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Old 25th April 2013, 01:59 PM   #1
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Default MKP DC-LINK vs electrolytic power supply 4 tube amp

Hi all...

I am considering replacing a 15 years old power supply in a tube amp with MKP film. The variety of DC LINK caps and the price are compelling.

This amp uses 6 x 4700 @100 for the end result of +-800uf @600vdc for the B+ (480vdc), followed by a choke to the screen supply of 47uf, followed by the preamp (4 x 12ax7) supply at 47uf. The power supply is rectified by 8 IXYS hexfreds feeding the 6 caps as noted.

This one channel of a dual mono amp with six 6L6GC per channel in triode or pentode. Output impedance of the transformer is high for a tube amp but also follows the concept of maximum power transfer to the load. I bring this up as there is a relationship between this and the power supply. Comments are welcome.

Size makes the difference here, and will require an outboard box, or a second box under the amp to house the MKP caps. Price: well very good grade electrolytics x 6, is about the same as a CDE 860uf@900vdc or WIMA 820uf@700vdc, about $100/$125 per channel.

The specs on these caps is very impressive. If I am going to go down this road,i have a choice of a large cap as mentioned above, or upwards of 10 smaller uf caps in parallel to reach the 800uf value. Opinions on one large vs many smaller to achieve 800uf is welcome.

Has anyone substituted this type of cap for electrolytics as the primary cap for a power supply in a tube amp, and felt it was worth it?
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Old 25th April 2013, 05:10 PM   #2
DF96 is offline DF96  England
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Quote:
Originally Posted by glassandlight
Output impedance of the transformer is high for a tube amp but also follows the concept of maximum power transfer to the load. I bring this up as there is a relationship between this and the power supply. Comments are welcome.
Sorry, you lost me there. Which transformer? Maximum power transfer?
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Old 25th April 2013, 09:30 PM   #3
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output transformer to the speakers. I am not sure many have any idea what the power supply would/could be to the main amp with such a complicated number of tubes, I sure don't. Yes, ohms law ....output impedance and speaker load near the same is maximum power transfer, and a lot of heat in the process. Many people leave out information, seem I did here, that confuses the original question. I brought it up since the output of the speaker transformer reflects back to the power supply....IMF, EMF and so forth.
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Old 25th April 2013, 09:50 PM   #4
DF96 is offline DF96  England
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What is complicated about (presumably) parallel push-pull? Arithmetic still works.

Amplifiers do not generally use the maximum power transfer theorem, because it leads to poor efficiency and bad bass response.

I still have no idea what it is that you are asking us to comment on, apart from your main point about a capacitor swap. I can't comment on that as I am happy to use appropriate electrolytics.
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Old 25th April 2013, 10:04 PM   #5
tomchr is offline tomchr  United States
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I don't understand what you mean by "This amp uses 6 x 4700 @100 for the end result of +-800uf @600vdc". Are you saying six 4700 uF, 100 V capacitors are connected in series, resulting in a cap of approximately 800 uF rated at 600 V?

What's the B+ voltage of this amp? 500 V? If that's the case, I suggest getting two 2200 uF caps rated for 350~400 V operation and connecting them in series. That should take up less space than the current configuration and would leave room for a good polypropylene cap to be connected in parallel with the electrolytic cap.

If the choice is between a large, say, 1000 uF electrolytic capacitor inside the chassis or a stack of polypropylene caps totaling 1000 uF in an external chassis, I would prefer the electrolytic inside the chassis. You'll waste all the advantage of the nice film caps by connecting them through long wires (even if it's "only" a foot or so).

So my vote is for an electrolytic can with a film cap in parallel.

Or better yet, a regulated B+ supply.

~Tom
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Old 25th April 2013, 10:33 PM   #6
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That may be what I do.

But, my original question was (more or less) has anyone used such caps and how did they work out.

I know I have choices for electrolytic caps, lots of very good ones. Space conservation is not a goal, sound is.

You mention 'long wires even a foot or so" I assume you mean parasitic effects which I can see for coupling caps, but I need enlightening on the affects and effects if we are talking about a power supply. The affects and effects of printed circuit board traces add plenty of parasitic artifacts as this amp sits now. I mean....There are many amps with outboard power supplies and most are owned by the 1% or home brew by some respected names in the DIY crowd.. What do you mean here?

