Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bootspratt on April 18, 2011, 10:59:19 am
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I recently got an early 70's Fender Princeton (no-verb) and I'm wanting to change out the filter caps. My amp has 3 caps in the can at 20uf/450v and one big cap in the chasis at 20uf/500v. Every Princeton and schematic I've seen shows 4 20uf/450v caps and no 500v ones. Do you think it would be OK to use a replacement can with 4 20uf/475v? Or am I risking an explosion. Tubes and More sells a perfect replacement can at 20uf/475v that would fit perfect and I wouldn't have that big cap in the chasis.
Here's pic of my amp: Thanks
(http://i178.photobucket.com/albums/w267/bootsypratt/P1020203.jpg)
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I don't think there's much practical difference between a 475 volt electrolytic and a 500 volt. However, I'd get on rated for 500 volts if it's available myself. Depends partly on what rectifier tube you've got in there. If you're using a solid-state substitute, 500 volts might not be enough safety cushion.
Is your amp the AA964 circuit or the AB1270 circuit? I'm asking because the AA964 circuit only has 3 filter caps on the power rail AFAIK. Check out Hoffman's schematic page for two versions of the AA964:
http://www.el34world.com/charts/Schematics2.php
You'll notice that the "reservoir cap" (first in power rail - supplies OT) is 40uf in the slightly later version.
Hoffman sells a JJ four section cap can that has 40-20-20-20 sections and is rated at 500 volts that is cheaper than the AES 4x20uf cap I can see. That might be a better alternative if your amp has 4 power rail filter caps.
BTW those reddish brown coupling caps on the board look a bit worse for wear & tear, especially the righthand one that looks like it's partly melted.
Also, is the "Ground" switch still wired with the "death cap" or does it serve another purpose? I think I see wiring for a 3-prong cord there. If so, please check and make sure it's wired properly. I worked on a Blackface Princeton Reverb last year that had a very nasty habit of shocking the guitarist (or amp tech).
Hope this helps,
Chip
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I use the JJ 40-20-20-20 in nearly every amp I build and that includes Plexi clones (except bass and single-ended amps). I've never had a failure. Keep in mind, I've never done an amp with SS rectification (again, except bass and single-ended amps). I doubt you'd have a problem with a Princeton... as I've replaced the cap cans in a couple of SF Princetons with the JJ's.
Dave
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I'm not exactly sure what circuit mine is, I can't find a schemo for the AB1270 circuit. But mine has a 5U4GB rectifier tube. Mine looks more like this circuit but with a 5U4GB: http://www.el34world.com/charts/Schematics/files/fender/princeton_aa964_schem.pdf
So if I got a 40-20-20-20 should I run the 40 cap to the rectifier? Will it change the tone?
My amp has the three way ground switch, and the wiring for the power chord is original but it only had a 2 prong plug. So I have the switch in the lifted (off) position, removed the death cap and installed a 3 prong plug using the original wire.
Not sure if the coupling cap on the right is melted, there is some silicon or wax there to hold the wires in place and it just look melted.
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That cap has a date code that suggests 1973.
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I use the JJ 40-20-20-20 in nearly every amp I build and that includes Plexi clones (except bass and single-ended amps). I've never had a failure. Keep in mind, I've never done an amp with SS rectification (again, except bass and single-ended amps). I doubt you'd have a problem with a Princeton... as I've replaced the cap cans in a couple of SF Princetons with the JJ's.
Dave
I pretty much use those exclusively also and I use them in 100watt SS rectified builds with not one failure. I wouldn't use it my Fender concert though. Plate voltage on that hovers around 500V.
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The picture is a new one for me. I've owned a number of blackface and silverface Princetons (with and without reverb). All had a 4 section cap can. However, the Mallory electrolytic is exactly the type Fender used and appears original. Looks like Fender got a good deal on some 3 section cap cans and used them.
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The picture is a new one for me. I've owned a number of blackface and silverface Princetons (with and without reverb). All had a 4 section cap can. However, the Mallory electrolytic is exactly the type Fender used and appears original. Looks like Fender got a good deal on some 3 section cap cans and used them.
That's what I'm thinking
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The picture is a new one for me. I've owned a number of blackface and silverface Princetons (with and without reverb). All had a 4 section cap can. However, the Mallory electrolytic is exactly the type Fender used and appears original. Looks like Fender got a good deal on some 3 section cap cans and used them.
This is the second picture I have seen someone post online of a Fender Princeton with this cap arrangement. My assumption is that week/month/whatever they ran out of the 4 cap cans and did this as a workaround. Leo reportedly regularly did that when he was running the show.
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I don't ever remember seeing a 3 section can in Princetons, only fours.
BTW, I have a note on the 40x20x20x20 JJcaps about the chassis hole size on Princeton and Champ chassis
http://www.hoffmanamps.com/MyStore/catalog/parts4htm
The terminals on the cap may be too close to the chassis unless the hole size is correct.