12B4A + LL1671 Linestage

[Fuling]

Hi,

I have a pair of LL1671/50mA interstage/line output transformers left over from a SET project that never was built (and probably never will be).
Tre transformers was meant to couple a beefy driver tube (6B4G or similar) to a triode wired 813 and allow peak A2 operation, hence the 50mA airgap.

I´ve realized that I´m much more in need of a decent line stage than another SE power amp, so my latest idea is to build a line stage based on these transformer.

A (wishful) spec list:

A single triode per channel
No negative feedback (*)
Low gain (x2-3)
Capable of driving fairly low impedances.

The preliminary plan is to use a pair of 12B4As with the transformers connected 2:1.
What bothers me a bit is the big airgaps and the resulting low primary inductance.
The LL1671 datasheet states 35H with 30mA airgap, so I estimate the inductance to 20-25H with 50mA airgap.
A 12B4A running "low and hot" (100V 35mA or so) should have an internal resistance (Rp) just above 1k.

A while ago I ran some tests on a choke loaded 12B4A gain stage. I used a 30H 200mA filter choke as plate load and an OP optimised for maximum output swing (300V 17mA).
IIRC this gain stage delivered full output signal down to 20Hz, so I guess it is possible to get away with 20-25H with a different OP.

* LL1671 has a 1+1+1+1:2+2 winding arrangment which gave me the idea to put one of the four primary sections between cathode and ground to get cathode feedback.
I believe this would decrease the effective Rp of the tube which might be useful at LF.

[Zen Mod]

just go for it

[Fuling]

just go for it

That´s what the guys at my "local" forum (hififorum.nu) keeps telling me, too.
Perhaps the bandwidth will be somewhat limited at the low end, but I´m sure it would sound great anyway.

[Zen Mod]

Originally posted by Fuling


That�s what the guys at my "local" forum (hififorum.nu) keeps telling me, too.
Perhaps the bandwidth will be somewhat limited at the low end, but I�m sure it would sound great anyway.

go to www.vt52.com and download few handy xls files ;

if you didn't already

[Fuling]

Yeah, vt52.com is the right place to find inspiration for this kind of project!
Thank you for reminding me of that page!

[Zen Mod]

Originally posted by Fuling
Yeah, vt52.com is the right place to find inspiration for this kind of project!
Thank you for reminding me of that page!

you can always click on my www button and see my WOT preamp ;
not exactly as your intention , but similar