Fender AB763 Filter Cap Voltage Ratings

[Bob M.]

Mr. Sparkle,
With a 350-0-350 output power transformer, you'll need to put your electrolytic filter caps in series. I'd recommend using 100uf @ 350 volts. Two of these caps in series will yield an equivalent of 50uf @ 700 Vdc. Please make sure to use the bleeder/balancing resistors. Copy a SF Twin Reverb schematic and you'll be fine. With any sort of steel guitar, you'll want a very stiff filter, use the same as above on the screens. Then, depending on the B+ on the PI and the preamp plates (which I don't know here), I'd probably suggest single electrolytic [email protected] Vdc. Axial electrolytic caps at high voltages are now getting really expensive so in a homebrew amp, I'd probably opt for radial caps to save money.
For steel guitar, consider using a solid state (diodes) rectifier. Steel guitar amps have more in common with keyboard applications or bass guitar applications than most guitar amps. You'll want a stiff power supply with not very much sag. In my amps, I have this rectifier choice on a switch so I can use either type of rectifier, depending on the circumstances at hand. More likely, you'll be using the solid state option most of the time.
Fender placed the standby switch in different places in the filter string in different eras of designs. Sometimes, the 1st filter is on the hot side of the standby switch with all subsequent filters downstream of the standby. Other schematics show all filters downstream of the standby switch, depending on the era of when the amp was built. With this second design, sometimes you'll get a noticeable 'thump' when switching from standby to play.
If this amp is dedicated to steel guitar use, consider using 4x 6L6 power tubes. You might be very glad you made this design choice, especially when playing outdoor gigs where the extra power of 4x power tubes is frequently needed for a clean sound. You also can incorporate a switch in your design to standby two of those power tubes so you have the more familiar Vibroverb power amp compliment. I also do this on a few of my amps and it works well.
Good luck, have fun,
Bob M.

 

[enorbet2]

Greetings. Vintage Heretic here. Before I "speak the heresy" let me say that Tweed amps are my favorite amps of all time. I've owned several, gigged extensively with them and other amps, restored, modded, repaired... you name it. I've likely done it with many vintage amps BUT there are some things that were designed and built down to a price that can benefit from upgrades on a one off.... especially in the power supply. For example, while replacing center-tapped heater windings with even the lousy plus or minus 10% resistors can function OK, the substitution wasn't made for better quality but for lower cost. A center-tapped secondary is better and DC heaters are even better - much less hum and zero effect (other than all but eliminating non-musical intermodulation harmonics) on tone.

In addition to heater supply compromise for cost, IMHO 16uf filter caps are simply insufficient to do a decent job of filtering ripple and that's even worse with solid state rectification. In my experience it is better to build a solid but supple (low ESR) filter circuit. If you find that level of articulation reduces sag too much you can adjust negative feedback and B- to get it back but with less noise and bad-sounding distortion, not to mention a hedge against parasitic oscillation.

Do what you will, but blind reverence is rarely a wise decision. Progress does exist. You just have to choose and balance.

 

[Phrygian77]

@King Fan I don't know what happened, but whatever it was caused members to show up and comment in a thread that they may not have seen otherwise.

Edit: Well now I'm even getting my threads mixed up.

 

[One-Eyed Jack]

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 AA763 and AB763 schematics I have for the 1964 Vibroverb show 20uf caps rated at 525VDC. The AB version changes the parallel 20uf/525VDC capacitor bank to series 70uf/350VDC caps, giving it a higher voltage rating. So, the original circuits aren't underrated on the face of it. The current F&T 22uf caps are rated at 500VDC, very usable with a B+ of 465VDC.

I'm not sure where you are finding Fender using 16uf/450VDC caps. The schematics I'm referring to above are from Schematic Heaven online and in Dave Funk's Tube Amp Workbook, and are the original Fender drawings.

I built a Vibroverb clone myself and it's really one of my favorite amps. I think one key to achieving playing satisfaction is finding an original, functional D130F and having the magnet recharged (but don't recone the speaker). I went through a number of other speakers, including a few Altec 418s (Series 1 and 2), reconed JBL D130s, JBL K140, vintage Jensen P15N, and Celestion Fullback. Find a D130F (maybe a D130) with a good original cone and have it remagged. Over the years, the magnetic field from the voice coil will demagnetize the AlNiCo PM and it should be refreshed.

 

[Wound_Up]

I don't have anything to add to the discussion. I just wanted to post that, I swear I just read this same kind of thing about Mesas? They used 450V caps that regularly see 500V and they end up being an issue down the road or something. I don't recall exsctly so I may have some of the details incorrect. But the extent of it was underrated caps that ended up causing known issues.

 

[Ellen Faye]

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....

I just finished a single channel Vibroverb. Your transformer is really spot on for that build. The differences in each variation of those amps is slight, 6L6, 6V6, speaker arrangement etc. If you have that transformer just build the Vibroverb because that is what you are going to end up with anyhow. And the filter capacitor arrangement takes care of that high inrush voltage. And if you use that arrangement, you are essentially building that amp.

