I added an extra cap in my 5C8 (tried 16uf first now 8uf) so now it has a 5D8 power string except I made the last resistor 100R instead of the 10K in the 5D8. I wanted to affect the voltages in the preamp as little as possible.
Question: anything wrong with using just 100R? Does more resistance equal better decoupling and better noise reduction or can I just use any value I want and 100R is fine?
Power String Decoupling Question
Moderators: pompeiisneaks, Colossal
Re: Power String Decoupling Question
If this is for the last stage, then it is drawing very little current. For that matter, just about any stage below the PI is drawing very little current and the PI doesn't draw much either. This means you can probably go up to 5K or 10K without that much voltage drop.
You can predict the result fairly easily using Ohm's Law. Figure the voltage drop from the PI node to the last B+ node. Use the sum of the dropping resistors. I=V/R...once you know I, substitute the R value, hold I as fixed, and backsolve for V. It should be a reasonably good estimate. Or, maybe you just try something and see what happens.
The larger value resistors, improve stage decoupling and that is a good reason for using them. I just learned this lesson the hard way.
You can predict the result fairly easily using Ohm's Law. Figure the voltage drop from the PI node to the last B+ node. Use the sum of the dropping resistors. I=V/R...once you know I, substitute the R value, hold I as fixed, and backsolve for V. It should be a reasonably good estimate. Or, maybe you just try something and see what happens.
The larger value resistors, improve stage decoupling and that is a good reason for using them. I just learned this lesson the hard way.
Re: Power String Decoupling Question
Going downstream, the B+ droppers are filtering out 120Hz, so it makes sense for the resistors and caps to sized so that there's at least a few dB of attenuation.
So 8uF has capacitive reactance of 166 ohms at 120Hz, so coupled to a 166 ohm dropper it would reduce the 120Hz by 3dB.
However, as I see it, for audio decoupling, the goal is to filter the whole audio bandwidth from the B+ supply, both going upstream and downstream.
So the B+ should be decoupled to a lower frequency than the signal path is coupled to, taking into account stage gain.
So if a B+ node feeds cascading gain stages of max potential gain 60dB coupled so that they're -3dB at 7Hz, then it may be beneficial for the decoupling upstream and downstream along the B+ (as applicable) to be greater than 60dB at 7Hz.
That may explain how an amp can motorboat without having noticeable 120Hz hum.
Pete
So 8uF has capacitive reactance of 166 ohms at 120Hz, so coupled to a 166 ohm dropper it would reduce the 120Hz by 3dB.
However, as I see it, for audio decoupling, the goal is to filter the whole audio bandwidth from the B+ supply, both going upstream and downstream.
So the B+ should be decoupled to a lower frequency than the signal path is coupled to, taking into account stage gain.
So if a B+ node feeds cascading gain stages of max potential gain 60dB coupled so that they're -3dB at 7Hz, then it may be beneficial for the decoupling upstream and downstream along the B+ (as applicable) to be greater than 60dB at 7Hz.
That may explain how an amp can motorboat without having noticeable 120Hz hum.
Pete
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Re: Power String Decoupling Question
You'd think by this point I'd figure that out. I feel real dumb. I still need a lot of hand holding. But it worked out perfect. 10K ate up like 20V across the dropping resistor though the pre valves dropped only a few volts. I settled w/ 4.7K, had hardly any drop on the tube plates and called it day. Amp seems quieter too, though I have dirty power and today is a good day.Phil_S wrote:If this is for the last stage, then it is drawing very little current. For that matter, just about any stage below the PI is drawing very little current and the PI doesn't draw much either. This means you can probably go up to 5K or 10K without that much voltage drop. You can predict the result fairly easily using Ohm's Law.
BTW the 5C8 is now an all octal 6SC7 5D8, it's quiet and it sounds spectacular. Damn I wish I lived in a house in the woods! I must go update my post on it.
Last edited by rp on Tue Feb 18, 2014 2:36 pm, edited 1 time in total.
Re: Power String Decoupling Question
Yeah, well someone pointed this out to me a couple of weeks ago for a troublesome build with hum. Sometimes our attention is elsewhere and we don't apply what we already know. That is what TAG (and other groups) is for. There are days when things hide in plain sight and require someone else to help. Don't think twice. What's important is that your amp gets an A+!rp wrote:...You'd think by this point I'd figure that out...