Post Phase Inverter Cathode Follower - Pointless in anything under 100W?

[gingertube]

Yes that is it.
Current Source loaded Mosfet source followers plus the Baby Huey feedback scheme, balanced shunt feedback from output tube anodes
Cheers
Ian

Thanks, so from your design I see that there's no need to double the bias voltage to get linearity, 1.5 times is enough with that design of CCS-loaded source-followers.
And the negative side is also useful to give more swing to the PI.

EDIT
I've two questions more:
- why going to two different CCS designs for the PI and for the source followers?
- have you tried to implement the R3&R4 as shown here: http://www.elektronik-kompendium.de/pub ... rr2pol.htm

The PI needs the "best" CCS. I started with a single transistor CCS with a LED reference. Going to Cascode was noticable (particularly improved top end clarity). The theory supports that - a single transistor CCS has its impedance limited by "EARLY eFFECT".

The Source followers don't need fantastic CCS's and in fact I originally ran resistor loads. Using the CCS enabled me to increase operating current in the folower a bit, without needing extra voltage on the -ve supply,
the "ring of Two" transistor CCSs were "good enough".

As a "Rule of Thumb" the -ve supply should be at least X3 the bias voltage.

Cheers,
Ian

[chief mushroom cloud]

Pointless.

[roberto]

Pointless.

It's the more pertinent comment ever seen in a thread.
Talking about a power amp, the text feedbacks to itself throught the description of itself until subject, adjective and object complement merge into one single entity.

[roberto]

The PI needs the "best" CCS. I started with a single transistor CCS with a LED reference.

So that configuration is better than others with more components? So I don't get why overcomplicating it with more components.

As a "Rule of Thumb" the -ve supply should be at least X3 the bias voltage.

Thanks, but the schematic shows a way lower voltages, considering it is biasing EL34 with just 10 Ohms on the cathodes.
Do I miss anything?

[chief mushroom cloud]

Pointless.

It's the more pertinent comment ever seen in a thread.
Talking about a power amp, the text feedbacks to itself throught the description of itself until subject, adjective and object complement merge into one single entity.

I would elborate....but it would be..........pointless.
Maybe.
This is how I skin my cats......
I just raise the PI B+ ,double PI B+ cap, up the PI coupling caps to 1kV or more, and stick a 12BZ7 in the PI socket.
Or in the case of lower power amps (</=50W)....just throw a 12BZ7 in there or decrease PI coupling cap values by 25%.
Sure, it might change the tone slightly....but that's why we have tone controls...isn't it.
How come no one had this problem back in 60s & 70s?
I'm all for this SS tomfoolery, but we are talking about guitar amps and not a bomb bay door motor drive circuit....right?
Or are we.....¬_¬

[gingertube]

The PI needs the "best" CCS. I started with a single transistor CCS with a LED reference.

So that configuration is better than others with more components? So I don't get why overcomplicating it with more components.

As a "Rule of Thumb" the -ve supply should be at least X3 the bias voltage.

Thanks, but the schematic shows a way lower voltages, considering it is biasing EL34 with just 10 Ohms on the cathodes.
Do I miss anything?

You need to be able to swing the output tube grid voltage negative enough to drive it to cut off WITHOUT the cathode/source follower going into cutoff, that is, the buffer should remain in Class A Operation.
The -ve rail change is something that Marc did when he did the PCB layout for EL34 Outputs. My view is that it is a bit low.
Cheers,
Ian

[roberto]

The -ve rail change is something that Marc did when he did the PCB layout for EL34 Outputs. My view is that it is a bit low.

Thank you very much Ian, so that voltage is ok for EL84s, not so much for EL34s. I think he just used a standard bias tap for it, but twice that voltage is better. Good to know in case I'll decide to build that amp.