 

[Ellen Faye]

The AA763 and AB763 schematics I have for the 1964 Vibroverb show 20uf caps rated at 525VDC. The AB version changes the parallel 20uf/525VDC capacitor bank to series 70uf/350VDC caps, giving it a higher voltage rating. So, the original circuits aren't underrated on the face of it. The current F&T 22uf caps are rated at 500VDC, very usable with a B+ of 465VDC.

I'm not sure where you are finding Fender using 16uf/450VDC caps. The schematics I'm referring to above are from Schematic Heaven online and in Dave Funk's Tube Amp Workbook, and are the original Fender drawings.

I built a Vibroverb clone myself and it's really one of my favorite amps. I think one key to achieving playing satisfaction is finding an original, functional D130F and having the magnet recharged (but don't recone the speaker). I went through a number of other speakers, including a few Altec 418s (Series 1 and 2), reconed JBL D130s, JBL K140, vintage Jensen P15N, and Celestion Fullback. Find a D130F (maybe a D130) with a good original cone and have it remagged. Over the years, the magnetic field from the voice coil will demagnetize the AlNiCo PM and it should be refreshed.

Could you tell us more about having a speaker "remagged". I was under the impression a speaker was toast when the Alnico magnet lost its magnetism.

 

[Phrygian77]

@Ellen Faye I think it can be done during the reconing process, but I don't see how it could be done without reconing.

 

[mistersparkle]

I just finished a single channel Vibroverb. Your transformer is really spot on for that build. The differences in each variation of those amps is slight, 6L6, 6V6, speaker arrangement etc. If you have that transformer just build the Vibroverb because that is what you are going to end up with anyhow. And the filter capacitor arrangement takes care of that high inrush voltage. And if you use that arrangement, you are essentially building that amp.

Totally agree, and have adjusted my schematic and BOM to reflect the Vibroverb power supply circuit (that I foolishly didn't notice was different from other 2x6L6 AB763 circuits).

 

[Lowerleftcoast]

I was under the impression a speaker was toast when the Alnico magnet lost its magnetism.

Alnico magnets can be charged or discharged. They can also have their poles swapped if one desired to do so. Alnico is found in many guitar pickups as well. Over the natural life, an Alnico magnet loses very little charge. It is one of the most stable magnetic materials. Alnico is easy to charge or discharge. I wouldn't worry about an Alnico speaker losing charge unless it were exposed to a strong magnetic field. Something like placing two speakers directly back to back. Under normal use the magnet will not lose enough charge during your lifetime to worry about.

By the way... don't put your Alnico pickups near the back of a speaker.

 

[Silverface]

and I note that in all of the AB763-based schematics the five 16uF filter caps are all rated at 450VDC

What is throwing you off is that Fender's tweed, brown, white, BF and most early SF amp schematics had voltage notations and parts values based on 112VAC input voltage.

Modern USA input voltage is generally 121VAC. To account for that I replace all filter caps with voltage capacity at LEAST 10% higher and adjust other voltage critical parts as needed. I've found that just changing rectifiers to try to lower overall voltages doesn't get the same tonal results.

 

[mistersparkle]

What is throwing you off is that Fender's tweed, brown, white, BF and most early SF amp schematics had voltage notations and parts values based on 112VAC input voltage.

Modern USA input voltage is generally 121VAC. To account for that I replace all filter caps with voltage capacity at LEAST 10% higher and adjust other voltage critical parts as needed. I've found that just changing rectifiers to try to lower overall voltages doesn't get the same tonal results.

Thanks. I ended up using two F&T 80uF/450V caps in series for the reservoir, and three F&T 22uF/500V for the filter caps. The amp is finished and works great. All measured voltages are well within the capacitors' rated working voltages, even with my AC line voltage of 126VAC.

 

[Silverface]

even with my AC line voltage of 126VAC.

Yikes!

That's the highest line voltage I've heard of unless a new service was installed and an error made.

Did you check to outlets for polarity and ground? I've seen plenty of 3-prong sockets with no ground wire connected and/or reversed hot and neutral wires.

 

[mistersparkle]

Yikes!

That's the highest line voltage I've heard of unless a new service was installed and an error made.

Did you check to outlets for polarity and ground? I've seen plenty of 3-prong sockets with no ground wire connected and/or reversed hot and neutral wires.

I was going by the cheap, little voltage/current meter I built into my light bulb current limiter, but so far that meter has agreed with my legitimate DMM. Today the voltage is 124V, as shown in the pic, but it was 126V the day I checked my amp's DC voltages. I also just checked the meter on my 240V electric car charger and it reads 248V, so all my meters are in agreement. Perhaps I'll check the voltages in some neighbors' homes and if we're all high maybe I'll contact the power company. This high voltage could explain why the circuit breaker in my central vac unit trips on occasion. Thanks for bringing it to my attention. I always thought my 125V (+/- 1V) was normal.

 

[Silverface]

I was going by the cheap, little voltage/current meter I built

That DIY meter is not enough, and it sounds like you may be unaware of safety precautions.

You need a decent multimeter - high voltage caps should not be touched at all until you've checked for residual voltage, even if you think you are discharging them safely. If you are going to even do minor amp work you need a good multimeter.

