Swapping 12ax7 with 2au7 on this circuit?

[Siemens EL34]

Hey guys... your opinion would be really helpful on this. I have this old little tube mixer full with 12ax7s. All3 preamp tubes are 12ax7, output gain 12ax7 , only the headphone amp is 12au7. The preamps are microphone preamps. Huge.. but i mean insane amount of gain. If I turn a pot like 1cm up, an SM57 is screaming.

My question is as I am not a techy... can all of these be swapped with 12au7 to get less gain? I do not know if anything like bias, current etc..needs to be adjusted? I do not want to damage anything before starting swapping out them. Thanks for your help!

[shooter]

maybe a little tinkering 1st;


pull v3 v4 and gator clip/wire like the sketch
get you closer to the gain you want?

[PRR]

Try one of the preamps with 12AU7. I bet the gain hardly goes down at all. The gain is set by NFB, the tube hardly matters.

Shooter's plan will work, though hiss level will be high.

The thing just has WAY TOO MUCH gain. This is typical of older gear sold on contract (better to be twitchy and hissy than to fall short). Almost 90dB. Every time an ignorant contract stuck me with one of these too-hot mixers I had to do a stage-by-stage re-design.

[AmberB]

You could probably add negative feedback to each stage to lower the gain until you get it where you want it.

[PRR]

You could probably add negative feedback to each stage to lower the gain until you get it where you want it.

Mebbe. But it already has gain like 625 in 2 stages cut down to about 30, 21:1 of NFB. Too much NFB, the response gets peaked at the ends of the bandpass, then any little thing makes it motoboat or squeal sub/super-sonic.

My butcher-sense says take out the stage I've tagged "25". Run the mix-network right to the Master pot.

All pots should be Audio Taper. And you should not be afraid to run them below "5". I've seen gear which was all Linear, and was just-right for some narrow range of small input levels, and unmanageable for anything more.

If the output is driving anything less than 1959 studio gear or long transmitter lines, it should be padded way down. Put 100 Ohms or less directly at the output jack, as long as the next input is happy.

[Siemens EL34]

Thanks guys for the replies, really appreciate it. And PRR... your are indeed right about the date ( and everything else too! ) ..this mixer was built around end of 1950s/early 1960s by someone for a radio station. I did trace some original documents down where most likely this mixer was mentioned. ( attached)

I would like to hook this mixer up with the "bridge in" input of a Magnegord (again 1950s) reel to reel preamp, leaving the input TX out for the mic, i am guessing this "bridge in" high Z. ?

I will also check the pot's..not sure what type of pot's are in there those 100k/s audio tapers or linear.

The thing is, I would really want this to work 100%. I did some test recordings, even with this huge gain, the sound coming out of this is incomparable to any other mic pre I have. I want to save this. Each input TX is a 1kg RCA and the output is also a huge RCA TX.

Again, thanks for you all your suggestions! Will start with the pot and changing them if these are not audio tapers, then as suggested, padding the output. Might try 12au7, but that according to PRR will not change much, so I will lave that out. Will come back here with some updates :) Cheers guys!


...after the above, checked the pots... Type J, TP39268 100k