[Russell Dawkins]

....."But here's the rub - those 2 big @ss soup can sized capacitors?  They are rated for 450v.  Not bad, except the voltage from the transformer is 530 Volts!  Now, even though they are connected in series, there is not a leveling resistor(s) in place between them.  That means there is a high degree of danger that they will blow up, Yes, BLOW UP LIKE HAND GRENADES!!!!."....

I stumbled upon this old thread with this comment and was curious on the technical point raised. I understand that identical  capacitors in series have double the voltage rating and the same capacitance. That would seem to imply that two 450 V rated caps have an equivalent rating of 900 volts and would thus be safe. Does the lack of leveling resistors negate this?

[Russell Dawkins]

double post

[dB Cooper]

Surprised to see defense of an amp that uses inadequate parts. You don't need an EE degree to know that using switches at 21 times their rating and high voltage caps at 75V over max ratings= bad engineering. Plus a build that looks worse than the first Dynaco I did at 14yo? Bad build quality isn't dangerous per se, but is that what buyers paid for?

[rpf]

How do you read that as a defense of the amp? Russell was merely asking a technical question.   

[Guy 13]

Hi all.
For the last 20 years, I've been involved with photographic flashes
with banks of capacitors from 100uF/350V to 6400uF/600V each.
I've seen many capacitors blow up, fortunately,
they are all inside a sturdy casing, therfore, no arm,
but after opening the casing to replace the capacitor(s)
I saw the damage it did and I would be really worried
if that same capacitors would have been outside the casing,
like on some amplifiers.
Having anything voltage under rating is looking for trouble sooner or later.
I've seen some capacitors blow up in the same room as I was
and for the small ones,
it's like a BB gun, but for the big ones, it's pretty scary.
Up to now I have not seen any amateur and professional photographer's flashes
with a known approval sticker and I am sure the same thing applies for the amplifiers, pre-amplifiers, etc...
I don't like the way the OP was written, but the intention was to warn/scare potential buyers of that brand and that's good to me.
Now, the facts are there, the components were under rated,
and that equal potnetial trouble.
Yes, AC got rules, but this is in the interest of the AC members and potential buyers.
What's wrong with that.
When GM had defective ignition switches and people posted on Facebook thet problem, was that wrong, I think not, that's how often, it get people doing something to save lives.
O.K. enough. I said what I wanted, now send me to the intergalactic waste bin, I don't care, I am going to bed an have some sweet dreams...

Guy 13
   

[JohnR]

I understand that identical  capacitors in series have double the voltage rating and the same capacitance. That would seem to imply that two 450 V rated caps have an equivalent rating of 900 volts and would thus be safe. Does the lack of leveling resistors negate this?

Hi Russell, balancing resistors are used to equalize the voltage across the capacitors. Without them, different leakage currents through the caps would cause the voltages to be, well, out of balance.

[mgalusha]

Hi Russell, balancing resistors are used to equalize the voltage across the capacitors. Without them, different leakage currents through the caps would cause the voltages to be, well, out of balance.

Exactly and I've seen the results more than once. I repaired a 211 amp that had ~1050V B+, while it had balancing resistors in the power supply one of them failed. If one cap has a lower resistance than the others, it receives more voltage and bad things can happen. The rebuilt power supply received higher wattage resistors to withstand the voltage.

[avahifi]

And of course after you series two capacitors (with the appropriate voltage balancing resistors) you need to know that two capacitors in series end up having one-half the capacitance of one capacitor of the same value with the appropriate voltage rating.

In other words, to equal the capacitance and voltage rating of a single capacitor by using lower voltage rated parts, you will need four capacitors in a series and parallel array to equal one appropriate capacitor.  In many applications you will simply run out of real estate to manage to do this.

Frank Van Alstine

[JohnR]

In other words, to equal the capacitance and voltage rating of a single capacitor by using lower voltage rated parts, you will need four capacitors in a series and parallel array to equal one appropriate capacitor.

From the pics, this seems to have been done in an additional RC filter stage.

I don't understand why the B+ needs to be so high in the first place, though. I found a schematic online, which may not be this exact model, but it looks to be a grounded cathode amplifier followed by a White cathode follower. I have two tube amps that I drive headphones with (these are in "prototype" form), one is an Aikido (12SN7) with an OPT using about 240V B+, another a Broskie cathode follower buffer (6h30) using just 150V. Many moons ago I built a mu-follower amp from headwize.com which would have had about 240V. Here's a pic of the internal wiring of that one:

[Russell Dawkins]

And of course after you series two capacitors (with the appropriate voltage balancing resistors) you need to know that two capacitors in series end up having one-half the capacitance of one capacitor of the same value with the appropriate voltage rating.

In other words, to equal the capacitance and voltage rating of a single capacitor by using lower voltage rated parts, you will need four capacitors in a series and parallel array to equal one appropriate capacitor.  In many applications you will simply run out of real estate to manage to do this.

Frank Van Alstine

Thanks for the clarification, Frank - it's been a long time since I had to get creative with capacitors and had forgotten this fundamental series value calculation: C1 X C2/ C1 + C2.

One last point - the wiring may look messy in the first picture of reply #23, but I wouldn't presume bundling the wires as in the second picture would necessarily create a better sounding device. Notice that many of the wires in the first pic are crossing approximately at right angles or at least steep angles. This is good for reducing induction between conductors carrying and AC signal (music). The worst scenario is closely spaced parallel conductors, as in bundles, which maximizes inductive coupling and signal transference from one conductor to the other–the point being, of course, that the prettiest wiring is certainly not necessarily the best, sonically.

I have to be aware of this when I am laying my microphone lines, as the signal is extremely small from my ribbon mics. If on a location recording I have to cross an AC power cable, and I sometimes cannot avoid it, I make sure to cross at nearly right angles, always. I have only once in 25 years had a problem with hum.

[JohnR]

Hm, post #23 is actually a different amp than post #1.