NAD: 1968 Super Reverb

sds1

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Also for the sake of argumentation and for my edification, is a cap's lifespan dependent on how long ago it was made, or how long ago it was installed?
It's based on the manufacture date. The caps do age just sitting on the shelf. Then there are aggravating aging factors, the main one I think is heat. The datasheets often say 15 year service life. There is obviously no way to predict the actual end of life. Just put a reminder on your calendar for this time next year or whatever, I'm sure they will hang on for a while longer.
 

itsGiusto

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It's based on the manufacture date. The caps do age just sitting on the shelf. Then there are aggravating aging factors, the main one I think is heat. The datasheets often say 15 year service life. There is obviously no way to predict the actual end of life. Just put a reminder on your calendar for this time next year or whatever, I'm sure they will hang on for a while longer.
Another issue is that the site I usually order from doesn't stock 525 or 600v 20uf caps that are less than like 10 bucks a piece. But I guess these 500v ones have a surge voltage of 550v, at least:
 

sds1

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and all of them looked and measured just fine.
Maybe you are well-equipped, but I don't have the gear needed to test caps at their services voltage, so the tests I can perform (with my little Peak ESR tester for example) are not reliable enough. So, I go by date only. 🤷🏻‍♂️
 

sds1

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Another issue is that the site I usually order from doesn't stock 525 or 600v 20uf caps that are less than like 10 bucks a piece. But I guess these 500v ones have a surge voltage of 550v, at least:
Not a lot of choices out there anymore. Has nobody come up with a retrofit board/assembly to install radial caps in the Fender doghouse? 🤔

The IC's are only $4 each:

I would use those, but other folks would not. :)
 

itsGiusto

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Not a lot of choices out there anymore. Has nobody come up with a retrofit board/assembly to install radial caps in the Fender doghouse? 🤔

The IC's are only $4 each:

I would use those, but other folks would not. :)
Haha, yes, that's true. I don't have much negative experience with Illinois caps other than this bizarre incident:

However simply because everyone has trash talked them so much, I almost instinctively overlook them when I'm shopping on sites. I like, forget to even consider them.
 

Lowerleftcoast

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About 10 years ago, I had a tough time finding a datasheet for those blue caps. When I found it, it listed their 65C temp rating and also listed a very short life expectancy. Iirc, the shelf life was only 6 months. About two years ago I looked up the datasheet again. I could not find that old datasheet. I found a newer one with much better specs. I never heard they had a *new and improved* product but the datasheet sure had better specs. They do have a good reputation so I wouldn't be too worried about using them. I just have an aversion to the price.

Imo, E-cap life is similar, as they say, to car tires. Little use or no use may make them look good but they age. At a point they will not be safe. For longest life an E-cap should be charged and discharged regularly.

Back in the day, E-caps would be *formed* over several hours to get the best performance out of them. Manufacturers claim modern caps do not need to be *formed* but if they have been on the shelf or in a closet for extended periods of time, imo, they should be.
 

Jasonpatrick

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IC caps are garbage. Why cheap out on the most important part. 🤷‍♂️ Get FnT and be done with it. 500v is fine. I’m assuming you ain’t gonna be sitting there with the standby on like a moron for hours but even if that was your jam, you would be fine :) you are throwing money away getting the sprague 600v.. and imho, Why is IC garbage? I have never seen more failed caps in newer fenders then with this brand. Bulging leaking garbage.
 

SoK66

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Another issue is that the site I usually order from doesn't stock 525 or 600v 20uf caps that are less than like 10 bucks a piece. But I guess these 500v ones have a surge voltage of 550v, at least:
For that amp you don't need caps with a voltage rating higher than 500v. The old Spragues that were 600v rated wouldn't fit inside the doghouse cover anyway. I haven't seen any available in years. I usually get caps & resistors from either Antique Electronic Supply (same outfit as Tubes & More) or from Doug Hoffman at Hoffman Amps. Both are good suppliers with fresh stock.
 

SoK66

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I wouldn't just replace the blue molded caps willy nilly. Test for DC leakage & act accordingly. Nothing wrong with the brown " turd " caps. Plenty of older Fender amps, mine included, have them & they work & sound great. 😀
In all the years I've been fiddling with Fender amps I've never come across a blue molded cap that was leaking DC or failed. The old yellow Astrons in the tweeds and brown amps are another story. The brown blobs got a bad rep early on, but if they're still working and close to spec they're fine. The Cornel Dubillier (CD) ceramic caps in the OPs (and my) '68 SRs are very good and stable, the older red Sprague ceramic caps found in brown & black panel amps are very prone to leaking and drift. Easy to check.
 
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itsGiusto

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Okay, I could use some advice.

I took some voltages in the amp to see if I may want to do as @SoK66 suggested here and swap the 5U4 out for a GZ34, like would have come stock with this amp. Here are some of the voltages I found. Voltages were taken with amp fully on, all tubes and speakers plugged in, amp not in standby, and with volumes set to 0.

Voltage at 5U4 input: 359v AC
Voltage at 5U4 output (B+): 444v DC
Voltage at both power tube plates: 440v DC
Voltage at first preamp tube plate: 270v DC


Compare these against this schematic:
Super-Reverb-AB763-schematic1024_1.jpg


I find this confusing, because, the voltage input to the rectifier tube is correct (360v AC), and as @SoK66 suggested, the fact that there's a 5U4 there means that B+ is lower than it should be (444v instead of 465v). However, if this were the case, I would also expect that the preamp plate voltage would be lower as well, but instead we have a voltage of 270 at the plate, which matches the schematic perfectly.

