Center Tap Full Wave Rectifier - CT tied to DC instead of ground

[Pat Maki]

I'm repairing an Altec 525 power supply as a means to learn more about tube mic circuits. All the reading I've done on CT full wave rectifiers show the CT going to ground. Can someone please explain why the CT of the top secondary is referenced to 260V of rectified DC from the other secondary in this circuit? Why doesn't the CT just go to ground?

 

[shabtek]

There is a limited permissible cathode to heater voltage, data sheet might indicate it…the cathode is at 200vdc in this schematic/cathode follower configuration…in a typical plate follower the cathode will be close to 0vdc and filament supply can be referenced to 0v

 

[scott2000]

Why doesn't the CT just go to ground?

Look up heater elevation... elevating heaters....
I pretty much always do it.... Feels like a cheat code for noise most of the time...

 

[Pat Maki]

There is a limited permissible cathode to heater voltage, data sheet might indicate it…the cathode is at 200vdc in this schematic/cathode follower configuration…in a typical plate follower the cathode will be close to 0vdc and filament supply can be referenced to 0v

Thanks @shabtek. 5840 data sheet says cathode to heater voltage max is 200v. I think I get it, so it's relative voltage between the two. Since the heater voltage is elevated to ~260v and the cathode is at 200v, then the cathode to heater voltage is 60v and well below the 200v max. Is that the correct analysis?
And thanks for pointing out this is a cathode follower. Most of my reading on tube circuits til now has been on plate follower....apparently. The cathode follower config is blowing my mind open a bit right now..will do some deep diving here to get an understanding of how it works. From my initial reading this morning, that definitely helps me understand some things that were confusing when I presumed this was a plate follower...Thx!

 

[Pat Maki]

Look up heater elevation... elevating heaters....
I pretty much always do it.... Feels like a cheat code for noise most of the time...

Thanks @scott2000 . Love Blueglow's vid's but hadn't seen this one. Very helpful. Sounds like the purpose of elevating the heater voltage is reduce noise in the circuit.
I guess I'm still not clear on why the center tap of the top secondary ties to 260v DC from the secondary...wouldn't that send AC from the CT of the top secondary into the circuit and/or send the 260vdc into the pre-rectified AC in the top secondary? Sorry, newb here....just trying to get my head around this...

 

[scott2000]

Sounds like the purpose of elevating the heater voltage is reduce noise in the circuit.

I believe the reason is more along the lines of what shabtek posted but noise is more often than not a benefit imo... maybe why it's used where it doesn't necessarily need to be.

..wouldn't that send AC from the CT of the top secondary into the circuit

interesting question. Not sure . Guess any ac after rectification is being shunted to ground...filtered out

or send the 260vdc into the pre-rectified AC in the top secondary

the ac is rectified? That's how it's dc

Using dc as a ground reference is unclear to me how that works as it relates to transformer winds... Like I said, interesting question..Not sure..lol