mono sex with phono. question to PJ

[xcortes]

hi paul, a while back you mentioned that:

i plan to mod a SEX amp to mono, with the other side running a 6SL7 in the classic RCA phono preamp circuit. The cabin was my wife's folk's; there's a pile of opera excerpts and musicals, all o

is that something you end up doing? or at least designed? i'd like to pursue this idea further.

thanks a lot!

[Paul Joppa]

I've not done this yet, but I still plan to do it. We don't use the cabin much in the winter, but come spring I may take the parts up and see if I can assemble it there. The existing system has a classic Harman Kardon receiver, which unfortunately has a big hum problem - that's a motivator!

The design is simple; I'd replicate the SEX amp on one channel, and copy the RCA circuit on the other side. Most RCA tube manuals have the version for the 12AX7, but it was first published for the 6SL7. The level control would have to be larger; IIRC it calls for a 500K control. I have not decided how to implement the cartridge stereo-to-mono conversion.

[xcortes]

I have not decided how to implement the cartridge stereo-to-mono conversion.

why don't you just get a mono cartridge?

thanks. i'll look into that circuit and see what i can come up with.

[xcortes]

Here's an interesting implementation by Joe Esmilla. I'm only going for the RIAA but if you have lot's of old records:

(Almost identical to the classical RCA although this schematic has an additional gain stage):

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fmembers.myactv.net%2F%7Eje205d%2FvarEQph-1.jpg&hash=ef0f2bebb9bae7ff94d186d71c8ea512ce15052a)

And a RIAA one, this time with battery Bias on the 6SL7s (not that we need those with the Sex DC heaters):

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fmembers.myactv.net%2F%7Eje2a3%2FJELsc-phonopre1.jpg&hash=67e33b91452297c1c47ab4cdff58e833551dd90d)

Additional question:

The 6SL7 heater current is 0.3 Amps vs 0.9 of the 6DN7. Shouldn't be a problem, right?

And the cool thing is that the pins of both tubes are the same so the conversion shouldn't be that complicated.

[Paul Joppa]

...
The 6SL7 heater current is 0.3 Amps vs 0.9 of the 6DN7. Shouldn't be a problem, right?...

Change the 0.1 ohm resistor in the heater power supply to 0.22 ohms.

[xcortes]

thanks!