Tube DIY Asylum

RE: Increase L in pi network

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Sidewinder

In Reply to: RE: Increase L in pi network posted by Triode_Kingdom on September 22, 2017 at 08:09:20

Hello TK,
Thanks for the reply. My amp is running fine. There's no audible hum and it sounds smooth and pleasant. What's missing is a bit of speed and impact(slam?) that I want more of.

The choke measures 101ohms. The proposed 25H replacement is 205 ohms. So, what do you think?
Regards,
David

Follow Ups:

RE: Increase L in pi network - Triode_Kingdom 21:01:56 09/22/17 (2)
Bear in mind I'm only advising regarding the supply. What you're asking of the amplifier may not be related to this. Many things can influence system dynamics, including components and phenomena external to the amplifier proper.
Based on a basic SPICE analysis, the supply you're using now exhibits a very smooth output impedance curve. There is only one resonance peak, and it is well below the audio band at 6 Hz. The plot is below:

I also plotted the supply with the 25H choke you're considering. That pushes the peak much lower in frequency (off the page in fact), but it does nothing to reduce impedance at 20 Hz and above. The best means to reduce the impedance in that area - and push the peak away at the same time - is to install a larger value at C2. The plot below represents impedance values using a 100uF cap:

My conclusion from these plots is that the 25H choke will not materially benefit your amplifier. I'll also say that although the additional capacitance in the second plot might appear beneficial, the reduction in impedance at low frequencies must be considered against the load presented by the output stage and the efficiency of the overall system. Because this is a SET, the output stage does not make the same dynamic current demands at low frequencies as push-pull AB1. Its loading on the supply is relatively constant regardless of output power and frequency. However, many SETs are driven into clipping at least occasionally during normal use, and a larger second cap can be helpful in dealing with that.
I have to disagree with Eli's comment regarding SS rectification in this case. Additional anode voltage might be beneficial if the amplifier is being regularly tasked beyond its limits. Otherwise, the natural characteristic of Class A to always draw the same average current renders the benefits of SS as it relates to push-pull designs irrelevant.

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Buy Chinese. Bury freedom.

Edits: 09/22/17
- RE: Increase L in pi network - Sidewinder 23:17:37 09/22/17 (1)
  Thanks TK for taking the time to help me out. Based on your analysis and recommendation I will for now hold off in increasing the choke value and instead increase the final C value to 100uf.
  Regards,
  David
  - RE: Increase L in pi network - 91derlust 12:51:28 09/23/17 (0)
    I think that is a good decision. Please, let us know how you go with that.
    
    Cheers,
    91.
    
    "Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
RE: Increase L in pi network - Eli Duttman 18:16:41 09/22/17 (0)
"What's missing is a bit of speed and impact(slam?) that I want more of."

Switch to SS rectified B+ using "noiseless" high PIV Schottky diodes. It's most definitely not an accident that amps famous for good bass, like the McIntosh MC-275 and the Harman/Kardon Citation II, employ SS rectified B+.

Eli D.

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