Lowering G2 voltage with power zener diodes

[6A3sUMMER]

Connecting Pin 1 to Pin 8 can give some surprising results . . .

Most KT88 and most 6550 have a metal shell around the base.
Many KT88 and many 6550 connect that metal shell to Pin 1.

With a jumper from Pin 1 to Pin 8, the cathode voltage may be present on the KT88 metal shell, and the 6550 metal shell.
At your own risk . . . do that with Cathode Self Bias, or worse yet either Cathode Follower, or the use of Unity Coupled Output Transformers . . . Ouch!

As far as I know, the EL34 and 6CA7 are the only tube types that need to have Pin 1 connected to Pin 8.
Newbies beware.

Safety First!

 

[6A3sUMMER]

To make my post # 41 more accurate:

I said:
"As far as I know, the EL34 and 6CA7 are the only tube types that need to have Pin 1 connected to Pin 8."

But what I should have said, is for the 6L6x, 6V6x, 5881, KT66, KT77, KT88, 6550, and similar type direct replacement audio output tubes in those families . . .
Only the EL34 and 6CA7 and similar type direct replacement audio output tubes need Pin 1 and Pin 8 connected.

With similar type direct replacement tubes, I am referring to the 6000, 7000, and 8000 series that are direct plug-in replacements.

 

[trobbins]

Just a side note that any voltage rail derived from zener droppers will have a substantial level of ripple on it if the main B+ has significant ripple - even when there is filter cap on the G2 rail. This could be a hum issue for any PI or gain stage powered directly from the G2 rail, in which case a resistor in series with the zeners may be needed unless another RC is used to power other stages.

 

[TonyTecson]

I added a schematic to an earlier post, Tony. The 160V, 320V, 640V supply. What do you think of it? 34

EDIT: I think 34 won't work, but this will?

BTW 4.5 Henry choke is there because I have some and they cost me 5$. Probably it can be left out, and the second cap could be a zener network instead, too.

One of them will, damn it!

i like #34 better...

the choke is a good idea too...i use that i my amp builds as well..
in this amp, i used two chokes, one for the main B+ and another choke for the G2 supply...

you can save on two parts, a resistor and a cap if you just stack up the caps for the 640v supply to the output of the 320v B+

 

[TonyTecson]

Connecting Pin 1 to Pin 8 can give some surprising results . . .

Most KT88 and most 6550 have a metal shell around the base.
Many KT88 and many 6550 connect that metal shell to Pin 1.

With a jumper from Pin 1 to Pin 8, the cathode voltage may be present on the KT88 metal shell, and the 6550 metal shell.
At your own risk . . . do that with Cathode Self Bias, or worse yet either Cathode Follower, or the use of Unity Coupled Output Transformers . . . Ouch!

As far as I know, the EL34 and 6CA7 are the only tube types that need to have Pin 1 connected to Pin 8.
Newbies beware.

Safety First!

those black full metal types do have pin 1 connected to metal shells, it was common practice that time, i am not aware that kt88 that have metal bases have connections to pin 1, but i will look at it one more time...thanks for the heads up..

 

[Mother of PEARL]

Originally tertrodes were designed to have g2 between 250 VDC and 300VDC. Schade mentions that ideally g2 should be 15 - 20V below the plate for optimum functioning. Attached document uses a string of 5.6V zeners due to the best temperature coefficient. I am using a single 30V zener in my current SE UL design resulting in a 20V difference between plate and g2. Works just as good.

Hah, the pic is my little stunt from the mid '90s, nice to see it still getting some air time

 

[Francois G]

Hah, the pic is my little stunt from the mid '90s, nice to see it still getting some air time

Hah, “little stunt from the mid ‘90s”. And I took it for gospel 🤪 BTW, what diodes would you use now?