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What is the function of 12AX7 tubes in Eico ST40 amplifier? Phono preamp? Is there any reason in upgrading these tubes for vintage tubes if I don't have a TT?
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Look at the ST40 schematic. The 12AX7s are used in a low level magnetic device preamp. Like any number of units from that time frame, equalization is switchable between RIAA (records) and NAB (tape). Frankly, switches in lines carrying mV. level signals are most undesirable.
If all of your sources are line level, inexpensive tubes, serving as place holders, are quite sufficient.
Personally, I'd simplify source selection so all I/Ps, except 1, are line level. The exception gets hard wired to a decent phono circuit, like the tweaked RCA setup I'm associated with. Be well aware that AC filaments in phono sections, like EICO employed, are HIGHLY problematic. Only a 7025 or genuine equivalent, such as the Sovtek 12AX7LPS, has a fighting chance to yield satisfactory hum levels. 7025s are 12AX7s whose heaters are spiral wound and hum bucking.
To get good flexibility with multiple sources, as indicated above, the OEM source selector switch gets replaced. Mouser stock # 105-14572 gets the job done well for just under $5. :> D
Eli D.
Thanks for the info. I have been pleased with the 6dj8 unit so far, but now is the time to try out several designs on my bread boards before I commit to a single design.
Not a big fan of the sound of the 12ax7, but I haven't used any for a while, so maybe it's time to listen again.
Actually I was pleasantly surprised at how low the hum level was with the 6dj8 with ac on the filaments. I may go ahead and switch to dc after I listen for a while though to see if how much difference there is.
Thank you.
Indeed, the seller completely reworked the phono stage (I quote him below):
"Installed a new additional filament transformer and associated circuit to allow the upgraded phono section tubes to be heated with filtered DC rather then AC which doomed the original phono section to hum."
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