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RE: LM317 for PP cathode biasing?

Posted by Alex Kitic (A) on February 21, 2015 at 10:13:15

In Reply to: RE: LM317 for PP cathode biasing? posted by DAK on February 20, 2015 at 10:18:52:

Hi DAK,

No other changes than those explained above in my post. Basically, the main difference in operating a PP amp is whether any of the output devices can switch off during some part of the operating cycle (class AB or B) or the output devices never switch off during any part of the operating cycle.

If you use one LM317 for both output devices (tubes) the pair will draw a set constant current, but either tube will draw as much current as it "wants to" statically, and dynamically one of the devices can switch off - therefore possible class AB or B operation.

If each device has its own LM317 (as if it were 2 SE amp output stages) each device will draw a set constant current and will never switch off: therefore class A operation.

Logically, class A operation yields less output power than would be possible with class AB or B, and without entering further into details like what would be the optimal primary impedance, current, etc. for a given output stage in a given class... it is more than obvious that if you plan on modifying some amp by inserting a CCS device under the cathode(s) of output tubes, this is most probably going to affect output power as well.

I am gladly going to pass on commenting the LM317 as suitable or not for CCS duty under the cathode of output tubes, and the white noise thereof, etc. I gladly leave this type of discussion to people I like to call "armchair DIYers" (no pun intended) who build one (new) amp for every approx. 10k posts they make. That's a good way to keep oneself busy - posting without building, and it helps getting things into the right perspective (like, white noise and other stuff people who build real life amps relatively often actually do not tend to notice all that much)...

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