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I don't want to provoke a debate, but I am puzzled about the urge to jack up filter cap values, almost astronomically, with cap boards, etc.

The original ST70 design had 20/20/20/30-uF caps. A cap board is being offered that jacks these values up to 60/195/82/82uF. Ye gods!

Increasing the cap values very significantly will increase the B+ voltages and cause extra wear and tear on the rectifier tube, which there is too much of already.

Are people experiencing hum with the stock-value, or modestly increased, filter cap values?

I've heard that a major increase in cap value is needed to supply power through loud passages. Is this really known to be necessary? The caps are being recharged every 1/120th second.

dcriner wrote:I don't want to provoke a debate, but I am puzzled about the urge to jack up filter cap values, almost astronomically, with cap boards, etc.

The original ST70 design had 20/20/20/30-uF caps. A cap board is being offered that jacks these values up to 60/195/82/82uF. Ye gods!

Increasing the cap values very significantly will increase the B+ voltages and cause extra wear and tear on the rectifier tube, which there is too much of already.

only the first capacitor section might increase the B+ a bit and stress the rectifier if it is the input capacitor right off the rectifier. increasing capacitance after a choke or resistor won't have a noticeable effect on B+ or rectifier other than to slow the rise.

that said, 60uF right off a 5AR4 is pretty heinous and isn't recommended by me. not that I have a ST70. ideally it'd be a choke input power supply but there isn't exactly the space on the ST70's chassis for a big filter choke.

dcriner wrote:Are people experiencing hum with the stock-value, or modestly increased, filter cap values?

I've heard that a major increase in cap value is needed to supply power through loud passages. Is this really known to be necessary? The caps are being recharged every 1/120th second.

transients in music tend to happen instantaneously. what else is there to say?

Hi All,

Anyone who's worked on antique radios knows that the input capacitor was usually 4 to 8 µF in the early ones, and 8 to 16 µF in the "later" ones (1930s). The reason was simple: anything electrically larger was physically impossible to fit on the chassis. The result was that these radios all had power supply hum that was clearly audible.

As electrolytic trchnology was advanced during WWII, values of 40 and 50 µF became common. At that point, the problem of rectifier arcing started showing up in the 80s, 5Y3s and other directly heated small rectifiers. That's when the spec sheets started including input capacitance values which were typically 10 to 40 µF. And the audible hum was still there

When solid state rectifiers became affordable in the '70s, the arcing problem obviously went away (there were other problems, but that's another story), and the values of input capacitances started creeping up. When the sintering method of electrolytic construction developed by, I believe, Sanyo and Matsushita went public, the capacitance density went through the roof.

Today, it's not uncommon to see input cap values of 200 or even 500 µF. The ideal power supply has an internal resistance of 0Ω. Lithium batteries come close, but 500 volts' worth of lithium batteries would be a wee bit expensive. The realistic alternative is to use solid state rectification and high values of capacitance to get the internal resistance as low as prectically possible.

This now makes the weak link of the supply the power transformer. If its secondary can't supply the rectifiers with enough AC for them to feed the raw DC the caps want, the internal resistance of the supply goes right back up until it equilibrates with the time constant created by the filter network.

I am using the 'upgrade' multi-cap from
AES, in order from the rectifier,
40->80->30->20.

The larger cap values are supposed to
significantly increase the dynamics
of the musical content, particularly
in the bass. The bass response of
my amp is almost too much.

The B+ in my amp is ~25V higher
than the putative stock values. I
did not test the B+ stock; I never
even turned it on until I gutted
the amp and rebuilt it.

The 60mfd value on the CChong
board is the factory limit of the
rectifier. Using a cap of such a
high value seems ill-advised.

My amp does not hum, but I'll
be using the 20/20/20/30-uF
can from this time forth.

The main reason your (and everyone elses) B+ reads higher is because the line voltage in your home is higher than it used to be.

I checked my wall plug and it read 127v. :o

The AES cap will work fine the way you installed it: 40-80-30-20. I would install an inrush limiter also!

mine goes from around 122 to 126 on a rare occasion. is there any reason that voltages are so much higher than before?

A little research on Google shows that 60 mfd is the maximum suggested value for the first cap coming off a GZ-34. Most maximum values are conservative, so 60 mfd is probably just fine. After the choke, anything can be used, so the more the better. The ancient can caps were limited in value by physical sizes obtainable in the 50's ... we're past that, so why not take advantage of the technology, and reduce hum and give the amp better dynamics.

TerrySmith:
'The main reason your (and everyone elses)
B+ reads higher is because the line voltage
in your home is higher than it used to be.'

'I checked my wall plug and it read 127v'

Ok. Mine is 120v, only 3 volts greater
than the manual indicates for proper,
factory operation. My guess is that
in the '50s and '60s, the AC line
voltage, commonly available, was
not 117V. Probably, it was ~110V.

I took a reading from the 'D' lug on the
multi-cap. It was 464V. So, how
much greater in percentage is
464V than the manual's value of
438V.

464 - 438 = 26

26 / 438 = .059 = 5.9%

A value of 5.9% greater than the
manual is not worth loosing any
sleep over.

The more I read about rectifier circuits the more I think the '70 is under choked and overcapped for that part of the amp. I'm reading about choking with 3H to 10H chokes is standard engineering for these tubes with much less capacitance, for improved tube life and better hum filtering. This is advice from the tube data sheets I'm reading.

Tim

Chokes are good, much better than caps. But caps are cheap...

I don't know about ''cheap.'' These HV darlings are pretty dern high. $40/4 or $75/4. I'm wondering if we could get away with puting another c354 in there in series and swapping the OPT circuit to the 30mf. lug and puting the rectifier on a 20mf. lug.

Hmm. A Hammond 10 Henry choke rated for 300 mA will cost you over $55. The C354 is cheaper, but doesn't offer a whole lot in the way of real storage. They're rated for 1.5H, but sometimes I wonder how close they come to spec.

dcriner wrote:The original ST70 design had 20/20/20/30-uF caps. A cap board is being offered that jacks these values up to 60/195/82/82uF. Ye gods!

I have a cap board that with caps is less than the AES replacement cap. it makes 22/22/22/47uF.

For a choke, get the Hammond 159S - it's bigger and it's only $26.

Cheers!

Those other chokes are too big to fit under the chassis.

I think I've got a 159S around here somewhere... it doesn't exactly fit in the same space as a C354.