[rotarius]

Hello, I just acquired an older FET/Valve hybrid preamp that I am quite happy with.  However, I have it paired to a Rotel amp with a 1v/33 Kohm input spec and I would like to lower  the preamp gain a bit if I could.  Can the 12ax7 tubes be replaced by 12au7 without any issues for that purpose?

Darius

[avahifi]

Which model Fet Valve preamp do you have?  If it is an EC chassis (big chassis with tone controls) then all you need to do is run it with the "mute" button pushed in which is actually a low gain function.

Changing tubes will not lower the gain, that is set by the feedback loop, not the open loop gain of the tube.  The original Fet Valve preamp line circuit is not biased for a 12AT7 tube and you must stick with a 12AX7 tube in this application.

You could lower the gain on a Fet Valve SL preamp somewhat by installing a 1000 ohm resistor from output to ground on both channels at the output jacks, although this may limit the drive capability into the 33K input impedance of your Rotel amplifier.

Frank Van Alstine

[rotarius]

Thanks for the quick response frank.  I might try to solder the resistors or perhaps buy some inline attenuators.  One more question, in the future if I decide to vertically biamp my speakers what is the minimum recommended input impedance for the amps?  I have the standard SL version of the preamp, btw.