Could a 12AX7 in parallel with resistor replace a 12AY7?

[stiltamp]

Just an idea!
The internal resistance of a 12X7A triode will diminished by a 39kOhms resistor in parallel to the plate and cathode terminals to about 24kOhms.
In that case the amplification factor is diminished to 1,6 mA/V * 24 kOhms = 38.
The internal resistance of the 12AY7 triode is 23kOhms and the amplification factor is 40.

What do you think about, could this replacement work?
Any experience?

[roberto]

Could a 12AX7 in parallel with resistor replace a 12AX7?

12ax7 or 12ay7?

[stiltamp]

Could a 12AX7 in parallel with resistor replace a 12AX7?

12ax7 or 12ay7?

Thanks, I corrected the mistake.

[gingertube]

stiltamp,
No experience so FWIW.

Gain of the stage (with bypassed cathode resistor) is :

Gain = mu [(RL)/RL+rp]
For 12AX7 with 100K anode load resistor and rp = 62K and mu of 100 (typical) you get a gain of 62.

For a 12AY7 with a 100K anode load resistor and rp = 25K and a mu of 40 (typical) you get a gain of 32.

Both cases ignore the loading by the following stages grid leak resistor to ground (1 Meg typical).

In High Gain preamps you will often see an interstage attenuator often with quite low resistance values or even just a low value resistor to 0V. The effect of this resistance is to reduce the effective value of the anode load resistor RL which reduces the gain, in fact some authors describe this as "gain dumping".

To get the 12AX7 gain down to 12AY7 levels you would want an effective RL of about 30K (gain calcs give 32.6) which would require a "gain dumping" resistance of about 43K (we would of-course use preferred value of 39K).

The thing is that it will not sound the same. The amount of distortion you get in a triode stage is proportional to the change in rp (as the anode current swings up and down with signal) vs the effective anode load resistance.

The 12AY7 has an rp of less than half the 12AX7 and so its delta rp with signal current is likely to be much smaller and would normally give about half the distortion of the 12AX7.

With an effective RL of 30K I would expect the 12AX7 to give 3 times the distortion of the same 12AX7 with 100K. That is, a "gain dumped" 12AX7 (to 12AY7 gain levels) would have at least X3 the distortion - of a non-gain dumped 12AX7

Combining these two (estimates) I would conclude that a "gain dumped" 12AX7 would probably give you something like 6 times the distortion of a 12AY7 in the same place.

Hope this is helpful, values not exactly full accuracy but then not exactly plucked from the butt either.

Cheers,
Ian

[tubeswell]

But if you ran the paralleled 12AX7 stage through 47k plate load, with an unbypassed 820R Rk, you'd have a gain of around 32-34 I'd guess, and a combined plate resistance of 31k. It could sound a bit chimier than an ordinary 12AX7. If you want more chime, you could experiment with a treble boost (or stepped treble boost) bypass for the Rk, and even add a 100pF bright cap to the vol control following the stage

[VacuumVoodoo]

All this will do is shunt DC plate current and put the triode into close to cut-off.
You can use this DC-shunt with higher resistance values, close to the value of plate resistor to reduce gain by a couple dbs but it shifts the DC operating point quite dramatically.
It does not change the triodes internal plate resistance but changes the load seen by that triode and source drive resistance as seen by following gain stage.
Same thing happens when you use low grid leak resistance on that following stage but it's AC coupled to the preceeeding so it doesn't divert its DC op-points.
You can Spice it and see for yourself.

[stiltamp]

Thanks to all for the interresting and helpfull explanations.

[gingertube]

For simplest "close approach" to the 12AY7 gain and sound I would try a 12AT7 without a cathode bypass cap.
Cheers,
Ian

[tubeswell]

Speaking about tube comparisons, I recall acquiring this article from Old Tele man (Ampage forum) a while back. Worth a read for pre-amp comparisons

[katopan]

A lot of guys are using a 12AX7 with split plate resistors to lower the gain in place of the normal 12AY7 in V1 of their 5E3 Tweed Deluxe amps.