[scootsit]

Hey guys, I was just reviewing the Max thread, thinking about this. What do caps CA4/5L/R on the Max do? I noticed that on the original hybrid, they are only 100uF, but on the max, Tomb, you recommend a minimum of 1000uf. How do these caps influence the sound?
 
Much appreciated!
 
(And so the tweaking begins!)

 

[tomb]

Quote:

Hey guys, I was just reviewing the Max thread, thinking about this. What do caps CA4/5L/R on the Max do? I noticed that on the original hybrid, they are only 100uF, but on the max, Tomb, you recommend a minimum of 1000uf. How do these caps influence the sound?
 
Much appreciated!
 
(And so the tweaking begins!)

They are the amplifier "power" caps.  I think the generally regarded standard that developed during the PIMETA-PPAV2-Millett Hybrid days was 470uf.  The PIMETA had (has?) four of them, for instance.  After awhile, people even started putting 1000uf caps in a CMoy (220uf's were original).  We tried many combinations over the years and finally settled on the largest, highest-quality cap that would fit in the spaces on the MiniMAX - the Pansonic FM 1800uf.  On the signal side, we use 1000uf and 470uf Muse ES's, both at 25mm height (the largest available at the voltages needed in both cases for a "boutique").  On a MOSFET-MAX we go all the way to 2200uf UHE's because we don't usually have the 1" - 25mm height restriction that exists on the MiniMAX.
 
A lot of this is trial and error and there isn't a real science involved - it's all design and empirical knowledge.  In a normal scenario, you'd want a combination of large cap/smaller cap.  The large cap is thought to support the current impulse needed to provide strong bass, while the film caps (referred to a "bypass" caps in the Milletts) provide quick response for transients.  This is even more critical in the Millett MAX designs because of the way the power supply is completely shared among the tube plates, the tube heaters, and the heavy-ClassA-biased diamond buffer.*  The large capacitors help to cut down on the voltage variation within the amp due to those heavy loads and yet still respond to the wild transients in a music signal.
 
Many designs isolate the power supplies for tube heaters from the tube plate circuit.  In hybrids, the power supplies are often isolated between tube circuit and solid-state output buffer.  Those arrangements might work better in a Millett Hybrid, but then it wouldn't be a Millett Hybrid anymore.  The use of strong, large caps has worked pretty well through the years, though.

 
 
* Keep in mind that the original Millett Hybrid never had a Class A, discrete output buffer.  That uses much larger current than the monolithic chip buffers used in the original.

 

[scootsit]

That's sort of what I thought, that they provided a filter to the power supply. All right, thanks for all the info. Next Mouser order, I'll get a bunch of caps of different sizes and play. You recommend Panasonic FM (It's FC now, right?) and Nichicon UHE?
 
Edit: All of my bypasses (both in and out of the signal path) are the 0.1uF Wimas.
 
Edit again: You would not want audio grade boutiques here because the low ESR would pass too much current to ground, right?

 

[tomb]

Quote:

That's sort of what I thought, that they provided a filter to the power supply. All right, thanks for all the info. Next Mouser order, I'll get a bunch of caps of different sizes and play. You recommend Panasonic FM (It's FC now, right?) and Nichicon UHE?
 
Edit: All of my bypasses (both in and out of the signal path) are the 0.1uF Wimas.
 
Edit again: You would not want audio grade boutiques here because the low ESR would pass too much current to ground, right?

I'm not sure that I agree with the use of the word "filter."  That's more appropriate to use for the actual power supply caps - whatever is used in your linear-regulated power supply.  In the MiniMAX and MOSFET-MAX, those are the CR1-A,B caps.  They actually smooth out the remaining wave tops left after the rectifiers (and thereby filter out more ripple).
 
I tend to think of the amplifier circuit power caps as devices that level out the variances between the power rail and ground ... after the actual power supply.  All of the amps that I mentioned previously had their own separate power supplies with their own caps, but the amplifier circuit design (without power supply) had its own caps.  The ones you're asking about are the power caps within the amplifier circuit itself. Another way of thinking about it is decoupling the voltage supply from ground.  IOW, maintaining that total differential between V+ and Ground, and not letting it get smaller.
 

 - probably guilty of this by now, but I wanted to convey the proper answer to your question (at least from my perspective). 
 
 

 

[scootsit]

Got it!
Edit: You just answered the question I was going to ask!

 

[tomb]

You want low ESR ... absolutely.  The reason that you don't want boutique caps is that their ESR performance is often (always?) much worse than a quality power cap.  At the same time, a quality power cap may sound like cr*p if directly in the signal path.

 

[tomb]

Who the heck said FM's were discontinued?

 

[scootsit]

I thought they were, and then I couldn't find them at Mouser. But, my mistake, they are still at Mouser. What is the difference between caps labelled low impedance and ones labelled low ESR?

 

[tomb]

Quote:

I thought they were, and then I couldn't find them at Mouser. But, my mistake, they are still at Mouser. What is the difference between caps labelled low impedance and ones labelled low ESR?

If Mouser carries them, its off the radar (Mouser does that with a lot of things).  DigiKey has always been the primary source for Panasonic caps, at least for as long as I've been in this hobby.
 
Impedance implies frequency dependency.  It is usually accompanied by a tangent value (not our PIMETA friend).  ESR is simply a straight measurement of effective resistance.  For our purposes, I believe they are more or less the same - but you're getting beyond my knowledge now.  I just know that if you compare the two columns of impedance and ripple for most power caps, the Panasonic FM is superior to almost everything on the market.  UHE's are better in some ratings, but it's not consistent.  Some of their caps are worse.  That's admittedly working from memory, though - I haven't done spec sheet comparisons for awhile now.
 
The challenge is finding spec sheets for those same values with boutique caps - they usually don't exist.  Why would a manufacturer publish ratings that look bad, though?  At any rate, that's why there's a dividing line between power caps and boutique caps.  Use them for the purpose they're intended and everything will be fine. 
 
 

 

[scootsit]

Thanks for the thorough explanation, always so helpful!

 

[scootsit]

I finally got a chance to take some pictures!
Here's the innards of the power supply:

 
Here's the amp by day:

 
I plan to swap that knob out for a black one off of a P-Bass or Telecaster.
 
And, here it is at night:

 
That LED inside is the color-changer. I just think it looks coolest with blue light. Anyway, it changes colors in there, which looks really, really cool!
I'm going to open the amp up to recheck the tube biases. I'll photograph the innards then.
 
Thanks a ton for the help, guys!

 

[tomb]

That's pretty cool to see a point-to-point Millett Hybrid!  Even Pete Millett's original was PCB-based.  Great job!!

 

[scootsit]

It's not entirely point-to-point (in the sense of say a bottlehead). It's built on Radio Shack strip board. That said, the traces on Radio Shack strip board are so crappy, it might as well be. Basically, You just completely cover every trace in solder because the traces have a tendency to come loose.
 
And occasionally, you use bridges made out of wire to go back over all the screwed up traces.
 
And, thank you, I am quite proud of it!

 

[Breck]

Wow, I love the lighting!  I'll assume it sounds as good as it looks and say nice job!

 

[scootsit]

Breck, it sounds pretty spectacular. It definitely lacks a certain something that my Mosfet Max possesses, but I certainly have no complaints given the simplicity of the build! If anyone is interested, I have all the board layouts in DIY Creator and can make JPEGs, just let me know.