Precision Power Supply Question

[David Root]

Is there any downside to over rating the individual B+1 caps? I have a pair of 220uF 450V MIEC axial caps, for use in a #102, the Robben Ford ODS, which will probably see 460V or so, with a 900V rating. (Yes I know #102 doesn't use the precision power supply but I have the board in stock so I don't have to make one).

I can't think of any electrical contraindications, and in the event that one cap starts to leak and dumps some of its voltage into the other cap the other cap won't croak.

Tonally and/or feel wise, it `might be a different matter in #102, precison supply being a bit tighter and lower impedance etc.

[Structo]

I didn't use the precision supply but I do have a pair of Sprague 320uF caps for the main B+ for 160uF.
And F&T 22uF caps for the remainder.
I really don't notice and extra stiffness in my amp but I may be used to it.


With your 220uF you would have 110uF for the mains.
The 47uF caps on the PPS are in series so they equal 23.5uF each pair.
But due to the redundancy, you will get better regulation.

[67plexi]

David, I always over rate the power supply filter board capacitors.
Two each 300uf @ 500 volts = 150uf @ 1000 volts.
Two each 47uf @ 450 volts = 23.5uf @ 900 volts.
My bias board 100uf @ 350 volts. I have had runaway bias with china crap capacitors.

Steve.

[Smitty]

At some point I believe increasing the voltage rating might affect the high frequency response. The higher the voltage the greater the effect. Hard to say if you'll hear it with just a slight bump in rating on just one node. If you went to 2000v caps on every node I'd bet you could hear it. I think that is one of the benefits of the precision power supply...lower rated caps.

[ayan]

A while ago I had heard/read the same thing, that PS caps operated best if at close(r) to their rated volatges.

Gil

At some point I believe increasing the voltage rating might affect the high frequency response. The higher the voltage the greater the effect. Hard to say if you'll hear it with just a slight bump in rating on just one node. If you went to 2000v caps on every node I'd bet you could hear it. I think that is one of the benefits of the precision power supply...lower rated caps.

[talbany]

As far as the actual life span goes

Electrolytic capacitor life is not infinite. It is controlled by a number of factors, but mainly the temperature at which it operates, which is set by two things: ambient temperature and internal heating. Ambient temperature is the biggest factor, so if you are running a capacitor in a hot tube amp, be sure to use one with extended temperature rating if possible. The second factor, internal heating, depends on the ripple current in the capacitor and it's internal resistance.


Capacitor life is directly proportional to the ratio of rated voltage/operating voltage. If you run a 500V cap that has a specified lifetime of 10,000 hours at 250V, it will instead last 20,000 hours. If you run it at 500V as specified, however, it will only last 10,000 hours, which is what it is rated for. So, as you see, there is no benefit to running a capacitor right at it's rated voltage, it is better to run them conservatively with respect to voltage and temperature for maximum lifespan.

Film capacitors and ceramic capacitors are even more dramatic - their lifespan is proportional to the rated/operating voltage raised to the 7th power and 3rd power, respectively, so a film cap rated at 10,000 hours at 500V run at 250v will last 1,280,000 hours, and a ceramic cap under the same conditions will last 80,000 hours, while an electrolytic will last 20,000 hours as I mentioned previously.


As far as feel/tone goes
I prefer to run plate caps close to there actual rating ( perhaps something to do with surface area)..

Tony

[David Root]

Thank you so much folks. I like the extended life feature of over rating the caps a lot. My screen and PI caps are 47uF/315V so I have 630V there. The internal buffered loop caps are each 22uF/450V and the V1 and V2 caps are series 82uF/250V for 500V rating. (I modified the board so that the loop pair are no longer connected in series).

Tony's point on the plate caps is particularly telling. I have a pair of 220uF/300V F&T caps on order. Now I don't know whether to use the 450V MIECs now on the board or wait until I get the F&T caps.

[martin manning]

Here are two passages from Nichicon Application Guidelines for Aluminum Electolytic Capacitors, CAT.8101C:

"2-9-4 Applied Voltage and Life
The degree that applied voltage effects the life of the
capacitor when used below the rated voltage is small,
compared to the degree that ambient temperature and
ripple current effects life."

[some data is shown indicating that for low-voltage caps there is basically no change in loss of capacitance vs. time-MPM]

Then:

"In regards to high voltage capacitors used in
smoothing circuits for power electronic equipment, the
leakage current decreases as the voltage drops and
lessens the consumption of electrolyte. In such cases, the
life of the capacitor may be extended. For more details,
please contact our sales offices."

[David Root]

Thanx Martin. So it's only a potential issue for the plate caps it seems.

[briane]

I'll say from experience - after 40+ dumble builds some with, some without the precision power supply....

I DONT LIKE IT ONE BIT!:evil:

Personally I find the precision supply dulls the amp down and takes away its 'breathing' or life. I would say dont do it unless you already know thats what you luv.

Sprinkler on toast as needed. Just my opinion from experience.

[rogb]

I'll say from experience - after 40+ dumble builds some with, some without the precision power supply....

I DONT LIKE IT ONE BIT!:evil:

Personally I find the precision supply dulls the amp down and takes away its 'breathing' or life. I would say dont do it unless you already know thats what you luv.

Sprinkler on toast as needed. Just my opinion from experience.

Interesting view point. I have just built a 50w 102 with PPS. I have been putting together a Mouser order so I ordered a few Sprague caps 20 and 40uf, to go with the 100uf I have to make a PS board like 102 Feedback City.

I said to Tony I would compare the 2 PS boards so now I will! Thanks for the nudge, Briane.

I got to say, I have been working hard to get some bloom going at sensible levels so along with some NOS resistors on the way, I will try it Tony's way.
It does take off best with my Tele, which is just a good sounding guitar through everything, and an old Squier Strat with an old Lace Silver in the middle, but not so much with my D. Allen equipped Strat

[Structo]

Fascinating discussion guys.

I have experimented with adding lots of capacitance on the power tube filters and it does change the "feel" of the amp.

Over filtered amps seem to be sort of less lively or bloomy. (if that's a word)
One author described the power supply of a tube amp as the heart and soul of the amp.
Probably because we are taking that current and voltage and amplifying it with a modulated signal riding on top of it.

Goes to show that there are a lot of opinions on ratings, ESR and voltage.

Perhaps the current data on E caps does not correspond well with tube amps since most solid state devices don't develop a lot of heat, with the exception of power devices such as power amps.
Although there are series of capacitors that are made to withstand higher temperatures.

As Aluminum Electrolytic capacitors age, heat and ripple current, the ESR (Equivalent Series Resistance) increases.

The higher the ESR gets, the higher the temperature generated inside the capacitor.
This is why we use balancing resistors on capacitors connected in series.
This helps equalize the voltage and leakage current through the resistors.

Capacitors have a lot of properties most of us gloss over.
At high enough frequencies, the capacitor will actually start looking like an inductor.
The frequency at which the capacitor transitions from looking capacitive to inductive is called the self resonant frequency.

Here is an interesting paper about it.

[rogb]

When I do a brief review of this amp Tom, brief as there are PLENTY of 102 types about now, I will compare the 2 PSs and how they feel to me and my guitars.
I don't think I will be able to record any clips unless I get to the studio as I have quite noise sensitive neighbours but I will try if I can.
So I will just try to explain how it plays at home levels and at jams when I can get things going in a band situation.

They will be even unhappier when my Marshall gets built in the next few weeks

[vibratoking]

Does anyone know when the transition was made from 'original' power supply to precision power supply?

[ToneMerc]

I've drifted away the normal PPS, if I use the PPS topology at all, it's only on the preamp.

TM