will this spice works in real word?

[singh santa]

Hello,
any inputs?
Many Thanks!
SS6CA7_8W_amp.JPG

6ca7_sch.JPG

[Ty_Bower]

Too lazy to look at the whole schematic, but 280 volts feels low for a 6CA7.

Where are you trying to take the output? Off the load on the cathode? Seems weird...

[singh santa]

Thanks ! Isn't it enought ?

[rotaspec]

Well, I am new to tubes, but I can see some problems here:

If the FET M1 conducts, the 6CA7 will have 500v on its screen and 280v on its anode. Can't be good. I also think you would need to exchange the 500v and 280v supplies.

Output transformer should be in the anode circuit to provide a load, and the cathode grounded through appropriate bias resistor. Otherwise, I don't understand the way it will operate as it is, other than as unity gain.

C5 should be referenced to ground, not the cathode as you have drawn. I also don't know if NFB will work via a bias LED, but I am new to bias and NFB circuits.

There are plenty of knowledgeable folks here that will probably point out errors in my interpretation more than yours

Gary

[singh santa]

well, here is what screen Voltage of 6ca7 (tp3)
Thanks for the input.
SS

[godfrey]

Wow! Nice idea

Are you getting enough gain from the input stage, though?
I see you're already bootstrapping R4. Maybe bootstrapping R12 as well will help?

One difficulty - what to do with the 6CA7 heater? If you want to keep it within 100V of the cathode, it'll need to be floating and bootstrapped to the cathode?

280V for the 6ca7 seems ok (see datasheet extract below).
What about the bias, though? Looks like you're relying on the resistance of the transformer? Nice if it works - need to check the dc resistance, though.

@Gary:
I think the output transformer is wired as intended for unity gain, so you get the good efficiency of a pentode, but with really low output impedance. You need huge gain and voltage swing from the input stage to make it work though.

Also: the mosfet is bootstrapped via C4 and C2 to keep the screen of the 6CA7 280V higher than the cathode.

[Yvesm]

Great "true" pentode cathode follower

Boostrap is a good way to increase both gain and output swing of the driver, but it also increases the output impedance of the power stage . . . wich is not necessarly what you wish !

Full CF power stage are not easy !
I've found that a single 6L6 with "only" 10% of the load in its cathode exhibit an apparent RP less than 2K Ohms w/o any other kind of feed back.



Yves.

[singh santa]

Originally Posted by godfrey

Wow! Nice idea

Are you getting enough gain from the input stage, though?
I see you're already bootstrapping R4. Maybe bootstrapping R12 as well will help?

One difficulty - what to do with the 6CA7 heater? If you want to keep it within 100V of the cathode, it'll need to be floating and bootstrapped to the cathode?

280V for the 6ca7 seems ok (see datasheet extract below).
What about the bias, though? Looks like you're relying on the resistance of the transformer? Nice if it works - need to check the dc resistance, though.

@Gary:
I think the output transformer is wired as intended for unity gain, so you get the good efficiency of a pentode, but with really low output impedance. You need huge gain and voltage swing from the input stage to make it work though.

Also: the mosfet is bootstrapped via C4 and C2 to keep the screen of the 6CA7 280V higher than the cathode.

Well here are some more pics.
I'll pay attention at cathode to filement there are +-200V.
Thank you some much.
SS

[singh santa]

Originally Posted by Yvesm

Great "true" pentode cathode follower

Boostrap is a good way to increase both gain and output swing of the driver, but it also increases the output impedance of the power stage . . . wich is not necessarly what you wish !

Full CF power stage are not easy !
I've found that a single 6L6 with "only" 10% of the load in its cathode exhibit an apparent RP less than 2K Ohms w/o any other kind of feed back.



Yves.

Hi Yves,
Thanks for your kindness also the opt_data no more paper works
Cheers!
SS

[mashaffer]

Yves, is that just a normal cathode feedback winding or some unique custom thing that you had done?

mike