Preamp level matching transformer question

[Blue Jinn]

Hello,

For some reason, I can't seem to get my head around this. I have a 12AX7 preamp built from a circuit in a 1991? EM. It's pretty conventional two stage with a "drive" pot on the second tube grid. Basically like the "McTube" with a Baxandall tone control between the two stages.

Anyway, has a pot on the output for volume control. Even with the volume control down it is still way too hot, so I was thinking of eliminating hte pot* and putting a 10k or 15k:600 trafo on the output, to knock the signal down, with a side benefit to lower the output impedance (can't find the circuit right now, but assuming it's fairly high as comes straight off the plate via a cap and a 2.2k resistor to the top of I think a 50k or 100k pot.)

I don't have balanced I/O on anything it's going to connect to so I planned to just pull the "+ side" of the trafo just past the blocking capacitor on the tube, and then run the "+ side" of the secondary to the tip on the TS connector, and then ground the other trafo connectors. The whole thing is in a metal chassis, and signal ground and chassis ground are all the same.

Or, would it be better to isolate the signal ground, and take + and - from the transformer secondary? (Grounding - at the console input.)

* I've read elsewhere that putting a pot in front of a transformer can have undesired effects.

 

[dementedchord]

sounds like a question for the guys at prodigy.com ...

 

[Minion]

If the Pot is all the way down to zero then there shouldn"t be any sound coming out at all , when it is at zero the signal is shorted to ground .....

It is possible that there is a resistor between ground and the ground lug on the pot in which case there would still be volume even if the pot was at zero ..... If this is the case then removing this resistor and replacing it with a wire link would make it so there is no volume when the pot is at zero .....

You can also just put a 2 resistor voltage divider at the output that will knock the signal down ........

Cheers

 

[mshilarious]

If you have a pot between two tube stages it has no effect on output impedance of the second stage. Could you post a schematic or something? Even a sketch?

 

[Blue Jinn]

I can't locate the magazine article at the moment, but this is the jist of the circuit, it is just that the gain is pretty dang high, and I have a 5:1 trafo I'm not using, and the Edcors are pretty cheap also, so I thought to swap the output resistor and pot with the transformer to tame the output (and lower the output impedance as well.)

 

[mshilarious]

Oh there are two pots, OK. Yes, a pot on the output is generally not the best idea ever, and a 12AX7 plate follower output impedance is way too high for a line driver. A transformer is a better idea, but you'll have to try them, even if you strap on a 10K load (such that you have a nominal 250K load to the tube), if the transformer isn't designed for a 50K source impedance then the frequency response might not be too great. And you have to see if a very light load is not damping the transformer correctly, but if you increase the load then you start to have trouble with the load on the plate.

And of course you give up gain . . .

Anyway, try it and see how well it works, it could be just the thing. If the 4:1s don't work, try a DI transformer instead.