Of note: I measured the values of the two dropping resistors in the dog house. They were 1k and 5.6k (even though the markings indicate 4.7k - it must have drifted over time). I didn't measure the choke, but I looked up the model (Mojo 777), and it seems to be the correct value (3H, 125C1A).

The dropping resistor confuses things even more: if the resistor has drifted to be increased in value, wouldn't that result in the plate voltage at the preamp tube being even lower? Why is the plate voltage at the preamp tube to spec, despite the dropping resistor difference and the rectifier tube difference?

Finally, given this, would it be a bad idea to put a GZ34 into this amp? Would the preamp tube voltage end up being too high?

Thanks, all!
 
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SoK66

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I would say those readings are dead on the money. A b+ of 440 is just about true to the schematic if memory serves me correctly, but bear in mind the schematic values assume an input voltage of around 117vac. At today's wall voltage of 120 - 125vac, with a GZ34 I'd then expect the B+ to be closer to 470 vdc. The voltages are always approximate and the schematics have a +/- 20% disclaimer, although no one believes in that much variability. The B+ is ahead of the dropping resistors, which affect the phase inverter and preamp voltages. But, yes, if those have drifted up in value (my '66 Deluxe Reverb's had drifted up almost 20%) it results in lower voltages downstream. Plate resistor resistance variability affects preamp and phase inverter voltage readings even more.
 

itsGiusto

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I would say those readings are dead on the money. A b+ of 440 is just about true to the schematic if memory serves me correctly, but bear in mind the schematic values assume an input voltage of around 117vac.
Not quite, in the schematic B+ should be 465. So, within 20%, but not quite exact.

At today's wall voltage of 120 - 125vac, with a GZ34 I'd then expect the B+ to be closer to 470 vdc. The voltages are always approximate and the schematics have a +/- 20% disclaimer, although no one believes in that much variability. The B+ is ahead of the dropping resistors, which affect the phase inverter and preamp voltages. But, yes, if those have drifted up in value (my '66 Deluxe Reverb's had drifted up almost 20%) it results in lower voltages downstream.
That's what's weird though, the voltage isn't lower downstream, it's higher than I expected, 270 on the preamp plates. Given that B+ is low, I'd expect that preamp plate voltage to be low, too, but it's not.

At today's wall voltage of 120 - 125vac, with a GZ34 I'd then expect the B+ to be closer to 470 vdc.
So do you think I can try putting in a gz34, then? Would it be safe? What plate voltage on the preamp tubes would be too high?
 

SnidelyWhiplash

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In regards to the Super, the 100k plate resistors are a mix of carbon & metal. One of each. I can't really read the value of the metal. Could it be possible that said resistors are the wrong value??? 🤔
 
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sds1

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Why is IC garbage? I have never seen more failed caps in newer fenders then with this brand. Bulging leaking garbage.
Why do we only hear about how crappy IC caps are in the context of the modern production Fenders? Does IC manufacture caps for Fender to a lesser specification? Is Fender the only company that buys IC's? Is CDE oblivious to the quality issues after all these years and simply refuses to address the problem? What about Fender, also oblivious plus ignored the issue on all subsequent production runs to this day? Does Mouser have a reputation of selling crap?

These are some of my questions for the folks who have drawn broad conclusions from a very narrow scope of issues with those parts.
 

SnidelyWhiplash

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IC EC caps did have a high failure rate years ago. A bad batch, if I'm not mistaken. It may not be fair to make broad generalizations, but it only takes one said failure to make one very wary & swear off that particular brand.
 

Jasonpatrick

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Why do we only hear about how crappy IC caps are in the context of the modern production Fenders? Does IC manufacture caps for Fender to a lesser specification? Is Fender the only company that buys IC's? Is CDE oblivious to the quality issues after all these years and simply refuses to address the problem? What about Fender, also oblivious plus ignored the issue on all subsequent production runs to this day? Does Mouser have a reputation of selling crap?

These are some of my questions for the folks who have drawn broad conclusions from a very narrow scope of issues with those parts.
Hey man, you do you and keep using the cheap parts in critical areas. I’m just throwing it out there. Also, it’s laughable that you mention Fender ignoring problems. How them blues Jrs doing with their screen resistors …. They change that up yet so it doesn’t roach the board?
 

sds1

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IC EC caps did have a high failure rate years ago. A bad batch, if I'm not mistaken. It may not be fair to make broad generalizations, but it only takes one said failure to make one very wary & swear off that particular brand.
Absolutely. But with like 5 choices of axial caps remaining for legacy applications, and some of those being ridiculously expensive, it seems like a shame to rule one of those choices out based on a false premise.
 

sds1

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Hey man, you do you and keep using the cheap parts in critical areas. I’m just throwing it out there. Also, it’s laughable that you mention Fender ignoring problems. How them blues Jrs doing with their screen resistors …. They change that up yet so it doesn’t roach the board?
What brand are those screen resistors? So we can be sure to anecdotally avoid those too.
 
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