What to learn from Francesca

[HiGain]

Any thoughts?

About the PS wiring. Without seeing any pics, a wise man (Gus) once mentioned the use of positive feedback in the power supply. What do you think?

KF goes out of way to press not just the grid wires, but cathode and plate wires to the chassis. Is this just noise reduction?

Wires feeding the power tubes right on top of each other. Given the use of the cap across the plates, I don't see the reason for capacitive coupling. What gives?

The cap across the plates appears to be some sort of monilithic ceramic. I have found to this type to be clear, yet produce a sound similar to even order harminics-- illusory bass sort of sound.

Why the carbon comp caps feeding the bias supply and bypassing the filter caps? Why not carbon film. Mojo?

Re: the standoffs... Are they steel? I believe this is supposed to impart a different resonance to microphonic points in the circuit than aluminum.

I'd be interested in your thoughts on these issues, as well as other observations you may have had.

Jake

PS thanks again for the great pics!

[doctord02]

I cant help but notice how KF has the PS wires routed/placed under the 1k/25W resistor... Some sort of induction happening here? Wonder why?

[Doug H]

I kind of wonder why he uses a 25W resistor at all, esp in the Liverpool. According to the voltages Mark posted on the Liverpool schem, you don't need near that much wattage for that resistor, even at full load.

Doug

[doctord02]

I think there was some speculation that it was intentional... For sonic or behavior reasons. Might even relate to my question about why the wires are routed under it.

Check the old yahoo group for some discussion (I believe...).

[dms]

Is that really a 25watt resistor? It is MUCH smaller than the 25w Xicon resistor I just received from Mouser... If it is a true 25w, does anyone know who makes (made)it?

Thanks, D

[doctord02]

Are you talking about the big cement resistor on the tag board? Thats about the same size as the xicon that I used...

[Funkalicousgroove]

My take is that WAY TOO MUCH is being read into this, My guess is the wires were routed under the resistor because that's how it went together, and he used a 25 watter because it's what he had at hand. There is a law of diminishing returns when fretting over minutia, ie, huge differences in layout/resistor wattages yield minimal difference in tone, and one is better off practising their guitar than moving wires in 1/16" increments in order to improve tone.

No Offense is meant by this, nor is it directed at any one person, just an opinion- try it on like a hat, if it doesn't fit take it off.

[dobbhill]

Could someone please educate me about the caps I often see across power supply diodes? Why and what values?
Please excuse my ignorance as I am not formally educated in electronics.
Thanks, Dobb

[doctord02]

Hey Dobb

The caps are there to minimise the chance of "switching noise" getting into the amp. Diodes can sometimes make a high frequency buzzy noise as they "switch" from conducting to non conduction as the AC pulse changes. The caps are around .001uF as I recall.

I'm not convinced that they are necessary, but since I was building it for myself and it was at most a 50 cent tweek, I figured what the heck...

Dave

[Moose]

and he used a 25 watter because it's what he had at hand.

I don't doubt that people overthink things, but there are two issues here.

First, assuming that you're not being a hyper-anal twat about it, thinking about seemingly simple minutae can help you determine what that point of diminishing return really is. Like, is one RV4 pot better than another? Maybe, probably not worth hassling. Is an RV4 better than a 1/4 watt like the Alphas? Definitely, and now it's a matter of deciding if the cost is worth it. Seems like a lot of thinking over a stupid volume control thinger, but it opens discussion about benefits of changes and associated costs, which is how we learn.

As for the 25W, I don't think it was just what was on hand. First, that value's been seen in more than one amp. Second, think about what that is. A WIREWOUND resistor. A piece of wire that's not completely conductive wrapped in a coil. Sounds a little like a choke, doesn't it? it is definitely an inductor.

My guess is that, since he didn't think the cost or weight of a choke was necessary, but since a higher wattage wirewound (which are inductive unless it specifically says otherwise on the datasheet) fits and only costs a tiny bit more, I'm betting he figured "what the hell, it'll kill more ripple than a smaller and less inductive resistor" and went overrated.

Does this mean you NEED a 25W? Hell, I don't know. Probably not. You could use a small choke with a similar series resistance. You could just assume the PI filters are plenty (which they are) to cut the ripple. But I learned about how big power resistors are made, so even wondering why this R was used was worth the effort.

[Doug H]

Thanks Moose, I bet that is what it is for- an easy way of getting some inductance to kill ripple. When you think about how those resistors are constructed and where it is installed in the circuit, it makes perfect sense.

Thanks again,

Doug

[Mark]

My thoughts on the 25 watt resistor are fairly simple, put the 25 watt resistor into the amp then record the amp, and then replace the resistor with one of the same value but a much smaller wattage and repeat the process.

Personally, I suspect the 25 watt resistor was there for reliability as Two Rock used a 10 watt resistor, and there weren't comments of night and day differences between a real Express (Ingrid) and the Two Rock Ruby.

My advice for what it is worth, suck it and see.

[Funkalicousgroove]

A 1K 25W resistor produces +/-100 nH of inductance at 1kHz depending on brand, I can't imagine that would have a profoud effect on sound or Ripple.

[HiGain]

Thanks for the interesting discussion, guys.

I went ahead and rewired part of my clone according to the Francesca pictures. Basically, I pressed the first three stages' wires against the chassis. I also tied the power supply node wires together to enhance positive feedback.

The result is amazing. Even MORE harmonic feedback and touch sensitivity. Better clean to mean, and notes just effortlessly sustain and begin to feedback, at any gain setting. Try it!

Jake

[Dai H.]

A 1K 25W resistor produces +/-100 nH of inductance at 1kHz depending on brand, I can't imagine that would have a profoud effect on sound or Ripple.

I figured the inductance would be too small to matter. As somebody mentioned you have to be careful to not overthink things. Re: the wattage, would it matter maybe in case of failure (just guessing--but to help survive a current surge before the fuse blew?). Marshall apparently used a far larger than needed wattage power R in place of the choke in some of their amps.