(Newbie) CSS Question

[bibster]

Hi,

Sorry to bother you all with (probably) a simple question, but I can't seem to find out if it's a good thing to do.

I'd like to make some (new) CCS's (As per Sy gets jiggy) for my linestage, as the old ones gave up.
I think they gave up because I tried to drop too much voltage over them, so I thought:

Why not put a 3k3 resistor before them?

B+=260V, Operating point is 100V/20mA (10 per half 5687).

This would leave me with 260-(3k3*(20+5)mA)-15v-100v = 62.5V swing (give or take a bit: 5mA for the LEDs, 15V for the CSS)

Well, my question: Is this the right way to do it? If I sim. it, it seems to work (4mA though the leds... should I replace the 47k by a 33k?)
But sims aren't real life, so...

Thanks, Paul

(By the way: I'd like to try Pimm's CCSes, but his site seems gone..??)

 

[SY]

You can certainly put in series resistors, but if you use the right device in the right position, they'll last longer than you will. If you can sketch out a schematic of the stage in question, perhaps the grises eminences can suggest appropriate transistors.

 

[zigzagflux]

Sure, I suppose you could do that, but it seems like you are dropping a lot of voltage unnecessarily.

Maybe your 260V B+ is too high if your operating point is only 100V. How much swing are you looking for?

 

[bibster]

Thank you, Sy...

Basicly, 2n3906 on top, mje350 bottom, 2 red leds, 2k & 47k (or 33k) on the left hand side...
I end up dropping (with a bit of ignorance..) (260 B+, 100V O.P.) 160V over the MJE350, @ 20mA --> allmost 3.5 watt... That's quite a large sink for such a small thinggy...
(That's why I thought getting rid of some 90V in a resistor)

Paul

(Ow er... some 30V swing is enough... so yes, my PSU is giving too much voltage, yes... )

 

[SY]

You got it- for 30V of swing, you only need to drop (assuming that's 30V peak or 60V p-p) about 70V or so across the CCS. If you can't easily drop the rail voltage then, yes, a series resistor can take up some of that dissipation.

Otherwise, a heatsink to get rid of that 3.5W will need to be about 15 degrees C/watt or better for reliability. That's not small, but it's not really big, either.

 

[zigzagflux]

Dropping your rail a little with an RC network will provide for a lower ripple B+, an added bonus (of course, the CCS does that anyway). Allowing SS to run cooler is always a good idea, and power resistors are cheaper and easier to mount than heatsinks. I always try to design to avoid heatsinks.

Not to threadjack, but it does get hot under that chassis. I normally target an absolute heatsink temperature of 80 degrees C, with ambient of 40C. Looks like that would produce 93C with SY's heatsink.

I would not go over 100C long term.

 

[Tubelab_com]

If you look at the SOA (safe operating area) curves for the MJE350 you get a maximum of about 210 volts at 20 mA continuously. Experience has told me to stay far away from the SOA limits, or the device will die unexpectedly.

In my amplifiers I often add a resistor in series with the CCS. This reduces the power dissipation in the CCS, lowers the voltage drop across the CCS, and slightly increases the total impedance (by the resistor value). There is another benefit:

All solid state, and some hybrid CCS's have a capacitance associated with them. Some of that capacitance will vary with the voltage across the CCS. Many believe that the voltage varying capacitance (which is essentially from the plate to ground) can cause low level phase distortion. A resistor in series with the CCS is a resistor in series with the varying capacitance, thus lowering its effect. For this reason you do not want a decoupling capacitor after the resistor. The resistor must be small enough to avoid starving the circuit under all operating conditions.

 

[zigzagflux]

For this reason you do not want a decoupling capacitor after the resistor.

Just to clarify: I was suggesting using an RC to provide a lower B+ voltage before the CCS, not after it. That is, instead of using a 260V rail, drop it a little lower to maybe 170. Then the CCS gets to operate with less headroom.

Your point on 'blocking' the CCS capacitance is well taken.

 

[Tubelab_com]

I also place the resistor on the B+ side of the CCS. I run a B+ as high as 500V in my SimpleSE amps which makes the CCS chip (IXYS 10M45) rather warm (10 mA to a 12AT7). I put a 10 K resistor on the B+ side of the CCS to drop 100 volts. The chip runs cool enough with a small heat sink. I tried a decoupling cap between the CCS and the resistor and could not detect any measurable differences. I could not hear any difference either, but one of my golden eared friends could pick out the "capped" circuit most of the time, and said that it sounded "smeared". The differences must have been small, because he could not always identify whether the cap was present (he did not know in advance). If I can leave out a part, and it sounds the same or better, I am all for it!