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!

 

[analag]

Today was a good day. I rebuilt one and finished another. Both sounding good. I'm liking this circuit a lot. It holds it's own among the tube mics.

 

[analag]

Staging a high gain mic pre. Since this thread began I have made seven microphones. Two of which are solid state. I might do a 8th mic because I found a cheap TLM107 on Aliexpress and I bought it for the body. If it looks good I'll gut it and go with a 5702w and a large core transformer. I find the 5702 to be a wonderful sounding tube. To me at lease.
I am a huge fan of the 990 style opamp and over the decades (about 2 decades) I have built and blown up quite a few. I have three channels of these with BJT input, in the studio and the units have no on switch. They are on all the time. Most of my other solid state mic pres are one gain stage only for that superior sound which is not a problem with the loud arse tube mics. With this I hope to be able to capture the sound of a housefly batting it's wings from a mile away, lol.