I can not see how a regulated B+ is going to help. Since the music rides on top of the power supply, floats along if you will, a regulated supply ( to me) will add silicon at the least or a very complicated tube power supply to handle six 6L6GC tubes and not sag or fry the rectifier tubes with to much capacitance to try to charge.
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Old 25th April 2013, 10:45 PM   #7
DF96 is offline DF96  England
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Long wires on coupling caps do no harm as little signal current flows. Very different for PSU caps or decouplers. Outboard PSUs create problems and are best avoided where possible. If necessary, good decoupling in the amp can offset the problems.

Remember that the 47uF caps may have a greater effect. Correct grounding and wiring is more important than component selection.
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Old 25th April 2013, 11:15 PM   #8
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well, you have a good point there. I had thought about the last two caps in the chain and upgrading the main supply with...gosh, Nichicon gold or Super Through or even low esr CDE caps, and concentrating on a film (like Stolen or better Audyn) for the last two. The bigest problem with this amp is...what seems to be headroom, or supply on demand in pentode...it gets a bid congested as if starved for power.

In never liked PCB for tube, point to point is my prefered and if i redo some oldpiece, I try to convert to a star ground if possible and practical.
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Old 26th April 2013, 12:00 AM   #9
tomchr is offline tomchr  United States
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Quote:
Originally Posted by glassandlight View Post
You mention 'long wires even a foot or so" I assume you mean parasitic effects which I can see for coupling caps, but I need enlightening on the affects and effects if we are talking about a power supply. The affects and effects of printed circuit board traces add plenty of parasitic artifacts as this amp sits now.
For a supply cap, what you need is a capacitor with low parasitic series resistance (ESR) and low parasitic series inductance (ESL). This is because the charging pulses that run from the rectifier and into the capacitor are rather large (amps in some cases). Now, polypropylene caps usually have low ESR and ESL - at least the good ones do. So your "block-o-cap" will have low ESR and ESL, but the connection from the "block-o-cap" to to the rectifier and ground will be a few hundred mOhm and probably 50~100 nH of ESL. This needs to be added as ESR, ESL for the capacitor. In addition, the connection will act as an antenna for all the charging current and is likely to create EMI.

Yes, there are amps that have a separate chassis for the supply. However, you will notice that they feed rectified (and sometimes regulated) DC to the amp. They don't have the rectifier in one box and the reservoir cap in another. And if it's a good design, the first thing on the supply line inside the amp chassis is a decoupling capacitor. Usually, using separate chassis is more trouble than it's worth.

I don't intend to start a religious war on PCB vs point-to-point wiring, but the blanket statement, "PCB = bad; P2P = good" doesn't sit well with me. There's a reason your cell phone isn't built using P2P wiring. And parasitics matter greatly for those...
A crappy PCB layout will result in crappy performance. Just as a crappy P2P layout will. A good PCB layout will result in good performance that can be repeated across millions of units made. All layouts (PCB, vector board, P2P, etc) have parasitics. Physics is annoying that way...

~Tom
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Old 26th April 2013, 12:23 AM   #10
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well Tom....thank you for all that. I need to sit and chew on it. Redesigning a power supply is not an easy task...that is what this is about is essence. I can not find a more complicated and data scarce info bank then capacitors. ( go find some meaningfull data on ATOM caps....the so called best there is, Vishay does not even mention leakage {yet, I have measured caps with superior numbers over ATOM} and so much more. I can not think of any component that has more voodoo and heresay linked to it than caps.

This is neither here or there (althoughi mentioned that a second chassis can go right under the main one for leads no longer than what is ther now), but still i am looking for the best sound I can get before I die of old age. I can still hear conversations from 200 feet when my grandson can't hear me sitting beside him and we are meeting eye to eye!!! I can hear the tiniest inflection that an artist's recorded on his plastic stone while he paid no attention to the engineer who steared his ship to the stars or into the rocks.

I should have been a spy for the US instead of being cursed with seeking the holly grail of music reproduction (Western Electric vs RCA) in a world where 90% or the orginal music is loss and played on a 1 inch speaker to rave reviews.
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