12AX7 12AY7 together in perfect harmony?

[Fred G.]

I'm wondering if I can drop the gain on 1/2 of a 12AX7 to get it to around the gain of a 12AY7 (having, let's say, 100K plate resistor, 820 Cathode R ) by messing with cathode resistor alone?

I'm in a situation where I have one socket and I'd like to have 1/2 12AX7 in one half and a "simulated" 12AY7 in the other half (i.e, an extra gain stage driving a 12AY7-type Tweed first preamp tube) - all by using one 12AX7. Is it do-able?

Thanks!

[Tom Phillips]

Originally Posted by Fred G.

I'm wondering if I can drop the gain on 1/2 of a 12AX7 to get it to around the gain of a 12AY7 (having, let's say, 100K plate resistor, 820 Cathode R ) by messing with cathode resistor alone? !

You could do that but you’d be messing with the bias point and could create some other undesirable effects. You could also change the value of the cathode bypass cap but that would alter the frequency response as well as the gain.

Originally Posted by Fred G.

I'm in a situation where I have one socket and I'd like to have 1/2 12AX7 in one half and a "simulated" 12AY7 in the other half (i.e, an extra gain stage driving a 12AY7-type Tweed first preamp tube) - all by using one 12AX7. Is it do-able? !

Here’s an idea I have wanted to try but have not done yet. Convert the plate resistor to a split load. This allows you to tap off a reduced (i.e. effectively lower gain) AC signal from the plate resistor.
You can see a split load plate circuit in the first stage of the fender stand alone reverb at http://schematicheaven.com/fenderamp...6g15_schem.pdf
In that case they added an additional 10k Ohms but that’s not necessary. You can use two resistors that add to 100k Ohms total which is the original plate resistor value. The important feature is that the output coupling cap is moved to the new tap point created by the split plate resistor. You could install a 100k Ohm pot temporarily, adjust to your liking then measure the resistance of each half and replace with fixed resistors for your final circuit.

Cheers,

Tom

[Enzo]

Or you can add a resistor or two between stages to reduce the signal level some, this is done all the time. If we make them variable, we call it a volume control.

[Diablo]

Originally Posted by Enzo

Or you can add a resistor or two between stages to reduce the signal level some, this is done all the time. If we make them variable, we call it a volume control.

Now that's funny....

[loudthud]

Look at this thread: Extra Gain Stage - Tremolo?

The 4.7K probably needs to be something like 2.7K to 3.3K for the gain of a 12AY7. Install a trim pot and adjust as needed.

[MWJB]

"I'm wondering if I can drop the gain on 1/2 of a 12AX7 to get it to around the gain of a 12AY7 (having, let's say, 100K plate resistor, 820 Cathode R ) by messing with cathode resistor alone?" Simple answer = "no". But, as Tom says, a split load plate resistor is one idea, this will really sound like a low gain 12AX7 (e.g. a 12AX7 with more sweep on the dial) than a true 12AY. You could also add an additional power supply dropping node (100K) between the 2 halves of the tube, run the lower gain side at lower voltage, also upping the cathode value slightly to say 2.2K/2.7K? Try the low gain side with/without its bypass cap.

These suggestions can be mixed & matched with earlier ones.