You don't have to buy an expensive Fluke (which I and most techs use), but can get a decent one that handles at least 700+VDC for $40-50. FWIW the $10 multimeters sold or given away by Harbor Freight are useful only as fishing line weights!

It seems your DIY voltage meter is reading high - and you need to check both AC and DC voltages, check capacitors for stored power (which can, in some amps, be enough to shock or kill you with the amp unplugged and turned off) test resistors, test for continuity, read current and do basic capacitor tests (an LCR meter is needed to test both capacitance and ESR accurately - not vital but certainly useful - you can find used ones for $80-100 at times) etc to work on amps.

Dummy loads with high-wattage (50-100 depending on the amps you test) and each reading 4, 8 and 16 ohm are pretty much a necessity unless you like to listen to loud buzzing or hiss through speakers (since tube amps need a matching impedance load - DC resistance is good enough) any time tubes are installed and they are powered up.

Understanding amp safety (only one hand in the amp, ever; how to safely discharge filter caps; what NOT to touch; before doing ANYTHING check o see if it's plugged in or not, etc) and those basic tools will get you by. Oscilloscopes are rarely, if ever, needed and for more advanced signal tracing (with a signal generator); tube testers are a waste of money unless simply checking for shorts or ONLY testing preamp tubes; and a Variac isn't needed, period.

 

[Wound_Up]

Yikes!

That's the highest line voltage I've heard of unless a new service was installed and an error made.

Did you check to outlets for polarity and ground? I've seen plenty of 3-prong sockets with no ground wire connected and/or reversed hot and neutral wires.

My old house was full of them. Apparently the owners updated the house and didn't pay attention when they were replacing the power outlets. Literally all but 2 outlets in the entire house had no ground(old house) and most had neutral and hot reversed. It made for some pretty loud humming and buzzing from my gear until I fixed a couple in my room to use.

I was able to find an outlet in my room that had a ground, and neutral & hot were reversed so i fixed those and only used that outlet from then on. That alone killed most of the noise.

Then I moved to a more modern house with a modern electrical system. No more outlets with no ground, thank God. The first time I plugged my amp in at this house, it was so quiet I almost couldn't tell the amp was even on! Quite the difference from the house I moved from.

 

[Wound_Up]

That DIY meter is not enough, and it sounds like you may be unaware of safety precautions.

You need a decent multimeter - high voltage caps should not be touched at all until you've checked for residual voltage, even if you think you are discharging them safely. If you are going to even do minor amp work you need a good multimeter.

You don't have to buy an expensive Fluke (which I and most techs use), but can get a decent one that handles at least 700+VDC for $40-50. FWIW the $10 multimeters sold or given away by Harbor Freight are useful only as fishing line weights!

It seems your DIY voltage meter is reading high - and you need to check both AC and DC voltages, check capacitors for stored power (which can, in some amps, be enough to shock or kill you with the amp unplugged and turned off) test resistors, test for continuity, read current and do basic capacitor tests (an LCR meter is needed to test both capacitance and ESR accurately - not vital but certainly useful - you can find used ones for $80-100 at times) etc to work on amps.

Dummy loads with high-wattage (50-100 depending on the amps you test) and each reading 4, 8 and 16 ohm are pretty much a necessity unless you like to listen to loud buzzing or hiss through speakers (since tube amps need a matching impedance load - DC resistance is good enough) any time tubes are installed and they are powered up.

Understanding amp safety (only one hand in the amp, ever; how to safely discharge filter caps; what NOT to touch; before doing ANYTHING check o see if it's plugged in or not, etc) and those basic tools will get you by. Oscilloscopes are rarely, if ever, needed and for more advanced signal tracing (with a signal generator); tube testers are a waste of money unless simply checking for shorts or ONLY testing preamp tubes; and a Variac isn't needed, period.

Aling with a decent meter, I'd suggest a capacitor discharge tool, also. I have one that's got a voltage readout on it so you can visually confirm that whatever you're working on is discharging/has been discharged as you discharge it. Most are good for anything up to 1000V. They also usually have a 2.1mm test port to check voltage of any standard power adapter.

 

[timfred]

Yikes!

That's the highest line voltage I've heard of unless a new service was installed and an error made.

Did you check to outlets for polarity and ground? I've seen plenty of 3-prong sockets with no ground wire connected and/or reversed hot and neutral wires.

I am not an electric utility engineer but I think utilities in the US consider sustained +/- 5% to be within tolerance and expect equipment to work correctly anywhere in that range. For 120V nominal, that’s 114-126V. These days I think they try to target the upper end of the range as insurance against line losses between the distribution transformer and your house.

 

[Phrygian77]

Aling with a decent meter, I'd suggest a capacitor discharge tool, also. I have one that's got a voltage readout on it so you can visually confirm that whatever you're working on is discharging/has been discharged as you discharge it. Most are good for anything up to 1000V. They also usually have a 2.1mm test port to check voltage of any standard power adapter.

View attachment 997597 View attachment 997598

That's one the most unnecessary tools that I've ever seen.