My SCA35 mod to apply DC to the preamp tube heaters

for the DIY ST35, the Dynakit and every other PP EL84

My SCA35 mod to apply DC to the preamp tube heaters

Postby wa2ise » Thu Feb 11, 2010 1:24 am

http://www.audiokarma.org/forums/attachment.php?attachmentid=194849&stc=1&d=1265871704
Been hanging out at AK for a few years, and just found this forum. (lol) Anyway, this is a mod I did to my SCA35 to get DC on the preamp tubes nearly "free" by using the preamp tube heaters as cathode resistors for the 6BQ5 output tubes. Yes, it adds up to 24V, and the 6BQ5 wants 12V, but I rigged up the grid bias to be tied to the point between both of the 12AX7 preamp tubes (after I rewired their heaters). This is similar to the old trick of using a voltage divider on a 5V regulator chip to make it produce a higher regulated voltage. The tubes draw enough cathode current to raise the grid bias to 12VDC, and that makes the cathode rise to 24V. No more heater induced hum in the phono preamp! Only downside is that the output tubes will see a little less effective B+, but as today's powerline voltages run higher than back in the SCA35's day, this isn't really an issue. And a dB or 2 less output power isn't that significant. As I don't blast my music anyway...
wa2ise
 
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