CCS 10M45 in Cathode of CF Driver

[danFrank]

Hello, I have a question about using the 10M45 as a CCS in a cathode follower driver configuration.
Basically, the circuit I have now is 1/2 6SN7 as a cothode follower directly coupled to the grid of a 6B4G output.

See attached picture. The amp is actually push/pull but I only drew the top half of the PP driver / output circuit to save space.

Basically, it sounds very good with the 15K resistor going to the negative voltage supply.

All voltages are regulated using Maida style HV regulators.

See my next post to see a picture of what I would like to change it to...

Thanks, Daniel

[danFrank]

What I would like to do is change the 15K resistor to a 10M45 CCS chip. Is this the correct way to implement this chip? See attached picture.
I have never used CCS circuits before and am willing to try any simple tweek if it makes the amp sound better.
I am aware that I will probably have to change the negative supply voltage to some other value to accommodate the voltages seen by the tube.
The current going through the 1/2 6SN7 is approx 8ma.

Thanks again, Daniel

[rdf]

Looks fine here. You might want to consider a 10-turn trimpot as part of the current set resistor to adjust the 6B4G's gird bias voltage.

[SY]

What rdf said. And you probably don't need the current set adjustment.

[danFrank]

Why no current adjustment resistor? Is it because the way the 10M45 is used here all voltages "should fall into place"?
Will the adjustable fixed bias I have on the grid of the 6SN7 do all the adjustments? Should using the 10M45 be an improvment in this circuit?

Like I said, I have never used any CCS circuitry before in my amps, so I am kind of hazy on the idea. I get the general theory of a CCS but have no idea how one acts in an actual circuit. I guess I should start building...
Thanks, Daniel

[SY]

If you use a fixed resistor, the current will be close. In some applications, it really does matter whether it's 8.0 or 8.1mA, and you need an adjustment, given resistor and chip tolerances. With this one, it's not critical.

[ErikdeBest]

Hi Dan

Interesting thread, it treats some questions I have myself.

Once you have set the desired current you won't have to adjust it (so a pot is kind of superflous, or could be used at the begin, but substituted for a fixed resistor afterwards). The 6SN7 will 'faithfully' (quoting MJ) follow any variation imposed on its grid (that is the way you are biasing your output tube). The 10M45 should made the operation of the 6SN7 more linear, reducing distortion (which already is low).

As I said, I have a related question, and as I believe it is also of your interest, I will put it here, and see if the more experienced jump in. In the present situation you are using a resistor to bias the 6SN7 cathode follower. When turning the amplifier on there is no current flowing through the 6SN7, so I imagine the grid of the output tube is held at B-, totally cutting the output tube. When the 6SN7 starts conducting the voltage slowly goes up (until the set bias point) and the output tube starts conducting. When using the 10M45s I wonder how this biasing works...will the voltage at the output tubes grid be negative when filaments are cold? I think so, but not sure.

When the 6SN7 is not conducting, the 10M45s still wants to draw current. Could this be a problem?

And another thing, I also want to play with the 10M45 chips, but can not find them in Europe. Where are you buying yours? Could you buy some more, and resell them to me? Do you have paypal?

Erik

[rdf]

Hi SY. Depends on how the CF is being biased off the left of the shown schematic, hence the 'consider' part. If the 6SN7 fixed bias is already variable then a trimpot on the 10M45 would only permit fine tuning current as you note. I still find it useful given how variable these devices are.

[SY]

Heck, for a p-p, you could even run the 6SN7 in class B. A CCS is probably over-engineering.

[EC8010]

Originally posted by SY
Heck, for a p-p, you could even run the 6SN7 in class B. A CCS is probably over-engineering.

But if you had a pair of Class B cathode followers they would draw pulses of full-wave rectified current at twice the signal frequency from the power supply, making it much harder to keep the rails clean. The greatest benefit of the CCS is probably that it keeps the rails clean.