2. amps-preamps
  3. 1298216608

lowering gain in a tubed pre-amp

My Nagra PL-L is wonderful with one small problem: in driving my Kharma MP-150 amps, it has a fairly narrow band of acceptable volume control (from about 7-10 on the volume knob).

I have read multiple threads on this topic, including one today in which Almarg and others have suggested attenuators and other things.

My question: since the Nagra is a tubed pre-amp (it uses two 12ax7 and one 12at7 tube). Can I reduce gain help the situation I describe above by subtituting lower gain tubes?

I guess my question really has several parts:

(1) what tubes can I use?

e.g., I know that 5751s are lower gain substitutes for 12ax7s, but are there other safe substitutions -- and when I tried some 5751s they seemed just as loud to me as the 12ax7s...

(2) will going with substantially lower gain tubes harm the pre amp?

(3) in general what are the sonic benefits/costs of lowering the gain of the pre-amp using different tubes

(4) Perhaps a stupid question, but my understanding is that most of us are using our preamps as attenuators and not as amplifiers. What I mean is that "unity" gain on a pre-amp (where the pre is neither amplifing nor attenuating the source signal) is typically where the volume knob is higher than most of us have it set. So for those of us in the predicatment I describe above: volume "control" only between 7-10 o'clock, we are squarely in the attenuation portion of the pre-amps function, so changing the gain of my tubes will make no difference? Perhaps why the 5751s sounded just as loud to me as the 12ax7s???

Last bit of information: I have a CD player with a volume control. It certainly seems like I could turn it down in order to have a wider range of volume control on the pre-amp. But the CD player seems to definitely sound best when its volume control is between 12 o'clock and 2 o'clock, and it seems to be more important than the pre-amp (meaning sonically it has been better to leave the CD player in this range and deal with the narrow range of volume control on the pre-amp than the turn the CD player's volume down a lot more, and have a bigger range on the pre-amp.

Suggestions, mentoring, and advice greatly appreciated.

Thanks.

--dan
dgaylin
onhwy61's avatar
onhwy61
5,227 posts
 
Can't what you want be done with the modulometers?
dgaylin OP
255 posts
 
Onhwy61 -- well, that was sure stupid of me. You are essentially correct: the modulometer per se does not lower the gain, but there are jumpers in the pre designed to do just this -- it's been a long time since I looked at the owner's manual.

So THANK YOU for pointing this out, and it will probably fix my problem. Much appreciated

That said, I'm still very curious about the general answers to may questions, so would appreciate any responses.

Thanks

--dan
mechans
3,048 posts
 
I can't guaruntee saftey but you may be able to sub the 12AT7 for the much lower gain 12AU7. The 12--7 miniature twin triodes have the same pin out. The problem you might encounter is that while the voltage will lose gain, but the amperage pass thru goes up. I would still stick with the 5751 because it is just a better sounding tube regardless of it's lower mu of 70 affects the volume.
Other than that your choices are civil peaceful resistance. Or realistically a different volume attenuator which is what I think most preamps use to alter the volume of the amps. I think power amp gain is fixed and the preamp signal is what is indeed manipulated. The best way to do that is to buy a different preamp because buying a more sophisticated volume attennuator and having installed will cost a lot and lower the value of the pre unmoded. I know this is not the answer you wanted, but I also own a preamp with "gain from hell". The venerable ARC SP-6B which I just gave up on using.
BTW Almarg is a very good technical source, I would be still afraid though that a fixed resistor might flatten the sound. Just superstition but the resistor has to maintain the overall "caliber" of quality the rest of the pre has.
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