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Millet 6B4G amplifier - power supply peculiarity

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Posted on October 14, 2020 at 22:41:22
LowRedMoon
Audiophile

Posts: 1091
Location: Central PA
Joined: February 28, 2006
 Greetings,

I'm building some push-pull 2A3 amplifiers, and I'm going to try a few already-published designs, including the JE Labs PP2A3 and the Pete Millet 6B4G, but with 2A3s.

In the Pete Millet push-pull 6B4G, the driver and phase inverter plates (all 6SN7) are fed from the same 250V B+ stage, without a separate RC filter for each stage. In every amplifier design I'd previously built or read, each stage had its own RC filter. I thought that aside from providing different required voltages, these filters decoupled the amplification stages.

Question to experienced designers: is running two stages from the same B+ like this a compromise in any way? Do you have any insight to why Mr. Millet designed the amplifier this way?
I know sometimes we build with what we have on hand and if it works, we go with it, but I imagine Mr. Millet was intentional in his design choices.

Thank you!

 

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The schematic, posted on October 14, 2020 at 22:44:21
LowRedMoon
Audiophile

Posts: 1091
Location: Central PA
Joined: February 28, 2006



 

RE: Millet 6B4G amplifier - power supply peculiarity, posted on October 14, 2020 at 23:34:45
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 The power O/P stage is decoupled from the small signal stages and that is
adequate.

If OCD will not let you rest, add a "check valve" diode, as was done in "El Cheapo".








Eli D.

 

RE: Millet 6B4G amplifier - power supply peculiarity, posted on October 15, 2020 at 05:38:33
GEO
Audiophile

Posts: 4749
Joined: April 7, 2000
Contributor
  Since: September 9, 2000
 I was use Electraprint's autoformer for phase inverting. I did that on a PP 46.

 

The driver stage is differential, posted on October 15, 2020 at 11:41:00
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 -And so should not be adding a lot of noise to the B+; its load will be fairly constant unless you need a new tube.

So its really about bypassing the voltage amplifier at that point, as far as noise (modulation) of the supply is concerned. I really don't see a problem with the way its set up.

 

RE: Millet 6B4G amplifier - power supply peculiarity, posted on October 16, 2020 at 10:02:17
LowRedMoon
Audiophile

Posts: 1091
Location: Central PA
Joined: February 28, 2006
 Ah, the El Cheapo. I think I've looked at that schematic more than any other. I tinkered with that design years ago, but life changes placed my DIY audio involvement on a back burner for many years, and I never got to complete a fully to spec version of it. I have a lot of admiration for that design, as well as the aesthetics of the schematic. Was that drawn using a simulation program? I'm learning the ropes on LT Spice, which I'm finding to be very accurate and useful, as well as enjoyable to use.
I briefly thought of building a dichotomously named El Cheapo with the 2A3s and 7K PP Heath iron I have, but it would have been a bit of a redesign, and I have all of the parts to build a Millett amp.
I started building about 14 years ago, but with many years out of practice, I still trust established designs over anything I would come up with myself. I am in fact a hobbyist, after all.

Would it be preferable to place the diodes before or after the dropping resistor to the front end circuitry?

 

RE: The driver stage is differential, posted on October 16, 2020 at 10:02:48
LowRedMoon
Audiophile

Posts: 1091
Location: Central PA
Joined: February 28, 2006
 Good input! Thank you!

 

RE: Millet 6B4G amplifier - power supply peculiarity, posted on October 16, 2020 at 10:04:04
LowRedMoon
Audiophile

Posts: 1091
Location: Central PA
Joined: February 28, 2006
 Perhaps I'll use a transformer phase inverter in the future, but for now, I'm working with parts on hand. Good ones are expensive.

 

RE: Millet 6B4G amplifier - power supply peculiarity, posted on October 16, 2020 at 10:41:46
GEO
Audiophile

Posts: 4749
Joined: April 7, 2000
Contributor
  Since: September 9, 2000
 Ok. Just a suggestion. Good luck and post pictures

 

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