Winter Projects

[analag]

Got a few things on my short list for when the weather gets frigid. Time to update the AOC. It's an old design that can be made a lot better, another tube mic pre is on the table as well as a tube mic PSU. The KT120 amp was a resounding success! The customer felt a 36hrs drive round trip was well worth it. I tell ya a great tube amp will beat a solid state amp like a red head stepchild everyday of the week and twice on Sunday...but it's gotta be built right! Anyway time to go play with all these Russian tubes I've been accumulating.

 

[analag]

AOC now uses 6N2P and 6N1P. Some work done to the gain reduction circuit. It can be subtle and clean or dirty and crushing much more than before. It should be even better after these tubes burn in! The 6N1P will be replaced by 6N23P eventually.

 

[pucho812]

I didn’t know she had tube gear or time as she is a congresswomen. Who knew

 

[Script]

Some work done to the gain reduction circuit.

Sounds interesting. Like what ? You plan on sharing more details ?

 

[analag]

Sounds interesting. Like what ? You plan on sharing more details ?

Still playing around with it. I have a few more things to try.

 

[Script]

I always thought that the biggest 'flaw' in that circuit (if anyone had the right at all to call it that) is the optocell being fed by AC. Had the idea of converting to DC in front of opto via germanium diode bridge but never tried so far. Or ditch the optocell entirely and use FET (different beast).

 

[analag]

That design is more than 20yrs old and it worked right up until I started messing around with it recently. If I were to do it over completely I would go solid state detector this time. As it right now, I believe it's better or different. It can do brutal limiting almost like a clipper or gentle control. I'm almost satisfied with it, but I want to see if I can further improve it while retaining circuit simplicity.

 

[Script]

: )

 

[analag]

Obviously the Vactrol likes current so changing the coupling cap from 0.1 to 1uF made a big difference in the compression curve. The gain of the unit is also significantly increased and self noise got lower. The 6N2P brings some 2nd harmonic action to the sound. Haha I like it!

 

[Script]

1uf changes the curve to what effect? I wouldn't know as I always had 1uf for C102 and C104 (inspired by Kubi mod,I think). Chunky Wima with bent leads fit onto the PCB by silentarts, as I have the B+ resistors off-board.

The 6n2p is equivalent to 12ax7 ? That'd be an amp factor of 20 (12au7) or factor of 60 (12at7) up'ped to factor 100 ? That's a lot of gain...

Even more THD / gain can be had by lowering tube NFB, I assume. It's been on my mod list for years.

 

[analag]

Sounds like you got if figured out. 6N2P is like the 12AX7, I can only imagine how much gain the LA2A has with it's 1:9 input transformer, two high gain stages cascading into each other and a 5:1 transformer on the output with considerably less NFB than the AOC.

 

[Script]

Avid reader but slow builder/modder here.

l should finally try less NFB for more 2nd harn. and complementarily reduce gain elsewhere. If I like it, might put it on a MBB switch (dual-gang pot difficult, possibly dangerous, I think) and label it 'Master / Mix' or 'Linear / Dynamic' or 'Pristine / Sloppy' or whatever ; )