Sceptre Schematic Filter Cap Specs -v- what my amp had in it

[MrCreosote]

I downloaded the 1968 schematic for the Scepter and found these callouts:

500v: 30uF, 525v
485v: 20uF, 525v
385v: 20uF, 525v
295v: 20uF, 525v
260v: 20uF, 450v

but these are what I found in my amp: (I'll try and post am image later)

500v: 20uF, 600v (Sprague Beaver Atom mounted in what appears to be a fractory clamp)
485v: 30uF, 525v*
385v: 20uF, 525v*  (* is the Quad cap)
295v: 20uF, 525v*
260v: 20uF, 525v*

The BIG SURPRISE is that my amp has a 20uF,600v Atom where a 30uF,525v is in the schematic.  This also implies that this particular cap was from the Quad Cap.  If I stick with the wiring of my amp, I could simply replace the 20uF,600v Atom or 1) use a 25uF,800v from Antique Electronic Supply, or 2) use a parallel pair of 20uF's to get 40uF.  I think since the price of the 25uF,800v is not much more than the 20uF,600v, I will at least do that.

I also like the idea of using a 600v on the 500v leg and think that would be a very good idea on the 485v leg too. Which leads me towards recapping with individual Atoms which would actually be more robust than getting the repro Quad Cap.

What is interesting in the schematic is the 450v cap on the 260v leg.  This would imply that the quad cap would have been used for the 4 higher legs and the single cap for the lowest, 260v leg.  Does anyone know if this was how the amps were wired from the factory?

If I do individual Atoms, I would use a 600v cap  485v leg, and 500v caps for the remaining legs.  I would reduce my amps wiring of the 30uF 525v to 20uF on that 485 let.

Using a 500v cap on the 385v leg would be a reduction from the 525v schematic value and my amp's wiring, however, if they were using 525v caps on a 500v leg, I would think the 500v on the 385v leg would be OK.

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As far as series caps, I think the only purpose for that is to increase capacitance values - otherwise Atoms would be the way to go.

[MrCreosote]

I just compared the Dynaco Stereo 70 schematic and the voltages at the quad cap are significantly lower:

305, 375, 415, 435v* (* is the 30uF cap)

While most of what I read indicated that the Sunn was a copy of the Dynaco, it is clear that this power transformer is different in the number of taps, so I wonder if the Sunn is really running at higher voltages than the Dynaco.

If that is the case, it would be a VERY GOOD IDEA to upgrade the voltage spec on the filter caps and NOT use the Dynaco quad cap.  I just find it amazing that there is a discrepancy of 500v (Sunn) -v- 435v (Dynaco) coming off the GZ34.

Also, in the Sunn schematic, where in the world does the unlettered 260v leg go and what is the "D" leg doing at 16.5v?  I'm not the best at reading schematics but I think the "D" callouts are a mixture of the unmarked 260v (which is next to the "C" and would be expected to be called "C") and the 16.5V defined as "D"

Anyhow, my head hurts and I have to start doing my taxes.....
Tom

(I can't stay away from this....):

To make matters even worse, the Stereo 70 MK II had significantly increased capacitances at essentially the same leg voltages:  (a/b = uF/v)

100/400, 100/400, 68/450, 68/450 -v- 20/525, 20/525, 20/525, 30/525

Further, they eliminated the GZ34 for a solid state diode rectifier.

Which probably reveals why the audiophiles are using the series capacitors and the diode rectifiers.

While all this suggests the voltage rating of the quad cap is a limiting factor, my amp was owned by the piano player of a very popular local band that performed out a lot.  And it served him well.  So maybe the quad cap is just fine for a replacement.  The higher cap values offered by the series cap mod probably improves sound.  Whether it is that dramatic probably depends on whether you have a "golden" or "tin" ear.

Of course, I have no idea which way I'm going to go:  Quad Cap, Atoms, or Series setup.

Arghhhhhhhhh....

[JoeArthur]

I think you are over-analyzing.

It is not unusual for musical instrument amps to be run at higher voltages.

Measure the actual voltage in your amp. Line voltage has varied over the years and could be higher than what was typical in the late 1960s, and an increase in line voltage would give an increase in B+ voltages.

I wouldn't increase the capacitance rating of the first cap (30mf) unless you switch to a solid state rectifier. Tube rectifiers do not like feeding a lot of capacitance.

[MrCreosote]

I think you are over-analyzing.

Guilty as charged!
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I wouldn't increase the capacitance rating of the first cap (30mf) unless you switch to a solid state rectifier. Tube rectifiers do not like feeding a lot of capacitance.

I found a lot of references that the tube rectifiers don't like a lot of capacitance.

I was really wondering if the Sceptre, according to the schematic, ever used the 30uF from the quad cap for the first cap since mine did not. 

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Maybe I should do what is easiest:  replace the quad cap with the new AES repro and upgrade the 20uF/600v to the AES 25uF/800v cap in the attempt to get closer to the factory schematic. 

The individual Atoms would cost about the same but would be more robust, but entail extra labor.

It is safe to say, I've ruled out the series cap mod since it would only save a little money and add a bunch to the labor.

