five 16uF filter caps are all rated at 450VDC. Meanwhile, I also note that all the amps using 2x6L6 output tubes in this circuit have power transformers with 350-0-350 secondaries. The rectifier tube is a 5AR4/GZ34 with relatively low voltage drop, and two of the five filter caps are placed in front of the standby switch (in parallel, not series as is done in some later Fender circuits).

Everything I know about electronics tells me that, if the standby switch is opened, the filter caps in front of the switch will charge up very close to the peak of the AC voltage being applied, which in this case would be about 500V, give-or-take. Am I wrong, and if not, how did Fender get away with 450VDC rated caps? And if I'm right, should I build my amp with a pair of, let's say, F&T 80uF/450V caps in series (with balancing resistors, of course) for the reservoir? Btw, I already have the 350-0-350 PT (Mercury FDP-20) and I don't really want to lower the B+. And no, I'll probably never use the standby switch, but still....
The first two caps are in series with large value resistors balancing them. That gives them 900v value.
 




New Posts

Top