The Mic

[KrIVIUM2323]

Transformers looks like edcor's to me.

Active load can vary ( it'll be different eg for a common ground circuit or a cathode follower) , but from what i've seen and deducted from what Analag shown, he used a solidstate CCS as anode load for tube.

As CCS acts like 'theorical' infinite load to the tube it then behave as biasing is defined by a straigth horizontal line on tube curve, giving a max amplification factor ( the max one from the tube used) and low distortion.

You can use other ways to bias than a resistor in cathode like a led or diode. Such ccs can be bjt, fet, depletion mosfet, casoded version ( mix and match),... a cascoded version of depletion mosfet offers stellar performance static and dynamic with minimal signature imho.

Works wonder but often seen as a not 'purist' approach.

 

[Emmathom]

Tr1nsformers looks like edcor's to me.

Active load can vary, but from what i've seen and deducted from what Analag shown, you use a solidstate CCS as anode load for tube.
As CCS acts like 'theorical' infinite load to the tube it then bahave as biasing is defined by a straigth horizontal line on tube curve, giving a max amplification factor ( the max one from the tube used) and low distortion.

You can use other ways to bias than a resistor in cathode like a led or diode.

Works wonder but often seen as a not 'purist' approach.

I think that's what I do but with a CRD instead of a plate resistor (so to force a definite current thru the tube).
Yes I've seen cathode set by 2 diodes in serie (UF4007 for ex.) so to give ~1v or 1,2v

 

[KrIVIUM2323]

What do you call a CRD Emmathom?

 

[Emmathom]

Current Regulation Diode
Same as zener but about current (but rated at max. 100v)
E-152 (1,5mA) E-352 (3,5mA) etc

@thor.zmt taught be to use them and can muuuuuch more debate on that subject...

 

[analag]

Nice work. Input & output line transformers (which brand ?) 15uF coupling tube output ??? Wow...

You didn't say anything about your "tube active load"... how do you set that ?

Regards

Edcor 600:10K input and 10K:600 output. Originally the second stage active loaded but I ended up ripping it out. At the current levels I'm running the tubes the DN2540 was a little too hot for my liking even with stepped up heatsinking measures.

 

[analag]

Transformers looks like edcor's to me.
Works wonder but often seen as a not 'purist' approach.

Transformers are some Edcors I had laying around from when I designed the Original Poor Man's 660...it was never a 670. I made a 670 type and it's much different from the PM660.

I have been playing with active loading for more than a decade. I find it useful on crappy tubes like 12AX7s and when tubes are operated in the curved portion of their characteristic curve. I use the "intelligent" plate resistor to get the tube out of trouble, so to speak. I think a perfectionist is greater than a purist. A coach is considered great when he/ she can get the player to rise above their perceived limitation, therefore if I can put some silicon on top of a tube and be rewarded with better performance, then why not. It's all about the sound.
In the mics I run the tube at precisely half the B+ and it's held at it. No cathode bypass cap is needed or I should say the current is steady but the voltage will fluctuate, so I normally go with some 220-470uF to maintain steady voltage. To my ears it improves the sound enough to justify it's use. In that last mic, I went active loaded and added feedback from the tertiary winding. It sounds the way I expected it to...even, clean, no egg frying noises or very reduced and no super expensive magical fairy dust tube needed to make it happen...just a little piece of silicon, lol!