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CCS 10M45 in Cathode of CF Driver

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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
 

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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
 

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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
 
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
 
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.
 
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Joined 2003
SY said:
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.
 
That's a good point. I'm a regulator snob.:D

Actually, I'd expect that even with a CCS, you still have to consider what happens on that rail. The CCS controls the swing in the negative-going direction, but at the positive-going point where you transition into grid current, the driver tube's current will equal the CCS current plus what the grid wants. Not as severe as class B, but it really does end up being at least an AB stage unless you strictly limit swing to the region where grid current needn't be supplied.
 
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Joined 2003
Yes, output stage excursions into A2 will also result in full wave rectified current pulses being drawn from the driver supply. I suppose it's a matter of degree; Class B drivers require current pulses throughout the entire cycle and at all levels, whereas A2 output stage excursions force shorter duration pulses, and only occasionally (we hope).
 
Hi Erik,
The amp I have, turns on voltages in three stages. The 6B4 tubes do not get any voltages (filament and plate) until the 6SN7's are warm and conducting. Then the 6B4 filament is turned on and then 30-45 seconds after that, the 6B4 plate voltage is applied. When I built the amp, I figured the driver would have to be fully conducting to avoid bias problems with the 6B4.
The 6SN7's have regulated fixed bias that is adjustable with a pot at the 6SN7's grid. The 15K cathode resistor in the original drawing is basically used to set the 6B4's bias correctly when the 6SN7 conducts 8ma, so I don't know if I would quite call it cathode bias.
As for the 10M45's, I have not ordered them yet, but will within the next week. You can purchase them from DIGIKEY.COM . They will ship out of the USA by mail which should not cost too much as these parts are not heavy. I think Digikey has a $20 $USD$ minimum order.
Thanks for the replys to my original question...
Daniel
 
Hi Daniel

Thanks for replying on my questions. I do have another one...have you ever tried to use a mosfet as source follower in the amp instead of the 6SN7? It is something I want to try, SY and tubelab use mosfets with succes, preferring them over tubes (for unity gain, buffer stages).

I already looked at digikey for the 10m45s. They even have a page for the Netherlands...what I see as a 'problem' is that shipping and handling costs amount to 31 euros. If I would buy 10 pieces of the 10M45s, those costs would be 3 times the product costs.

Erik
 
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