ECN Forum Forums Technical Discussion Electrical Theory and Applications 7800 Series Voltage Regualtor

That's what I'd go with too, but just watch the peak voltage on the capacitors to allow for the switch-on surge before the pre-amp tubes warm up and start to draw current.

Does the power supply use an indirectly-heated rectifier for the B+ supply by the way? If so, then the peak voltage might not be that much higher anyway because the rectifier cathode needs to warm up just like the preamp tubes before B+ will appear.

It's much more of a problem if you're using semiconductor rectifiers so that the full B+ comes on before any tubes are drawing current to "pull" it down.

P.S. Do you know the configuration/tubes used in the preamp? 325V sounds a little on the high side for a small audio preamp.


[This message has been edited by pauluk (edited 10-06-2006).]

Thanks to JoeTestingEngr for the following:

This is where you tube gurus can tell me if this concept works. The zener would probably be just under the max differential voltage. (35V???). The top pot would be a high R, just to set the biasing for the control grid for optimal performance. 24V/Rtop sets the ref. current(Iref). (Vmax-24)/Iref=(R fixed + R adj.) The output cap is there for stability and should be >= 1uF. The adjustment filtering cap might be more than 1 cap. If you use a large capacitance for a slower ramp-up, you would probably use a small disk for RF bypassing. The zener should double as the I/O protection diode and the diode will discharge the filter capacitance.

This circuit puts the adjustment pot closest to circuit ground. It works with the assumption that you would want a given high voltage +/- a certain amount. You would take your voltage range / I ref to determine the closest linear pot value and make up the rest of the total with the fixed resistor. You could also use a multi-turn pot and no fixed R or a low R "FINE" adjustment in series with a high R "COARSE" adjustment.

So this is where Paul and John and Mike should chime in with triode suggestions (cuz I'm clueless!) The last tubes I worked with were 1KW broadcast tetrodes and 55KW klystrons, and I'm powerless here. I'll vouch for the part past the tube. Even if this circuit doesn't fit your pre-amp, it wouldn't hurt to have it around to tweak your other tube circuits.

As always, HAVE FUN!!!
Joe

Joe, I agree with most points, but not with the "Iref" equations. Iref is indeed the current through the reference divider, but it's supplied through the 7824 pass element, not through Rtop.

For stability, Iref is normally selected to be at least an order of magnitude above I(Q), the quiescent current sourced by the IC through the COM pin. For the 7824, typical I(Q) is 5 mA, so Iref would be set at 50 mA or so. The current supplied through Rtop is something like 1 mA, and the rest is supplied through the tube cathode and the IC.

However, that 50 mA Iref is a lot of wasted power for a 10 mA load, so a different regulator exhibiting a lower I(Q) would be better. Consider the LM317, which is essentially a "7801.25" (1.25 V regulator) having a maximum I(Q) of only 100 uA. You'd then select Iref to meet the LM317's minimum-load requirement (12 mA max), which is conveniently two orders of magnitude above I(Q).

You could probably choose reasonable fixed values intead of using a trimpot for Rtop. Consider how this circuit works: The OUT pin of the regulator IC is always negative with respect to the IN pin, and it's this negative differential that operates the control grid. The IC compensates for greater loads by increasing the conductance between its IN and OUT pins, making the control grid voltage less negative and raising the tube's conductance.

The problem is that there's a limit to how low the IC can make its IN-OUT voltage; for the 7824 it's about 2 V, and for the LM317 it's about 3 V. This voltage applied negatively to the control grid may not be low enough to allow the tube to supply the required full-load current, so the Rtop divider is in place to reduce it. However, the divider ratio must not be so low that the tube can't be controlled at the minimum-load condition when the IN-OUT differential is greatest (limited by the zener voltage).

This is probably a lot more than you wanted to know... but it's part of HAVING FUN, right?

John, I regretted not using component numbers after I had zipped the email off to Paul. I was referring to the top fixed resistor as Rtop since the pot was a VR. I agree with you completely because I was thinking of the LM317 or 350 all along. But Josh already mentioned the 7824. If I had known it was 5mA,(Are you sure?), I wouldn't have mentioned it. PDs of the reference string resistors start making it more of a PS/Heater. My datasheet didn't show specific values for the 7824. Not knowing the biasing requirements for a given tube, I thought tying to the Regl output might be pinching off too much. I was wondering if a little R in parallel with the triode might not be good as well.
So do we agree that with the LM317 or LM350 and a suitable triode, Josh might pull it off? I haven't read any tube data in over a decade. What about the possibility of a little R between the cathode and regulator input to set I limit and help with grid biasing?
Joe

[This message has been edited by JoeTestingEngr (edited 10-10-2006).]

Joe, I think it would work fine using an LM317 without any further tweaking. (Yep, the Fairchild LM7824 data sheet says I(Q) is 5 mA typ, 8 mA max.)

Since there are so many ways to skin this particular cat, I don't think I'd bother using a triode for this job. I was thinking along the lines of a TL431 with a cascode NPN driving an N-channel series-pass MOSFET. Guess what? Somebody else beat me to it .

Happy soldering,
John

Wow, all those years of reading Japlish in Sony Broadcast equipment manuals hasn't prepared me for Russlish. C'mon, was that really how you were going to design it? I was thinking along the lines of a single supply jfet op-amp with a zener bleeder supply and a bipolar series pass. Just a protected voltage divider input and reference scheme. Should we have just considered the ye ol' LM723 ?

Although the text is not always crystal clear I think the schematic speaks for itself: It's nicely simple and effective, and since the only IC is in a TO-92 package, it's also dead-simple to breadboard.

Of course, in a world where even the AC power cord has to be specially braided, rhodium-plated neutrogen-free copper with argon-filled polygooglene foamed insulation for impedance matching of sub-nanosecond microphonic transients, we all know that semiconductors of any kind are evil. The only proper way to do this is with a separate ferroresonant transformer for line regulation and a mercury-vapor rectifier.

Coincidentally, one of my current projects is a small production run of replacement power supplies for the military. Since it was designed in 1975, of course it uses... LM723!