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Old 3rd June 2009, 08:48 PM   #1
Trout is offline Trout  United States
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Default Capacitor Voltage Rating Question

Hi Guys,

What is a good rule of thumb with the voltage rating on Electrolytic Caps.

Example,

Sprague Atom 20uf /500V cap in a tube rectified circuit.
Under load, Cap input voltage of 500-506V ?

Or is it better(more reliable) to series 2 lesser caps rated at say 300-450V?

I looked and looked for a tolerance spec on the voltage for the Sprague atom TVA 1906 but I can find nothing at their site.

Option #2 Is

Replace the GZ34 rectifier with a 5U4GB and utilize the larger voltage drop.

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Old 3rd June 2009, 09:14 PM   #2
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The life of a capacitor is a strong function of the applied voltage. For long life it would be desirable to limit the voltage to no more than 50% of the rated voltage. (You might be able to push this some, but I wouldn't unless there was a very good reason for doing so.) For your situation I would use 2 500 volt caps in series. You also need to use balancing resistors to account for differences in the leakage currents of the 2 caps.
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Old 3rd June 2009, 09:24 PM   #3
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Quote:
Originally posted by sawreyrw
The life of a capacitor is a strong function of the applied voltage. For long life it would be desirable to limit the voltage to no more than 50% of the rated voltage. (You might be able to push this some, but I wouldn't unless there was a very good reason for doing so.) For your situation I would use 2 500 volt caps in series. You also need to use balancing resistors to account for differences in the leakage currents of the 2 caps.

Another critical point I should mention, this is in a 50's guitar amp circuit.

Based on info I gathered from many of the vintage schematics, it appears it was common practice to apply voltages very close to the cap rating. (Perhaps a production cost thing)

Many of these amps from the 50's/60's are still going strong even past what is thought of as beyond life cycle for the part.

In this case, we have a PP 6l6GC amp, Cathode Biased, and 2 12AX7's (1 Pre and 1 split load PI )

At the first cap which is where the OT center tap is attached a GZ34 rectifier is giving me 503V.

The 5U4 lowers me to around 487V.
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Old 3rd June 2009, 09:45 PM   #4
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I'm an old guy, but I don't know anything about 50's guitar amps or the caps they used then.

I didn't think you would care for my answer (and others may not also), but voltage derating is very important; check vendor web sites on this. Temperature is also a key factor, so this means ripple current (I^2*ESR) is important. If the ripple current is very small (relative to the parts rating), then operating at a higher voltages (above 50% of rated) will not be as important.
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Old 3rd June 2009, 09:47 PM   #5
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A lot of care is needed in selecting a reliable capacitor for the position just after the rectifier.

this is not so much to do with the voltage rating, but the ripple current. The input cap may well see an rms ripple current of double the value of dc current.

If your 500V supply is feeding a pair of 6L6GCs, and you run them like the TungSol data sheet you'll draw more than 120mA dc at idle, and more than double that for 55W output.

If your capacitors are to last more than a few months, the ripple current rating should be 400mA or so as a minimum, and preferably >500mA.

I couldn't find a spec for ripple of the Sprague, but I bet it's nowhere near that. The usual run of 22/500V types run 100..200mA or so.

In that position, I use 3x 68uF 400V Samwha HJ in series, with 160K 0.5W balancing resistors. It has a 560mA ripple handling, and three give near to the 22uF you need. They sound great for vintage AC30s and other guitar amps. The nichicons and Panasonics are good too.

balancing resistors notes:

http://www.bhc.co.uk/application.htm#parallel
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Old 3rd June 2009, 10:50 PM   #6
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First off, I am asking these questions basically because I have always been concerned about reliability.

More Info,

The power transformer is rated at 325-0-325 200ma.(117V)
The OT is 4200 ohm primary, 4,8 or 16 ohm secondary.

It is a cathode biased amp, 390 ohm cathode resistor gives a reading of 37V at pin 8 of both tubes.

I had previously used 20uf/500V caps from F&T out of Germany and with a GZ34 rectifier my voltage reading at 1st cap 492V +/- depending on line fluctuations.

I switch to the Sprague Atoms this time around, all else being as equal as possible the Voltage Jumped to 503V +/-.
Part Number 65 degree C

This is the only data I could locate on the cap.
Sprague Atom

Right now I have a bit of a fit issue should I opt to series axial caps.
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Old 3rd June 2009, 11:05 PM   #7
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the series-connexion needs a "+ to - " joint in the middle. 50+50/500V multisections have a common negative, so sadly they can't be used. I think the JJs are only rated 200mA ripple, too.

The industrial 'snap-in' radial terminal types are the best sounding, so far as I can tell. And they have much better ripple current rating than 500V caps.

If you are really tight for space, but want proper reliability, the minumum you could use might be 2 x 400V 56uF Panasonic HA series

ECO2GA560AA


20mm dia x 30mm, 510mA ripple at 105 deg C.

balnce the voltage using 100K 2W resistors x2.

That should give 5000 minimum, and more likely 10000 hours use, even in a valve amp.
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Old 3rd June 2009, 11:10 PM   #8
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http://www.chemi-con.com/guide/pg27.php

UCC, which makes (I think) them there Spragues thinks overvoltage is a bad idea.

In the 50's, there would not have been 500+ volts on the cap. You are probably overvoltage on the filaments too. For reliability, you want to address both issues. Dip B+, dip filaments. Keep the 500V cap.
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Old 4th June 2009, 12:01 AM   #9
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Thanks for all of the great information!!


I ran into an issue just recently and suspected that it might be the cap ratings.

Occasionally when running the amp at about 45-60% output, I get this odd distortion when really hitting the strings hard.

Thing was it was not easily repeatable on the bench. For some reason when I use a 4 X 12 cab (8 ohm) I ran solid as a rock and no annomily, But when I put the amp back in the single 15" cab with an 8 ohm speaker it appears.

Pretty much the classic example of a bad cap in a vintage amp i guess. A raspy saggy distortion that fades with the note&volume.

I guess it was/is right on the edge of failure at that point. When I put in the 5U4 the slightly lower B+ must be just enough for it to not appear.

Interestingly, the filament voltages are not overly much, under load I read 6.5V which is not aweful.

Anyway, once again, thanks for all of the insight!

I'll figure out some way to get the extra cap fit in even if I have to mount it elsewhere.
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Old 4th June 2009, 01:37 AM   #10
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Back in the day - the 50 years ago we are talking about - caps came with a "working voltage" rating. That was what was on the can. It would sometimes even add WV after the value instead of just V. 450WV meant the cap was rated to work all day in a 450v application. SOmetimes the real rating was on the cap as well - as a surge rating. SO if you had a 450 supply to filter, you got a 450v cap.

SO if you had a cap with a 50WV rating, the surge rating was probably 63V.
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