Can a 12AT7 sound similar to a 5751 ?

[chopstuck]

I'm working on a couple amps and would like to modify circuits using the expensive USA 5751 and subbing one of my many 12AT7's. The tube would be in an interstage position between the first gain stage and the inverter. What bias and impedance issues do I face ? I'm looking to optimise the circuit for the different charictaristics since the mu seems quite close. Ive got at least a dozen different AT's lying around.

[David Root]

In that position the 12AT7 will have a lower Zout, which going into a typical LTP PI doesn't matter as that will have Zin of 2-4Meg. What it does do is provide a wider DC and AC swing allowing a large clean signal to hit the PI. This seems to make a significant difference in getting max. clean power output.

Downside is the 12AT7 really needs more current than the 5751.

[chopstuck]

Forgive a dumb question - what needs to be done to supply more current if the PT is up to it ....change the dropping string resistors to a lower value ?

[tubeswell]

If the tube unless it is cathode biased, then decreasing the size of the cathode resistor will increase the current at the same time as lowering the bias voltage making it run hotter, and making it go into grid-current-limiting more easily.

Whereas raising the HT voltage (up to a point that doesn't exceed the rated maximum plate voltage) will give more headroom, and enable you to bias it colder (less current) with a larger cathode resistor without going into cutoff.

A 12AT7 runs at a lower plate voltage/higher plate current compared to a 5751 at the same HT voltage.

[Ears]

Forgive a dumb question - what needs to be done to supply more current if the PT is up to it ....change the dropping string resistors to a lower value ?

You are still only talking 7 to 10mA per triode or so for the AT. The AX7 drawing about 1ma per triode.
But in the preamp string this extra draw may require tweeking the resistors somewhat, no biggie.

[David Root]

I notice your original question also asked how it will sound compared to a 5751. The higher current should give you a rounder tone especially if you put the quiescent operating point more or less in the middle of the load line, where you will get the wide swing, some even harmonic distortion but likely no clipping.

If you pick a lower or higher bias point you will get more odd harmonics and maybe start clipping top or bottom of the signal. Mixing this in a chain of stages gives a more complex tone. Depends what you like.