LS5 Tube Layout

Mibix9,

Thanks for your last post. I learned something new. I did not know ARC used the 12BH7 tube.

Also I was in error when I said the pin-out was the same for the 12BH7 and the 6922 tube. Pins 4&5 of each tube is for the filaments but the 6922 is 6.3V and the 12BH7 pins 4&5 is 12.6V.

The 12BH7 is a dual voltage filament tube and can be wired for 6.3V or 12.6V.

For use with a single 6.3V power transformer winding, (pins 4&5 tie together), 6.3V is applied to pin(4&5) and pin #9.

The LS5 came in 3 flavors MKI, MKII and MKIII. Each of them had a different Tube set. The original MKI, which is the one in question used 6x12BH7 and 4x6922. Thanks to Steve at greatnorthensound for verifying my original assumption of the tube placement was correct.

Here are the tube complement descriptions from ARDB:
LS5: (4) 6922, (6)12BH7
LS5 MKII: (10) 6922
LS5 MKIII: (8) 6922/E88cc dual triode

There is a 4th incarnation, which greatnorthernsound offered as a mod, bringing the LS5 up to Reference level. Steve has enough parts to do one more mod, so I am going to give that a try.

Thanks for your responses.

Audio research Corp. LS5 Preamp??
Specs say the preamp uses ten 6922 tubes.....
http://www.arcdb.ws/LS5/LS5.html

Though the tube pin out for the 12BH7 and the 6922 are the same, I do not believe the two tubes are interchangeable.....
http://www.drtube.com/tubedata.htm

The circuitry for the preamp was built around the 6922 tube characteristics. The preamp should sound best using 6922 tubes or tubes from the same family, 6922/E88CC,7308/E188CC.

Using the 12BH7 could cause the filament winding of the power transformer to fail due to overload.
12BH7 heater current is 0.6 amp @ 6.3V.
6922 heater current is 0.3 amp @ 6.3V.