[Soundmasterg]

The Dynaco that the Sunn 200S was patterned after was not the Stereo 70, it was the Dynaco Mark 3. There's a voltage difference in these by quite a bit, with the Mark 3 being very close to the 560+ that the Sunn is. The Stereo 70 is closer to a stereo version of the smaller Sunns that used the EL34 instead of the 6550/KT88 version Sunns. The Stereo 70 has four EL34's and two output transformers since it is stereo.

The Atom caps are old technology and many modern caps will be a lot smaller, AND outperform the old Atoms in ESR and leakage tests. I also heard a rumor that the Atoms have been made for several years by putting smaller caps inside the larger Atom container...I don't know if that one is true or not. I've had great luck with the F & T brand, and with the Xicon brand, and they're quite a bit cheper than the Sprague Atom series. The main reason the Atoms have stuck around is that they fit best in the vintage Fenders IMO. They're aren't RoHs compliant, so their price may go up even more in the future as less people use them. I try to stay away from the Illinois brand of caps btw....they're the only ones I've ever used where a couple blew up when brand new, and yes I hooked them up right and didn't over voltage them.

The AES replacement can cap will work, but at 525v is over voltage and won't last as long as series connected caps for a higher voltage rating, or the Atom 600v ones will. Two series connected 350v caps will essentially give you a 700v cap, and they're aren't THAT hard to hook up. Whichever way you go, I'd suggest at least a 500v rated cap for the 3rd section, and at least 600v rated caps for the first two sections. The GZ34 tube rectifier can take up to 60uf but I'd be wary of doing that with modern GZ34 types. For bass, it works better with a solid state setup with higher initial cap values anyway, and the Weber Copper Cap WZ34 model emulates the sag of the tube rectifier so it keeps the voltages in a safe range. The higher value caps in the first two stages make the amp quicker to respond to bass notes, whch audibly tightens up the bass response of the amp quite a bit.

Take a look at the Dynaco Mk. 3 schematic and you'll see something closer to the Sunn 200S.

Greg

[Soundmasterg]

Forgot to add that the SDS boards that Triode Electronics sells are a great option of you want quick and easy. They will give you a higher voltage rating and install pretty easily inside the amp. Just disconnect the can and leave it in place if it didn't make too much of a mess. Make sure to clean up all the goo from the cap as that stuff is corrosive and will eat away at the metal and the rest of the parts inside the amp.

[MrCreosote]

Soundmasterg,

The only 600v brand I could find was Atom.  I ran into F&T's and I think Xicon is actually Xicon/Atom.  Now AES sells the "ET" caps like 25uF/800v of which I have no idea who makes them.  Hopefully, they are not just a series pair.  The very best selection of high voltage capacitors I have found was AES and they didn't have 600v F&T's.  (I did contact AES and they said their new Dynaco 30-20x3/525 is good for the Sunn.)

A series pair would be real easy and cheap without voltage sharing resistors.  With the resistors, I'd have to recalculate a resistor network for all the legs coming off the rectifier (right?  I don't know if I could get away w/o the sharing resistors.)

The Dynaco can in my amp is not over-voltage - it is NOT wired to the first leg so the 30uF/525v is on the 485v 2nd leg so that should be a good thing.  I took a picture of this.  Does it look factory?  According to the schematic, it looks like the single cap should be the bottom 260v leg 20uF/450v cap, not the top 500v leg.  And then there is no mention of a 20uF/600v cap in the schematic which my amp has.

My latest thinking is the AES quad with the 25uF/800v "ET" cap from AES.

[JoeArthur]

A series pair would be real easy and cheap without voltage sharing resistors.  With the resistors, I'd have to recalculate a resistor network for all the legs coming off the rectifier (right?  I don't know if I could get away w/o the sharing resistors.)

DO NOT use series caps without voltage sharing resistors. And no, it is not necessary to recalculate the other B+ resistor values.

[Soundmasterg]

The thing you have to realize about the schematics is that when those were written and when the Sunn amps were made the wall voltage was lower, more along the lines of 110v to 115v. Nowadays it is 125v and can creep up to 140v at night when the load is lower. There isn't a linear relationship as far as the DC voltages inside the amp in relation to what the line voltage is, so those votlages on the schematic are not accurate, and are too low for what the voltages in the amp really are. AES is going to say their can cap will work for the Sunns because it is the same as the original can mostly, but they aren't taking ito account the different wall votlages of today. The can will work, but won't last as long as the other solutions because it is over voltage at all times. AES also sells lots of Illinois caps, so if a brand is not listed, it is usually Illinois which you shouldn't use except as last resort. Xicon are the light blue ones whereas Illinois are usually grey. Xicon are not the same as Atoms. Atoms are very large compared to modern caps, almost twice the size in some cases.

If you're going to use the Sprague Atoms, it will work, but the series connected caps (two 350v caps) will give you a 700v rating and will be cheaper than the one Atom 600v cap. F & T don't make 600v caps...Atom is the only one I know of doing that with electrolytics. If you go with the series connected caps, you NEED the 220k 1W resistors in parallel with each of the series connected caps so that the caps will share the voltage and not have one try to hog all the voltage and blow. You won't have to reconfigure the dropping resistors at all in the amp. The 220k resistors are only working in between the series connected caps and don't interact with the rest of the dropping network. Even with the voltage sharing resistors, the series connected caps are still quite easy to hook up. It isn't rocket science as they say.

Greg