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RE: New concept using circuit to replace cathode bypass capacitor

15.181.199.141

In Reply to: RE: New concept using circuit to replace cathode bypass capacitor posted by Eric Chan on April 17, 2024 at 22:16:31

Dear Eric,

This is the music return to ground, of the Finals tube.

You ask, how does it sound ?

I ask, who wants to listen to what is being suggested .....two solid state devices, and a Zener diode and three different Shunting Rs added to perform this music-return to ground function ???

Does not "KISS" rule in good circuit design???

What about use of a single R and a single C ??

Why not apply quality parts ????

The Rk in this direct couple's second stage could be a nice sounding power resistor, a better wirewound. Mills MRA-12s, or maybe ARCOL ( Ohmite ) HS series??

The Ck can be, (no electrolytic, but) a high quality FILM type, maybe a WIMA DC Link, possibly also with ultra-highest-quality film cap bypassing thoughtfully applied.

"A simple ground return path, of highest quality parts, to honor the music."

Finally, let us all consider your proposed ECC99's op point :

Is 90 VDC on the plate of the ECC99 the optimum value ? No Eric - not at all IMHO.

This tube, acting as a driver, will "come alive" with 190 VDC to 210 VDC as the Ea, as this circuit's Direct Coupling voltage.

Look-up and notice the 12AX7s' Ea's / VDCs in William Z. Johnson's ARC SP3-A1 build of 1972. The information is all posted in the Preamp's (on line) Owner's Manual. Wm.Z listened (and he knew), way back then, how to do it!

Such higher VDC op points seem mainly forgotten .... or such art is lazily ignored.

You asked for opinions. Hence this response. An honest opinion.

Have fun building / listening. Vatron

Follow Ups:

RE: New concept using circuit to replace cathode bypass capacitor - Eric Chan 08:34:29 04/20/24 (3)
Thx for opinion Vatron, very detailed

However, the ECC99 may work as preamp tube at plate voltage around 100V, very little high harmonic as long as you run more than 12mA
- RE: New concept using circuit to replace cathode bypass capacitor - Tre' 14:31:54 04/20/24 (2)
  Can you please show me what's magic about 12 ma? Even at 12ma, a vertical load line would suck.
  A high load impedance (horizontal load line) is very important if you want the lowest harmonic distortion. and if you want/need much total swing you will want more than 100vdc plate.
  Tre'
  Have Fun and Enjoy the Music
  "Still Working the Problem"
  
  Edits: 04/20/24
  - RE: New concept using circuit to replace cathode bypass capacitor - Eric Chan 17:44:42 04/20/24 (1)
    Hi,
    
    The first Stage ECC99 only need to produce 3V RMS to reach 30W at 845, lying on 100V 12mA OP is quite straight in the curve
    
    I show the 6H30 schematic front end but the concept is the same as ECC99
    - Yes - Tre' 19:00:56 04/20/24 (0)
      Only needing 3 volts RMS makes it easy and the CCS presents a very high impedance load for the tube resulting in a horizontal load line. That tube can be operated at a lot of different plate current levels and remain linear because of the horizontal load line.
      As long as the spacing between the grid lines is equal, following the load line, then the tube is not producing harmonic distortion.
      It's not about the straightness of the grid lines on the plate curves, it's about the spacing between those grid lines following the load line.
      You have to plot a load line. If there is no load other than the CCS (as in your first stage) then plotting a load line is easy. It's just a horizontal line with the operating point (voltage and current at idle) sitting right in the middle of it.
      The plate load for your second stage is the 220k grid resistor of the next stage. That is in parallel with the impedance of the CCS which is so high of an impedance that you can ignore it.
      Tre'
      
      Have Fun and Enjoy the Music
      "Still Working the Problem"
      
      Edits: 04/20/24 04/20/24
Agree - Triode_Kingdom 20:48:58 04/18/24 (3)
You can't beat a quality cap and simple resistor for a Class A stage like this.
- RE: Agree - ivan_terrible 22:58:19 04/21/24 (2)
  Also the heater cathode differential to worry about.
  One side is at 90-100v the other nearly at ground.
  
  I reckon I had a NOS dual triode fail in the heater circuit because of it.
  - RE: Agree - Triode_Kingdom 23:46:19 04/21/24 (1)
    Elevating the heater resolves that issue.
    - RE: Agree - ivan_terrible 02:25:42 04/22/24 (0)
      Not entirely because on most amps the AF amp heater is common to the output valves which may not like elevated heaters either.
      
      There is also the hum problem to consider too.
      On my submin triodes I found it essential to use DC heaters because hum levels on AC were so high.
      +
      for this reason I use a seperate winding on my toroids to feed the heater on the 12CT3 rectifier+series triode regulator / controlled warm up device used to feed a regulated screen supply, despite it theoretically having a H-K potential able to withstand 400V.

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