Critique my schematic 12ay7 linestage

[badman]

Please critique my schematic for a 12ay7 grounded cathode linestage using this trafo: http://jensentransformers.com/datashts/10kbd.pdf

I'd appreciate any feedback people may have. The trafo seems well-suited due to its high input impedance but modest ratio, so I keep a tiny amount of gain. Being an input transformer by nature, it wouldn't like DC so it's cap-coupled.

[GMuffley]

I built a line stage using the 12AY7 tube, but with a cathode follower rather than transformer coupled. I found that 280V B+, with 100K plate rsistor and 1.8K for the cathode resistor (with 470 mfd cathode bypass) worked very well for the gain stage. Given the palte resisitance of the 12AY7, you probably want more B+ and more plate resitance for lowest distortion and greater swing.

Your drawing has an inadvertent error--the output is shown coming off the grid rather than the plate.

[badman]

Originally Posted by GMuffley

I built a line stage using the 12AY7 tube, but with a cathode follower rather than transformer coupled. I found that 280V B+, with 100K plate rsistor and 1.8K for the cathode resistor (with 470 mfd cathode bypass) worked very well for the gain stage. Given the palte resisitance of the 12AY7, you probably want more B+ and more plate resitance for lowest distortion and greater swing.

Your drawing has an inadvertent error--the output is shown coming off the grid rather than the plate.

Thanks for the input! With the transformer, the lower output impedance of the lower plate resistor would not be the problem it is with the simple gain stage driving the output directly (as I'm using it now), so your suggestions make sense from that perspective. Attached is a corrected schema.

[Eli Duttman]

Dude,

Look at the 12AY7 data sheet. R_P is 25 KOhms. At least 3X the plate resistance, as the net load impedance, is necessary for low distortion. Your schematic shows a net load less than R_P.

Replace that 25 KOhm part with a 10M45S CCS, if you don't want to increase the B+ rail voltage.

[badman]

Originally Posted by Eli Duttman

Dude,

Look at the 12AY7 data sheet. R_P is 25 KOhms. At least 3X the plate resistance, as the net load impedance, is necessary for low distortion. Your schematic shows a net load less than R_P.

Replace that 25 KOhm part with a 10M45S CCS, if you don't want to increase the B+ rail voltage.

Eli, you seem to be thinking the trafo is 1:1. It's not. It's 40k:2k4, which will load the tube with about 50k

[Eli Duttman]

Remember, that impedances in parallel follow a reciprocal law, to yield the net. As far as AC is concerned, the 25 KOhm resistor and the trafo are in parallel, hence the catch word parafeed. The net must be less than the smallest item in the parallel set. There's good reason why the parafeed circuits you see show either a huge inductance or a CCS as the connection to the PSU.

[badman]

Originally Posted by badman

Eli, you seem to be thinking the trafo is 1:1. It's not. It's 40k:2k4, which will load the tube with about 50k

My mistake. NET load. I was ignoring the plate resistor. You guys can see why I'm needing the help, my tube chops aren't too impressive.

[Arnulf]

Yup, listen to Eli

I'm curious though, what is your signal source and what kind of peak swing can it deliver ?

Have you considered turning things around, i.e. your transformer at the input, reversed (for voltage gain), and tube wired as cathode follower ?

[sakis]

Originally Posted by GMuffley

I built a line stage using the 12AY7 tube, but with a cathode follower rather than transformer coupled. I found that 280V B+, with 100K plate rsistor and 1.8K for the cathode resistor (with 470 mfd cathode bypass) worked very well for the gain stage. Given the palte resisitance of the 12AY7, you probably want more B+ and more plate resitance for lowest distortion and greater swing.

Your drawing has an inadvertent error--the output is shown coming off the grid rather than the plate.

is there a chance that you post a schematic of your application ??? i have so many original motorola 12AY7 and i constructed afew things with them but i was never very happy with them

kind regards sakis

[badman]

Hi Guys, Muffley:

280V and 100k do seem like more optimal operational points, though I'll probably juice up the cathode resistor a little higher, maybe 2.5k. Annoyingly, I'll have to change from a 6x4 fullwave to a solid state bridge to get the voltage needed. C'est La Vie.

After this, I'll still have a lower load on the tube, 2x Rp, but that's a heckuva improvement.