Cheap Lunch?

[dsavitsk]

I have been meaning to build an el34 or maybe 2a3 drd/free lunch amp for a while. I have most of the parts, but I really have just been too cheap to buy the plate loading chokes. So, I started thinking that I might be able to swap out the chokes for a CCS that would work about the same and cost $3 instead if $200 making it a cheap free lunch, which makes no sense. Moreover, the CCS will drop however many volts it needs, so it seems that one could do away with the resistor divider in the output tube's cathode and just let the CCS set the voltage.

So, here is a preliminary schematic awaiting comments.

[jeff mai]

Don't forget the resistive divider helps set the bias for the output stage. Also, a choke load allows the plate voltage on the input tube to swing above B+; the CCS needs to have enough voltage drop across it so it doesn't drop out when the plate swings more positive. If you can get it to work it will cost more in terms of higher B+ required.

[mach1]

The 5842 will only be able to swing (Vg-kEL34 - CCS insertion loss). Not enough for full power. Ideally a driver should be able to swing 1.5 - 2 x the Vg-k of the oputput tube.

That's why chokes are used.

pm

[dsavitsk]

Originally posted by jeff mai
Don't forget the resistive divider helps set the bias for the output stage.

Here, the current through the output tube is set by the voltage drop across the CCS, so the resistive divider is not necessary.

Originally posted by mach1
The 5842 will only be able to swing (Vg-kEL34 - CCS insertion loss). Not enough for full power. Ideally a driver should be able to swing 1.5 - 2 x the Vg-k of the oputput tube.

This doesn't strike me as a huge deal. I am more interested in the DC coupling part than the extra power part that DRD provides. It is a SET, which is hugely inefficient, so making it slightly more inefficient isn't a concern. I never run amps anywhere near their max. But, the EL34 might not be ideal. I probably need something that is biased a bit higher.

[rdf]

To expand on what Jeff said, at DC a choke's inductance equals zero. It's the driver's static curent through the choke's residual resistance which is used to set the output tube's bias. A CCS maintains megaohms impedance at DC, leaving no way to properly bias the output.

Edit: Bad timing. How is the voltage across a CCS controlled?

[jeff mai]

Try it and let us know how it works. Use stunt dummy tubes first. There might be some gotcha at turn on.

[dsavitsk]

Originally posted by rdf
To expand on what Jeff said, at DC a choke's inductance equals zero. It's the driver's static curent through the choke's residual resistance which is used to set the output tube's bias. A CCS maintains megaohms impedance at DC, leaving no way to properly bias the output.

Edit: Bad timing. How is the voltage across a CCS controlled?

The constant current across the driver tube forces the CCS to drop the right amount of volts to maintain the voltage on the driver's plate. I'll say that a different way ... the operating point of the driver is determined by the convergence of 3 things -- the current, the bias, and the plate voltage. Usually, you think of specifying the voltage and the bias, and the current falls into line. But, there is no reason not to do it the other way. By specifying the current and the bias (which is quite specific due to the LED), the tube will set its own plate voltage, and so long as the voltage on the of of the ccs is higher than this, the ccs will drop the appropriate number of volts. This, then, sets the bias on the output tube. I think it is just a matter of finding the right combination of tubes, which I have not really done yet.

Originally posted by jeff mai
Try it and let us know how it works. Use stunt dummy tubes first. There might be some gotcha at turn on.

No doubt I just thought of this while in the shower today, so I can still be convinced it is unworkable.

[dsavitsk]

Here's a simple example that might be useful for pushing some headphones ...

[rdf]

You're on fire. I see what you're saying, if a little mixed up between which tube's terminals your explanation referred. As long as the voltage across the CCS doesn't fall below the minimum required to operate though it doesn't matter where its positive terminal is connected, the 5687's plate voltage is completely independent of the voltage on the other end of the CCS and therefore the output tube's operating condition. Given any tube/LED pair the driver's plate voltage will be set entirely by the CCS current. The DC interaction between driver and output cathode voltage that acts to stabilize a free-lunch is lost.

I wonder if replacing the 30K with a bypassed adjustable CCS as well would get some stability back. The 12B4's cathode voltage would then seem to be free to 'float' to a stable op point. A second cap from 12b4 cathode to ground will also be required to maintain the Ultrapath PS rejection. However the only thing then to be gained from tying the driver CCS to the output cathode is saving power supply draw.

[dsavitsk]

Originally posted by rdf
You're on fire.

I hope this doesn't mean that building this is going to cause me to be so ...

Originally posted by rdf
YThe DC interaction between driver and output cathode voltage that acts to stabilize a free-lunch is lost.

I am going to need to keep studying to understand this.

Originally posted by rdf
wonder if replacing the 30K with a bypassed adjustable CCS as well would get some stability back. The 12B4's cathode voltage would then seem to be free to 'float' to a stable op point. A second cap from 12b4 cathode to ground will also be required to maintain the Ultrapath PS rejection.

I also thought about making the output stage parafeed with a plate CCS of it's own and a WE cap connection.

Originally posted by rdf
However the only thing then to be gained from tying the driver CCS to the output cathode is saving power supply draw.

No small thing in the above example -- 40mA v. 70mA seems significant enough to think about (at least in as much as one of them could be built with stuff in my closet, and the other would require ordering yet another power transformer). But, the real idea was to get rid of a coupling cap. Maybe it isn't that important ...